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Health-risk factors and the incidence of hypertension: 4-year prospective findings from a national cohort of 60,569
Thai open university students
Journal: BMJ Open
Manuscript ID: bmjopen-2013-002826
Article Type: Research
Date Submitted by the Author: 06-Mar-2013
Complete List of Authors: thawornchaisit, prasutr; Faculty of Health Sciences, School of Medicine; Lerdsin Hospital, Medicine de Looze, Ferdinandus; Faculty of Health Sciences, School of Medicine
Reid, Christopher; Monash University, School of Public Health and Preventive Medicine Seubsman, Sam-ang; Sukhothai Thammathirat Open University, School of Human Ecology Sleigh, Adrian; The Australian National University, National Centre for Epidemiology and Population Health
<b>Primary Subject Heading</b>:
Epidemiology
Secondary Subject Heading: Epidemiology, Public health
Keywords: Hypertension < CARDIOLOGY, General diabetes < DIABETES & ENDOCRINOLOGY, Risk management < HEALTH SERVICES ADMINISTRATION & MANAGEMENT, Adult nephrology < NEPHROLOGY
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Health-risk factors and the incidence of hypertension: 4-year prospective findings
from a national cohort of 60,569 Thai open university students
Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University of Queensland, 288
Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of Medicine, Lerdsin Hospital,
Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected]
Telephone: 662-585-8015
Fax: 662-912-0121
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc1, Christopher M Reid, PhD2, Sam-
ang Seubsman, PhD3, Adrian Sleigh, MD4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia, 2 School of Public Health and
Preventive Medicine, Monash University, Melbourne, Australia, 3 School of Human Ecology, Sukhothai
Thammathirat Open University, Nonthaburi, Thailand, 4 National Centre for Epidemiology and Population
Health, ANU College of Medicine, Biology and Environment, The Australian National University,
Canberra, Australia.
Health-risk factors and the incidence of hypertension: 4-year prospective findings from a national cohort of 60,569
Thai open university students
Key words: hypertension; socioeconomic status; body mass index; smoking, drinking
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Article summary
Article focus
To evaluate the role of health-risk factors in 2005 Thai Cohort Study (phase I) such as age, personal income,
educational attainment, obesity (body mass index), physical activity, diabetes mellitus, kidney diseases, high
lipids, cigarette smoking and alcohol consumption in the development of hypertension in phase II of the
Thai Cohort Study 2009.
Key Messages
Overall, the incidence of self-reported hypertension in Thai adults was 3.5% and the rate in males was more
than twice the corresponding rate in females (5.2% vs. 2.1%).
Marriage or having a partner increased the risk of hypertension in females but not in males. Planned
exercise, averaged across the 4-year follow-up, was protective for males. In males, higher LPPA (≥15
sessions / week) associated with lower incidence of hypertension however this was not observed in females.
In males, current-smokers had a higher risk of developing hypertension than non-smokers while in females,
smoking (which was rare) had no apparent hypertension effect. Among males, regular drinking increased the
risk of incident hypertension
Strengths
In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. The influence of risk factors on hypertension incidence is clearly
demonstrated since this is a large longitudinal cohort covering a 4-year period. As a consequence, the
independent risk factors for hypertension in Thais can be identified.
Our cohort represents Thais well in geography and socio-economy so the results reflect the trend of health-
risk transition in the Thai population.
Limitations
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However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
Because of the better education attainment of STOU students combined with their representative income and
geographical location, they represent the next generation of Thais.
Abstract
Objective
As Thailand is transitioning from a developing to a middle-income developed country, chronic diseases
particularly cardiovascular disease have emerged as major health issues. Hypertension is a major risk factor
for heart attack and stroke and earlier cross-sectional studies have indicated that the prevalence is increasing.
This study evaluates the impact of a number of demographic and lifestyle related health risk factors on the
incidence of hypertension over a 4-year period.
Designs
This 4-year prospective study analysed the health-risk factors and their effects on incidence of hypertension
in the national Thai Cohort Study (TCS) from 2005 to 2009. 56,072 Sukhothai Thammathirat Open
University students who participated in both 2005 and 2009 questionnaire surveys and who were
normotensive in 2005 were included in the analysis. Adjusted relative risks of association between each risk
factor and incidence of hypertension were calculated by sex, after controlling for the confounding factors
such as age, SES, BMI and underlying diseases.
Results
The overall 4-year incidence of hypertension was 3.5% with the rate in men being remarkably higher than
that in women (5.2% vs. 2.1%). Hypertension associated with age, a higher BMI and having co-morbidities
in both sexes. In men, hypertension associated with physical inactivity, smoking, alcohol and fast food
intake. In women, hypertension was only related to having a partner.
Conclusion
In both men and women, hypertension was highly associated with age, obesity and co-morbidities while it
had no association with socio-economic factors. The cohort pattern of socio-economy and hypertension
reflects that the health-risk transition in Thais is likely to be at the middle stage. Dietary and lifestyle factors
are associated with the incidence of hypertension in Thais and may be amenable targets for community
based hypertension control programs.
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Introduction
Hypertension is a global public health challenge due to its high prevalence and the concomitant increase in
risk of stroke and cardiovascular diseases [1]. In 2007, according to the World Health Organization,
cardiovascular diseases caused 33.7% of all deaths in the world, while other chronic diseases were
responsible for 26.5% [2]. Globally, it has been estimated that premature deaths resulting from hypertension
annually are approximately 7.1 million, which account for 64 million disability-adjusted life years (DALYs)
[3]. Hypertension ranks third, after underweight and unsafe sex, of the six major risk factors contributing to
the global disease burden[3].
In Thailand, hypertension is the third major risk factor of the national disease burden and causes 600,000
DALY losses per year [4]. The prevalence of hypertension has been estimated from a number of cross-
sectional studies among adults and has been increasing over the past decades from 20% in 1985 to 22% in
2004 [4-6]. The emergence of hypertension has been the result of rapid socioeconomic development which
has resulted in one third of Thais migrating to urban areas [7]. Urbanization has been associated with
increased obesity as people become less physically active and adopt sedentary life styles such as spending
more time watching television, sitting and using computers [7 8]. These factors, in association with
Thailand’s ageing population [9], underpin the importance of controlling hypertension in Thai community.
As there is limited longitudinal research on health risk factors and hypertension incidence in Thailand [5].
The purpose of this study is to address that knowledge gap. We prospectively examined the influence of a
cluster of health-risk factors on the incidence of hypertension during 4 years of follow-up in the Thai Cohort
Study (TCS). This large national cohort of Thai adults has been followed since 2005 for risk factor trends
and associated disease outcomes, including hypertension.
Methods
Study population
The first TCS evaluation was conducted in 2005 by sending twenty-page health-risk questionnaires to the
200,000 distance-learning students enrolled at Sukhothai Thammathirat Open University (STOU) and
87,134 (44%) completed them and signed the consent forms. The cohort has been shown to be socio-
economically and geographically well representative of the Thai population [10]. However, they resided
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more in urban areas (51.8% vs. 31.1%) and on average were younger and better educated than most Thai
adults. Details of the baseline TCS methodology have been published [10]. This questionnaire focused on
socio-demography, habitation, work, health services, injury, sedentary habits, physical activity, transport,
underlying diseases, family history, personal behaviours, BMI, and consumption of foods, vegetables and
fruit. A 4-year follow-up was carried out in 2009. Overall, 71% of the cohort responded generating
longitudinal data on 60,569 participants.
Incident Hypertension
Incident hypertension was defined as being normotensive in 2005 and self-reported hypertensive in 2009.
Hypertension incidence was the dependent variable for analyses and the person-time denominator included
all individuals at risk - those 57,558 participants with longitudinal data who were negative for hypertension
in 2005, the beginning of the 4-year study period.
Risk Factors
Participants were grouped into three age categories: younger (≤30 years), middle (31-40 years) and older
(>40 years). Urbanization status was based on rural (R) or urban (U) residence, first when aged 10-12 years
old and again in 2005, producing four groups: lifelong ruralites (RR), urbanizers (RU), de-urbanizers (UR)
and urbanites (UU). Personal monthly income was divided into four categories: ≤7000 baht, 7001–10000
baht, 10001–20000 baht and >20000 baht. In 2005, one US dollar was equivalent to 42 baht so most of
participants had quite low incomes. Household assets were classified into three categories: low (≤30,000
baht), medium (30,001-60,000) and high (>60,000).
Sedentariness was assessed by screen time (hours/day spent before TV or computer) and (separately) sitting
time (hours per day spent sitting for any purpose). Incidental exercise was measured by the frequency of
housework or gardening, categorised as follows: ≤3 times per month; 1-2 times per week; 3-4 times per
week; most days. We measured planned physical activity at the beginning and end of the 4-year follow up
period. On both occasions, cohort members reported the number of sessions per week of strenuous and
moderate exercise for at least 20 minutes, and of walking for at least 10 minutes. We derived an “overall
measure” of planned physical activity calculated separately at both the baseline and the 4-year mark. The
measure was weighted as follows: (2 × strenuous + 1 × moderate + 1 × walking) sessions per week. This
weighting system is based on the recommendation of the International Physical Activity Questionnaire and
the Active Australia Survey as used in other analyses of the cohort data [11 12]. Finally, for each individual,
the “overall measures” of weekly exercise for 2005 and 2009 were added and then averaged by dividing by
2, creating a longitudinal measure of planned physical activity (LPPA).
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Smoking status was classified as never, ex-smoker or current-smoker. Alcohol exposure was classified by
four categories: never, ex-drinker, occasional drinker or current-drinker. Body mass index (BMI) was
calculated from self-reported height in metres and weight in kilograms as weight/ height2. It was divided
into 4 categories as follows according to Asian criteria:[8 13-16] underweight (BMI <18.5), normal (18.5 to
<23), overweight (23 to <25), and obese (≥25). Foods that could potentially influence blood pressure (deep
fried, instant, roast or smoked, soybean products and soft drinks) were assessed for consumption frequency
based on a five-point Likert scale ranging from less than once a month to once or more a day. Western-style
fast food exposure was noted on a three-point scale from less than once to more than 3 times per month.
Fruit and vegetable consumption were recorded as standard serves eaten per day.
Statistical analyses
All analyses were performed using SPSS software. The incidence of hypertension and its 95% confidence
interval (CI) were calculated for each value of each categorical variable in both male and female participants
and the influence on incidence by each variable was evaluated by Chi-square test. For statistical inference,
all p-values were 2 tailed and significance was set at 5%.
Relative risks in a large study of an uncommon disease (incidence less than 10%) can be accurately
estimated as odds ratios [17]. Accordingly, for each risk variable, the relative risk (RR) and 95% confidence
interval were estimated using logistic regression to calculate the bivariate odds ratio for hypertension.
Adjusted RRs (ARRs) were estimated by calculating multivariate logistic regression odds ratios. The ARRs
were controlled for confounding by age, marital status, SES, BMI, underlying diseases and personal
behaviours (cigarette smoking and alcohol drinking). A variable was included in a multivariate model if
bivariate analysis had indicated a statistically significant association with incidence of hypertension. Some
variables were included because earlier analyses reported elsewhere had shown significant or substantial
association with hypertension.
Results
Baseline Characteristics
Baseline characteristics of study participants self-reporting as normotensive in 2005 are shown in Table 1.
The highest proportion of males resided in the North-eastern regions and Central while there were more
females in the Central and Bangkok area. More than half of the male and female participants lived in urban
areas. Males had a higher monthly income than females whereas females had a higher educational
attainment than males. The distribution of household assets for males and females was similar and over half
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of them were in medium and high categories. The study population had a similar socio-demography, income
and location of residence to the general Thai population [9].
Incidence and risks of hypertension
Overall, the incidence of self-reported hypertension in Thai adults was 3.5% and the rate in males was more
than twice the corresponding rate in females (5.2% vs. 2.1%) (Table 2). In males and females, the incidence
of hypertension increased with increasing age. Marriage or having a partner increased the risk of
hypertension in females only. Educational attainment and personal income had no influence on the risk of
hypertension in both sexes. In males and females, the risk of hypertension increased with an increased BMI.
The risk of hypertension increased in both males and females who had diabetes mellitus, high blood lipids
and kidney disease.
Planned exercise, averaged across the 4-year follow-up, was protective for males. In males, higher LPPA
(≥15 sessions / week) associated with lower incidence of hypertension however this was not observed in
females.
In males, current-smokers had a higher risk of developing hypertension than non-smokers while in females,
smoking (which was rare) had no apparent hypertension effect. Among males, regular drinking increased the
risk of incident hypertension; patterns of relative risks were similar for females but, reflecting the low
frequency of female drinking, did not reach statistical significance. In males, the risk of hypertension had a
direct association with the frequency of instant food consumption but there was no such association detected
in females.
Over the 4-year period, a variety of factors that could have been risks for hypertension were shown not to be
in this setting (data not shown). Thus, in males and females, factors that had no statistically significant
influence on the longitudinal risk of hypertension incidence were urbanization status, non-participation in
housework or gardening, high levels of television and computer watching, sleeping and sitting time, and
food consumption habits that included deep fried food and soft drink, Western fast food, roasted or smoked
foods, fruit, vegetables and soybean products.
Discussion
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This large prospective cohort of adults living all over Thailand has been the first studied nationally for a
wide array of risk factors for incident hypertension. Overall, the 4-year incidence of hypertension from 2005
to 2009 was 3.5%, notably higher in men (5.2%) than women (2.1%). In both sexes, age, obesity and co-
morbidity with diabetes mellitus, high blood lipids and kidney disease strongly linked to hypertension
incidence; in males, instant food consumption, smoking and drinking alcohol had a moderate adverse effect
while planned exercise averaging 15 or more sessions per week over the 4-year period was substantially
protective, reducing risk by 22%. In females, having a partner linked to an increased incidence of
hypertension.
The age effect on hypertension incidence noted by us is consistent with findings of many other population
studies of hypertension including (for example) a longitudinal study in China [18] and cross-sectional
studies in Korea [19], Taiwan [20] and the USA [21]. The noticeably greater incidence of hypertension in
men than in women that we found in Thailand has also been reported from many parts of the world
including neighboring Malaysia [22] and distant USA [23] as well as in a recent global review [24].
In men and women of the Thai cohort, a progressively higher BMI was positively and significantly
associated with an increased risk of incident hypertension over the four years. This finding was similar to
that reported for other prospective cohorts in the US [25 26] and Finland [27]. Furthermore, after seven
years of follow-up in a US study [28], weight loss associated with a decreased risk of incident hypertension.
Among a cohort of US nurses, the risk of incident hypertension increased 5% for each kilogram addition of
weight [29], and rose 15% for each increase of one kilogram per meter squared of BMI in Taiwan [18].
These results suggest that the adverse effect of high BMI on risk of hypertension does not depend on
ethinicity or sex and is wide spread.
In our cohort, we noted a higher risk of incident hypertension among those with diabetes mellitus. This
finding is consistent with longitudinal studies in the USA [30 31] which reported, respectively, that in
diabetic patients the risk of incident hypertension rose 41% and 56%. As well, prospective cohort studies in
Japan showed that diabetes [32] and impaired fasting blood glucose [33] were associated with an increased
risk of hypertension incidence. Supportive cross-sectional results emerge from studies in diverse settings
including Bangladesh and India [34], Barbados [35] and the USA [36]. A causal association between
diabetes and hypertension is suspected and possible mechanisms include endothelial dysfunction and
vascular inflammation [37].
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We also found that high blood lipids significantly associated with increased risk of incident hypertension.
This supports the results from previous longitudinal studies in men [38 39], women [40 41] and adults [32
42]. The risk of hypertension increased with an increase of total cholesterol, triglyceride and low-density
lipoprotein cholesterol levels and decreased with an increase of high-density lipoprotein cholesterol level
[38 39 41].The mechanism by which dyslipidemia provokes high blood pressure is uncertain but could
reflect blood vessel dysfunction and damaged endothelium [43].
Among men and women in the Thai cohort, kidney disease strongly associated with increased risk of
hypertension incidence. This result is supported by cross-sectional data from the Czech Republic [44] and
the USA [45]. Microalbuminuria is a biomarker for renal hypertension [45] and the risk increases with the
severity of kidney impairment, revealed by the serum creatinine level [46 47]. Glomerular damage is more
closely connected with hypertension than tubular or interstitial disease [47]. Chronic kidney disease
facilitates high blood pressure through many mechanisms including excessive intravascular volume,
increased endothelin production, activity of the renin-angiotensin system and sympathetic nervous system
and decreased vasodilators, nitric oxide production and endothelial function [48].
In men we found that cigarette smoking modestly associated with increased risk of developing hypertension.
This is consistent with many other reports such as longitudinal studies in Japanese [49] and American men
[50] and a cross-sectional study in French men [51]. In addition, smoking Jordanian men had significantly
higher blood pressure than their non-smoking counterparts [52]. However, we also found that smoking had
no influence on hypertension in women. This result for females is compatible with reports from Korea [53]
and the USA [54]. In contrast, one prospective US study [55] reported an increased risk of hypertension
among women. It is likely that men tend to smoke more cigarette than women and the impact of smoking on
hypertension is a dose-response pattern [49].
Regular alcohol consumption was strongly associated with an increased risk of incident hypertension. This
result is supported by findings from longitudinal studies in Japan [56 57] and cross-section studies in
China[58] and Korea [53]. In Japanese men, alcohol consumption, particularly more than 200 grams per
week, significantly associated with an increase of blood pressure [59] while alcohol restriction was
noticeably related to a decreased blood pressure [60]. Among our Thai cohort females, alcohol consumption
had no influence on hypertension; this result was supported by longitudinal data from Japan [56] and cross-
sectional data from Korea [53]. In American females the quantity of alcohol consumption per week had no
effect on systolic blood pressure [61]. Males usually consume more alcohol than females and the
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hypertension risk is linear with a dose-response [57 62]. Our results suggest that regular alcohol
consumption, especially in males, is a significant risk factor for developing hypertension.
Marital status in males had no influence on hypertension which supports the results from more economically
developed countries, the USA and Korea [53 54]. However, in middle-income countries, results diverged:
married males, when compared to their unmarried counterparts, had a higher risk of hypertension in
Barbados [35] and a lower risk in Poland [63]. Married individuals had higher emotional and social support
and lower stress [63] and probability of being a current smoker [64]. We found incident hypertension was
relatively more frequent among married females, unlike the no-sex-link pattern in developed countries such
as the USA and Korea [53 54]. The influence of marrital status on hypertension varies from one country to
another country partly due to the difference of socio-economy and social structure. For our Thai cohort, the
pattern in males was similar to that in developed countries and in females it was not.
Participant educational attainment did not protect against hypertension in either sex. This is similar to cross-
sectional results for adults in developed countries such as Korea [53] and the USA [36 65]. However, other
studies in various settings and with diverse designs reported educational attainment protects against
hypertension in adults; these included a prospective cohort in middle income Thailand [5], a cross-sectional
study in middle income Malaysia [22] and a cross-sectional study in high income USA [66]. Opposite
findings have been reported in low income settings in both Bangladesh and India [34] where education was
a risk factor for hypertension. Taken collectively, these data suggest that income modifies the protective
education-hypertension effect, inverting it when income is low and boosting it when income is high. Our
middle-income cohort could be in the middle stage of this education effect as it is transiting from the low-
income pattern.
We found no association of personal monthly income and risk of hypertension in either sex. This was similar
to previous studies in middle-income countries including a longitudinal study in Thailand [5] and a cross-
sectional study in Malaysia [67] and in cross-sectional studies in high-income countries such as in Korea
[53] and the USA [21 36]. However, income had an inverse association with risk of hypertension in another
study in high-income developed countries such as the USA [65] while it had a direct association in a low-
income developing country such as Ghana [68]. The health transition in Thailand is likely to be in the
middle stage, according to the concept of a tipping-point for the relationship between socio-economic status,
body size and hypertension risk factors. High income, large body size and high blood pressure is seen in
developing countries while in developed countries, high income is a protective factor [69-71]. Indeed, the
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Thai cohort has already been shown to be at a transitional point for metabolic states because females had an
inverse relationship between income and BMI whereas males had a positive (traditional) relationship [14].
However, now all Thais benefit from universal health insurance so access to health care is not dependent on
their income making disease associations more attenuated than before [72].
In our cohort, the reduction of risk of hypertension incidence in males but not in females by a longitudinal
measure of planned physical activity was comparable with a prospective study in the middle-aged American
whites [73]. In another prospective study in middle-aged Finns, the hypertension protective effect of
vigorous physical activity or total energy expenditure from leisure-time physical activity was observed in
men but not in women [74]. Such physical activity among females may be less intense than among males so
having no effect on the risk of hypertension [74]. A large Finnish study [27] demonstrated that the incidence
of hypertension was inversely associated with the level of physical activity in a dose-response manner in
both sexes. In our cohort study, risk of obesity had an inverse association with quantified planned exercise in
both sexes but the effect was significantly stronger in males than in females [12]. As Thailand progresses
economically, more people migrate to urban areas and decrease their physical activity [7]. Therefore, in the
future, Thai people may encounter a higher risk of hypertension due to a lower physical activity.
Instant food - mostly combinations of carbohydrate, fat and salt - associated with an increased risk of
incident hypertension in males but not in females. Other foods, fruit and vegetables had no impact on
hypertension. This may be as a result of men comsuming more instant foods, such as instant noodles, than
women. For example a study in Korea showed daily instant food consumption in men and women averages
23.7 and 11.1 grams respectively [75]. Other studies showed that carbohydrate [76 77], fruit and vegetables
[78 79] consumption had no influence on hypertension. Most students consume instant noodles which lead
to a higher intake of energy, fat, sodium, thiamine, and riboflavin but a lower intake of protein, calcium,
phosphorus [75]. Koreans who favor instant noodles have a salt intake more than three times higher than the
daily recommendation (6.4 vs 2 gms) [75]. Daily salt consumption of Thai people has been reported to
average 10.8gm which is much higher than the recommendation
(http://www.worldactiononsalt.com/worldaction/asia/75286.pdf). High salt consumption was associated with
an increased risk of developing hypertension in a longitudinal study in Taiwan [80] and in cross-sectional
studies in China [81 82] while other reports revealed that salt restriction reduced blood pressure [83 84].
High plasma sodium, due to an overwhelmed capacity of the kidney to excrete excessive salt consumption,
leads to excessive intravascular volume and high blood pressure [85]. It seems reasonable to conclude that
high salt intake from instant noodle consumption is likely to be causally associated with an increased risk of
hypertension in Thailand.
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In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. Indeed, the study size along with its national representation are
important factors to consider when exploring study validity. The influence of risk factors on hypertension
incidence is clearly demonstrated since this is a large longitudinal cohort covering a 4-year period. As a
consequence, the independent risk factors for hypertension in Thais can be identified. Our cohort represents
Thais well in geography and socio-economy [9] so the results reflect the trend of health-risk transition in the
Thai population. Because of the better education attainment of STOU students combined with their
representative income and geographical location, they represent the next generation of Thais. It would be
reasonable to conclude that our 4-year follow-up results show the future hypertension trends for Thai people.
However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
But a study of the validity of self-reported weight and height in our cohort found that the accurary was quite
acceptable [86]. As well, we have conducted a validation study of self-reported hypertension (Prasutr
Thawornchaisit, 2013, under review) in a random age-sex matched sub-sample of the cohort reporting
hypertension (n=240) or no hypertension (n=240). We found the sensitivity was high (82%) and that
negative reports were usually accurate (86%). The findings from our study are likely to be robust and
represent well the future trends of health-risk transition in middle-income Thailand and similar ASEAN
countries.
In conclusion, older age, obesity and underlying morbidity due to diabetes, high blood lipids and kidney
disease were strongly associated with an increased risk of incident hypertension in both sexes while in
males, physical inactivity, instant food consumption, smoking and drinking alcohol had a moderate
association. Sedentary life-style and the consumption of friut and vegetables had no detected influence on
hypertension. The Thai health-risk transition is likely to be in the middle stage since education attainment
and income still have no net effect to protect against hypertension. Prevention should focus on obesity, high
blood lipids and high salt consumption and people should be encouraged to increase physical exercise and
consume low fat foods and less instant foods. A reduction of salt consumption should be a national policy by
limiting the concentration of salt in ready-to-eat and industrial foods. Smoking and drinking cessation
should be promoted for males. Thailand should also consciously aim to take advantage of its culturally
determined low rate of drinking and smoking among women and ensure that there is no attempt to
undermine this situation and develop the market for the female half of the population.
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Thai Cohort Study Team*
Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan
Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita
Sripaiboonkij, Pathumvadee Somsamai, Prasutr Thawornchaisit, Duangkae Vilainerun, Wanee
Wimonwattanaphan, Cha-aim Pachanee, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin
Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora,
Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim, Anthony
McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara Yiengprugsawan, Susan Jordan,
Janneke Berecki-Gisolf, Rod McClure.
Acknowledgements and Funding
This study was supported by the International Collaborative Research Grants Scheme with joint grants from
the Wellcome Trust UK (GR071587MA) and the Australian National Health and Medical Research Council
(268055), and by a global health grant from the NHMRC (585426). We thank the staff at Sukhothai
Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are
participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen
University for guiding us successfully through the complex data processing.
Conflict of interest
The authors declare that there are no conflicts of interest.
Ethical considerations
Ethical approval was obtained from Sukhothai Thammathirat Open University Research and Development
Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee
(protocol 2004344 and 2009570). Informed, written consent was obtained from all participants.
Authors' contributions
PT, FD, CR, SS, and AS jointly planned the study. PT performed the data collection and analysis and
drafted the manuscript. SS managed and coordinated the data collection. FD, CR, SS and AS participated in
the study conduction and manuscript preparation. FD, CR, SS and AS commented and corrected the
manuscript. All authors read and approved the final manuscript.
Data Sharing
No additional data available.
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Table 1 Characteristics of 57558 participants normotensive at the 2005 Thai Cohort Study baseline
Male Female Difference Factor n % n % P-valuea
Demographic data
Participants 25320 44.0 32238 56.0 <0.0001 Age (y) mean 32.9 30.0 <0.0001
b
(SD) (8.5) (7.6) Age group <0.0001 ≤30 y 11293 44.6 19229 59.6 31-40 y 9177 36.2 9638 29.9 >40 y 4850 19.2 3371 10.5 Married/partnered <0.0001 No 11060 44.8 18851 60.1 Yes 13607 55.2 12513 39.9 Regions <0.0001 Bangkok 3488 13.9 6057 18.9 Central 5712 22.7 8267 25.8 North 5180 20.6 5818 18.1 North-east 6142 24.4 5760 18.0 East 1515 6.0 1922 6.0 South 3103 12.3 4254 13.3 Urbanization status
c <0.0001
Rural–rural (RR) 11512 46,1 13895 43.6 Rural–urban (RU) 7891 31.6 9546 29.9 Urban–rural (UR) 1078 4.3 1344 4.2 Urban–urban (UU) 4495 18.0 7114 22.3
Socioeconomic status
Education level <0.0001 High school 13366 52.9 13135 40.8 Diploma 5749 22.8 9791 30.4 University 6153 24.4 9231 28.7 Personal monthly income (baht)
d <0.0001 ≤7000 8025 32.3 13890 44.1 7001–10000 5790 23.3 7578 24.0 10001–20000 7549 30.4 7194 22.8 >20000 3452 13.9 2851 9.0 Household assets
e (baht)
d <0.001
Low 9852 39.1 12434 38.7 Medium 8065 32.0 9863 30.7 High 7298 28.9 9804 30.5 aChi-square test b unpaired t test cLocation of residence (rural, R, or urban, U) at age 10-12 years and again in 2005. dAt the time of the survey in 2009, US$1 = 31 Thai baht eReplacement value in Thai baht: low ≤30,000, medium 30,001-60,000 and high >60,000
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Table 2 Hypertension incidence and association with risk factors in male and female participants.
Males Females HT(n) Incia%(95% CI) ARRb(95% CI) HT(n) Incia%(95% CI) ARRb(95% CI) Participants 1306 5.2 (4.9-5.4) 690 2.1 (2.0-2.3)
Demography
Age group
≤30 y 279 2.5 (2.2-2.8) 1 215 1.1 (0.97-1.3) 1 31-40 y 484 5.3 (4.8-5.7) 1.66 (1.39-1.98) 245 2.5 (2.23-2.9) 1.93 (1.56-2.39) >40 y 543 11.2 (10.3-12.1) 3.13 (2.57-3.81) 230 6.8 (6.0-7.7) 4.2 (3.26-5.42) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Marital status (married/partnered)
No 368 3.3 (3.0-3.7) 1 303 1.6 (1.43-1.8) 1 Yes 896 6.6 (6.17-7.0) 0.97 (0.83-1.12) 368 2.9 (2.65-3.2) 1.22 (1.03-1.45)
Socio-economic status
Education level
High school 697 5.2 (4.8-5.6) 1 303 2.3 (2.05-2.57) 1 Diploma 276 4.8 (4.25-5.4) 1.0 (0.86-1.17) 180 1.8 (1.57-2.1) 0.81 (0.66-0.99) University 331 5.4 (4.8-5.9) 0.93 (0.8-1.1) 204 2.2 (1.9-2.51) 0.87 (0.71-1.07) P-trend 0.83 0.61 0.463 0.1 Personal monthly income (baht)c ≤7000 268 3.3 (2.95-3.7) 1 217 1.6 (1.36-1.8) 1 7001-10000 229 4.0 (3.45-4.5) 1.0 (0.81-1.2) 142 1.9 (1.57-2.2) 1.1 (0.87-1.37) 10001-20000 497 6.6 (6.0-7.14) 1.1 (0.91-1.31) 196 2.7 (2.35-3.1) 0.97 (0.77-1.23) >20000 289 8.4 (7.46-9.3) 1.0 (0.8-1.25) 122 4.3 (3.54-5.0) 1.04 (0.78-1.39) P-trend <0.0001 0.527 <0.0001 0.783
BMI classificationd
Underweight 359 2.9 (2.62-3.2) 0.46 (0.27-0.78) 60 0.85 (0.63-1.15) 0.9 (0.67-1.21) (BMI <18.5) Normal 16 1.1 (0.56-1.6) 1 258 1.4 (1.2-1.5) 1 (18.5 ≤ BMI < 23) Overweight 333 5.9 (5.3-6.54) 1.66 (1.41-1.95) 129 3.9 (3.3-4.62) 2.32 (1.8-2.91) (23 ≤ BMI < 25) Obese 575 10.4 (9.6-11.2) 2.82 (2.43-3.27) 231 7.4 (6.4-8.27) 4.14 (3.4-5.05) (BMI ≥25) P-trend <0.0001 <0.0001 <0.0001 <0.0001
aIncidence of Hypertension
bAdjusted relative risks (ARR) were calculated from multi-variate logistic regression models of hypertension
adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification
Continued….
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Table 2 (continued...)
Males Females HT(n) Inci
a%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Physical activities
Weighted total sessions exercise-related physical activity (average between 2005 and 2009) 0-7 ses/wc 198 6.3 (5.5-7.1) 1 162 2.2 (1.8-2.6) 1 8-14 ses/w 589 5.2 (4.8-5.6) 0.9 (0.76-1.07) 352 2.1 (1.9-2.3) 1.0 (0.82-1.23)
≥15 ses/w 380 4.4 (4.0-5.0) 0.78 (0.65-0.95) 119 2.2 (1.8-2.6) 1.0 (0.77-1.3)
P-trend <0.0001 <0.029 0.905 1.0
Underlying diseases
Diabetes mellitus
No 1251 5.0 (4.7-5.27) 1 674 2.1 (1.9-2.26) 1 Yes 55 18.3 (13.9-22.7) 1.81 (1.29-2.52) 16 10.7 (5.7-15.8) 2.54 (1.41-4.56)
High lipids
No 983 4.4 (4.13-4.67) 1 576 1.9 (1.76-2.1) 1 Yes 323 10.8 (9.7-11.9) 1.4 (1.21-1.63) 114 5.4 (4.47-6.4) 1.38 (1.09-1.75)
Kidney diseases
No 1248 5.1 (4.8-5.33) 1 659 2.1 (1.9-2.26) 1 Yes 58 9.6 (7.2-11.9) 1.62 (1.19-2.2) 31 3.9 (2.52-5.2) 1.77 (1.19-2.63)
Personal behaviours
Smoking status Never 506 4.1 (3.7-4.4) 1 617 2.1 (1.9-2.23) 1 Ex-smoker 496 6.6 (6.0-7.13) 1.15 (1.0-1.32) 37 3.2 (2.2-4.22) 1.28 (0.88-1.87) Cur-smokerd 276 5.8 (5.16-6.5) 1.22 (1.04-1.44) 8 3.1 (0.97-5.2) 1.54 (0.74-3.2) P-trend <0.0001 <0.032 <0.008 0.23
Drinking status
Never 105 3.9 (3.2-4.7) 1 268 2.1 (1.8-2.32) 1 Ex-drinker 155 5.7 (4.85-6.6) 0.98 (0.74-1.3) 50 2.2 (1.6-2.85) 1.14 (0.82-1.59) Occ- drinkere 821 4.8 (4.45-5.1) 1.0 (0.8-1.25) 352 2.1 (1.9-2.36) 1.14 (0.96-1.36) Reg- drinker
f 212 8.6 (7.5-9.7) 1.61 (1.23-2.11) 9 4.7 (1.66-7.7) 1.83 (0.86-3.9)
P-trend <0.0001 <0.0001 0.471 0.254
Food consumption habit
Instant food
<1 time/m 335 6.3 (5.6-6.95) 1 168 2.8(2.36-3.2) 1 1-3 times/m 485 5.1 (4.63-5.5) 1.02 (0.87-1.19) 273 2.1 (1.84-2.3) 0.95 (0.77-1.17) 1-2 times/wk 280 4.4 (3.9-4.9) 1.09 (0.91-1.30) 153 1.9 (1.6-2.2) 1.04 (0.81-1.33) 3-6 times/wk 165 5.0 (4.3-5.75) 1.36 (1.1-1.69) 73 1.8 (1.4-2.22) 1.14 (0.84-1.55) ≥1 times/d 32 5.5 (3.6-7.33) 1.67 (1.11-2.52) 18 2.3 (1.3-3.35) 1.48 (0.86-2.53) P-trend <0.002 <0.008 <0.002 0.403 aIncidence of Hypertension
bAdjusted relative risks were calculated from multi-variate logistic regression models of hypertension adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cSessions / week dCurrent-smoker eOccasional drinker fRegular drinker
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STROBE 2007 (v4) Statement—Checklist of items that should be included in reports of cohort studies
Section/Topic Item
# Recommendation Reported on page #
Title and abstract 1 (a) Indicate the study’s design with a commonly used term in the title or the abstract 1
(b) Provide in the abstract an informative and balanced summary of what was done and what was found 2
Introduction
Background/rationale 2 Explain the scientific background and rationale for the investigation being reported 3
Objectives 3 State specific objectives, including any prespecified hypotheses 3
Methods
Study design 4 Present key elements of study design early in the paper 3
Setting 5 Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data
collection
4
Participants 6 (a) Give the eligibility criteria, and the sources and methods of selection of participants. Describe methods of follow-up 3
(b) For matched studies, give matching criteria and number of exposed and unexposed 3-4
Variables 7 Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if
applicable
4
Data sources/
measurement
8* For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe
comparability of assessment methods if there is more than one group
4-5
Bias 9 Describe any efforts to address potential sources of bias 11
Study size 10 Explain how the study size was arrived at 3
Quantitative variables 11 Explain how quantitative variables were handled in the analyses. If applicable, describe which groupings were chosen and
why
4-5
Statistical methods 12 (a) Describe all statistical methods, including those used to control for confounding 5
(b) Describe any methods used to examine subgroups and interactions 5
(c) Explain how missing data were addressed 5
(d) If applicable, explain how loss to follow-up was addressed 5
(e) Describe any sensitivity analyses 5
Results 5-6
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Participants 13* (a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, examined for eligibility, confirmed
eligible, included in the study, completing follow-up, and analysed
3
(b) Give reasons for non-participation at each stage 4
(c) Consider use of a flow diagram
Descriptive data 14* (a) Give characteristics of study participants (eg demographic, clinical, social) and information on exposures and potential
confounders
5
(b) Indicate number of participants with missing data for each variable of interest
(c) Summarise follow-up time (eg, average and total amount) 4
Outcome data 15* Report numbers of outcome events or summary measures over time 6
Main results 16 (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (eg, 95% confidence
interval). Make clear which confounders were adjusted for and why they were included
6
(b) Report category boundaries when continuous variables were categorized 6
(c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period 6
Other analyses 17 Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses 6
Discussion
Key results 18 Summarise key results with reference to study objectives 7
Limitations
Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from
similar studies, and other relevant evidence
11
Generalisability 21 Discuss the generalisability (external validity) of the study results 11
Other information
Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on
which the present article is based
12
*Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed groups in cohort and cross-sectional studies.
Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent reporting. The STROBE
checklist is best used in conjunction with this article (freely available on the Web sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at
http://www.annals.org/, and Epidemiology at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org.
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Health-risk factors and the incidence of hypertension: 4-year prospective findings from a national cohort of 60,569
Thai open university students
Journal: BMJ Open
Manuscript ID: bmjopen-2013-002826.R1
Article Type: Research
Date Submitted by the Author: 10-Apr-2013
Complete List of Authors: thawornchaisit, prasutr; Faculty of Health Sciences, School of Medicine; Lerdsin Hospital, Medicine de Looze, Ferdinandus; Faculty of Health Sciences, School of Medicine
Reid, Christopher; Monash University, School of Public Health and Preventive Medicine Seubsman, Sam-ang; Sukhothai Thammathirat Open University, School of Human Ecology Sleigh, Adrian; The Australian National University, National Centre for Epidemiology and Population Health
<b>Primary Subject Heading</b>:
Epidemiology
Secondary Subject Heading: Epidemiology, Public health
Keywords: Hypertension < CARDIOLOGY, General diabetes < DIABETES & ENDOCRINOLOGY, Risk management < HEALTH SERVICES ADMINISTRATION & MANAGEMENT, Adult nephrology < NEPHROLOGY
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1
Health-risk factors and the incidence of hypertension: 4-year prospective
findings from a national cohort of 60,569 Thai open university students
Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University of Queensland, 288
Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of Medicine, Lerdsin Hospital,
Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected]
Telephone: 662-585-8015
Fax: 662-912-0121
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc1, Christopher M Reid, PhD2, Sam-
ang Seubsman, PhD3, Adrian Sleigh, MD4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia, 2 School of Public Health and
Preventive Medicine, Monash University, Melbourne, Australia, 3 School of Human Ecology, Sukhothai
Thammathirat Open University, Nonthaburi, Thailand, 4 National Centre for Epidemiology and Population
Health, ANU College of Medicine, Biology and Environment, The Australian National University,
Canberra, Australia.
*Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan
Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita
Sripaiboonkij, Pathumvadee Somsamai, Prasutr Thawornchaisit, Duangkae Vilainerun, Wanee
Wimonwattanaphan, Cha-aim Pachanee, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin
Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora,
Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim, Anthony
McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara Yiengprugsawan, Susan Jordan,
Janneke Berecki-Gisolf, Rod McClure.
Running Head: Health-risks and hypertension incidence in the Thai Cohort Study
Key words: hypertension; health-risks; cohort; Thailand; socioeconomic
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Abstract
Objective
This study evaluates the impact of a number of demographic, biological, behavioural and lifestyle health-
risk factors on the incidence of hypertension in Thailand over a 4-year period.
Design
A 4-year prospective study of health-risk factors and their effects on incidence of hypertension in a national
Thai Cohort Study (TCS) from 2005 to 2009
Setting
As Thailand is transitioning from a developing to a middle-income developed country, chronic diseases
(particularly cardiovascular disease) have emerged as major health issues. Hypertension is a major risk
factor for heart attack and stroke and cross-sectional studies have indicated that the prevalence is increasing.
Study participants
Sukhothai Thammathirat Open University students (57,558) who participated in both 2005 and 2009
questionnaire surveys and who were normotensive in 2005 were included in the analysis.
Measures
Adjusted relative risks associating each risk factor and incidence of hypertension by sex, after controlling for
confounders such as age, socio-economic status, body mass index (BMI) and underlying diseases.
Results
The overall 4-year incidence of hypertension was 3.5% with the rate in men being remarkably higher than
that in women (5.2% vs. 2.1%). In both sexes, hypertension associated with age, higher BMI and co-
morbidities but did not associate with income and education. In men, hypertension associated with physical
inactivity, smoking, alcohol and fast food intake. In women, hypertension was related to having a partner.
Conclusion
In both men and women, hypertension strongly associated with age, obesity and co-morbidities while it had
no association with socio-economic factors. The cohort pattern of socio-economy and hypertension reflects
that the health-risk transition in Thais is likely to be at the middle stage. Diet and lifestyle factors associate
with incidence of hypertension in Thais and may be amenable targets for hypertension control programs.
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Article summary
Article focus
In a large Thai Cohort Study, we measured the effect of exposure to health-risk factors recorded in 2005 on
the incidence rate of hypertension over the following 4 years. This is part of a study of the health-risk
transition underway in Thailand. We investigated socio-demographic factors, co-morbidity, body size,
physical activity and health behaviour. Cohort members are Open University students, live nationwide, are
well educated and of modest means, representing the next generation of adult Thais.
Key Messages
Overall, the incidence of self-reported hypertension in Thai adults was 3.5% with the rate for males much
higher than females (5.2% vs. 2.1%). In both sexes hypertension incidence was associated with age, higher
body mass index, kidney disease, diabetes and high lipids, but not with socioeconomic status.
Marriage or partnering increased risk of hypertension in females but not in males. Planned physical activity,
averaged across the 4-year follow-up, protected males: ≥15 sessions /week associated with lower incidence
of hypertension. However, this was not observed in females.
In males, current-smokers had a higher risk of developing hypertension than non-smokers while in females
smoking (which was rare) had no apparent hypertension effect. Among males, regular drinking increased the
risk of incident hypertension.
Strengths
In Thailand, this cohort constitutes the largest longitudinal study of health-risk factors and hypertension
development. As a consequence, many independent risk factors for hypertension in Thais can be identified.
Our cohort represents Thais well in geography and socio-economy so the results reflect trends of the health-
risk transition in the population.
Limitations
Our study relies on self-report. There may be recall and reporting errors. Some risk factors may take more
than four years to produce hypertension and will manifest as the cohort ages.
Introduction
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Hypertension is a global public health challenge due to its high prevalence and the concomitant increase in
risk of stroke and cardiovascular diseases [1]. In 2007, according to the World Health Organization,
cardiovascular diseases caused 33.7% of all deaths in the world, while other chronic diseases were
responsible for 26.5% [2]. Globally, it has been estimated that premature deaths resulting from hypertension
annually are approximately 7.1 million, which account for 64 million disability-adjusted life years (DALYs)
[3]. Hypertension ranks third, after underweight and unsafe sex, of the six major risk factors contributing to
the global disease burden[3].
In Thailand, hypertension is the third major risk factor of the national disease burden and causes 600,000
DALY losses per year [4]. The prevalence of hypertension has been estimated from a number of cross-
sectional studies among adults and has been increasing over the past decades from 20% in 1985 to 22% in
2004 [4-6]. The recent Thai National Health Examination Survey (NHES) in 2009 reported the prevalence
of hypertension in adults was 21.5% [4]. The emergence of hypertension has been the result of rapid
socioeconomic development which has resulted in one third of Thais migrating to urban areas [7].
Urbanization has been associated with increased obesity as people become less physically active and adopt
sedentary life styles such as spending more time watching television, sitting and using computers [7 8].
These factors, in association with Thailand’s ageing population [9], underpin the importance of controlling
hypertension in Thai community.
There is limited longitudinal research on health risk factors and hypertension incidence in Thailand [5]. The
purpose of this study is to address that knowledge gap. We prospectively examined the influence of an array
of demographic, biological, health-risk and lifestyle factors on the incidence of hypertension during 4 years
of follow-up in the Thai Cohort Study (TCS). This is part of an overarching research program investigating
Thailand’s health-risk transition from high maternal-child and infectious disease mortality to increased
longevity and emerging chronic disease. The large national cohort of Thai adults reported on here has been
followed since 2005 for risk factor trends and associated disease outcomes, including hypertension.
Methods
Study population
The first TCS evaluation was conducted in 2005 by sending twenty-page health-risk questionnaires to the
200,000 distance-learning students enrolled at Sukhothai Thammathirat Open University (STOU) and
87,134 (44%) completed them and signed the consent forms. The cohort has been shown to be socio-
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economically and geographically well representative of the Thai population [10]. However, they resided
more in urban areas (51.8% vs. 31.1%) and on average were younger and better educated than most Thai
adults. Details of the baseline TCS methodology have been published [10]. This questionnaire focused on
socio-demography, habitation, work, health services, injury, sedentary habits, physical activity, transport,
underlying diseases, family history, personal behaviours, BMI, and consumption of foods, vegetables and
fruit. A 4-year follow-up was carried out in 2009. Overall, 71% of the cohort responded generating
longitudinal data on 60,569 participants.
Incident Hypertension
Incident hypertension was defined as being normotensive in 2005 and self-reported doctor diagnosed
hypertensive in 2009. Hypertension incidence was the dependent variable for analyses and the person-time
denominator included all individuals at risk - those 57,558 participants with longitudinal data who were
negative for hypertension in 2005, the beginning of the 4-year study period.
Risk Factors
Hypertension was defined as describe above, underlying diseases diagnosed by a doctor were also reported
including diabetes mellitus, kidney diseases and high blood lipids, all of which were investigated as
potential risk factors of hypertension.
Participants were grouped into three age categories: younger (≤30 years), middle (31-40 years) and older
(>40 years). Urbanization status was based on rural (R) or urban (U) residence, first when aged 10-12 years
old and again in 2005, producing four groups: lifelong ruralites (RR), urbanizers (RU), de-urbanizers (UR)
and urbanites (UU). Personal monthly income was divided into four categories: ≤7000 baht, 7001–10000
baht, 10001–20000 baht and >20000 baht. In 2005, one US dollar was equivalent to 42 baht so most of
participants had quite low incomes. Household assets were classified into three categories: low (≤30,000
baht), medium (30,001-60,000) and high (>60,000).
Sedentariness was assessed by screen time (hours/day spent before TV or computer) and (separately) sitting
time (hours per day spent sitting for any purpose). Incidental exercise was measured by the frequency of
housework or gardening, categorised as follows: ≤3 times per month; 1-2 times per week; 3-4 times per
week; most days. We measured planned physical activity at the beginning and end of the 4-year follow up
period. On both occasions, cohort members reported the number of sessions per week of strenuous and
moderate exercise for at least 20 minutes, and of walking for at least 10 minutes. We derived an “overall
measure” of planned physical activity calculated separately at both the baseline and the 4-year mark. The
measure was weighted as follows: (2 × strenuous + 1 × moderate + 1 × walking) sessions per week. This
weighting system is based on the recommendation of the International Physical Activity Questionnaire and
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the Active Australia Survey as used in other analyses of the cohort data [11 12]. Finally, for each individual,
the “overall measures” of weekly exercise for 2005 and 2009 were added and then averaged by dividing by
2, creating a longitudinal measure of planned physical activity (LPPA).
Smoking status was classified as never, ex-smoker or current-smoker. Alcohol exposure was classified by
four categories: never, ex-drinker, occasional drinker or current-drinker. Body mass index (BMI) was
calculated from self-reported height in metres and weight in kilograms as weight/height2. It was divided into
4 categories as follows according to Asian criteria:[8 13-16] underweight (BMI <18.5), normal (18.5 to
<23), overweight (23 to <25), and obese (≥25). Foods that could potentially influence blood pressure (deep
fried, instant, roast or smoked, soybean products and soft drinks) were assessed for consumption frequency
based on a five-point Likert scale ranging from less than once a month to once or more a day. Western-style
fast food exposure was noted on a three-point scale from less than once to more than 3 times per month.
Fruit and vegetable consumption were recorded as standard serves eaten per day.
Statistical analyses
All analyses were performed using SPSS software. The incidence of hypertension and its 95% confidence
interval (CI) were calculated for each value of each categorical variable in both male and female participants
and the influence on incidence by each variable was evaluated by Chi-square test. For statistical inference,
all p-values were 2 tailed and significance was set at 5%.
Relative risks in a large study of an uncommon disease (incidence less than 10%) can be accurately
estimated as odds ratios [17]. Accordingly, for each risk variable, the relative risk (RR) and 95% confidence
interval were estimated using logistic regression to calculate the bivariate odds ratio for hypertension.
Adjusted RRs (ARRs) were estimated by calculating multivariate logistic regression odds ratios. The ARRs
were controlled for confounding by age, marital status, SES, BMI, underlying diseases and personal
behaviours (cigarette smoking and alcohol drinking). A variable was included in a multivariate model if
bivariate analysis had indicated a statistically significant association with incidence of hypertension. Some
variables were included because earlier analyses reported elsewhere had shown significant or substantial
association with hypertension.
Results
Baseline Characteristics
Baseline characteristics of study participants self-reporting as normotensive in 2005 are shown in Table 1.
The highest proportion of males resided in the North-eastern regions and Central while there were more
females in the Central and Bangkok area. More than half of the male and female participants lived in urban
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areas. Males had a higher monthly income than females whereas females had a higher educational
attainment than males. The distribution of household assets for males and females was similar and over half
of them were in medium and high categories. The study population had a similar socio-demography, income
and location of residence to the general Thai population [9].
Incidence and risks of hypertension
Overall, the incidence of self-reported doctor diagnosed hypertension in Thai adults was 3.5% and the rate in
males was more than twice the corresponding rate in females (5.2% vs. 2.1%) (Table 2). In males and
females, the incidence of hypertension increased with increasing age. Marriage or having a partner increased
the risk of hypertension in females. Educational attainment and personal income had no influence on the risk
of hypertension in both sexes. In males and females, the risk of hypertension increased with an increased
BMI. The risk of hypertension increased in both males and females who had diabetes mellitus, high blood
lipids and kidney disease.
Planned physical activity, averaged across the 4-year follow-up, was protective for males. In males, higher
LPPA (≥15 sessions / week) associated with lower incidence of hypertension; however, this association was
not observed in females. In males, current-smokers had a higher risk of developing hypertension than non-
smokers while in females, smoking (which was rare) had no apparent hypertension effect. Among males,
regular drinking increased the risk of incident hypertension; patterns of relative risks were similar for
females but, reflecting the low frequency of female drinking, did not reach statistical significance. In males,
the risk of hypertension had a direct association with the frequency of instant food consumption but there
was no such association detected in females.
Over the 4-year period, a variety of factors that could have been risks for hypertension over a longer period
were found not to be in this setting (data not shown). Thus, in males and females, factors that had no
statistically significant influence on the longitudinal 4-year risk of hypertension incidence were urbanization
status, sedentary lifestyle (non-participation in housework or gardening, high levels of television and
computer watching, sleeping and sitting time), and food consumption habits that included deep fried food
and soft drink, Western fast food, roasted or smoked foods, fruit, vegetables and soybean products.
Discussion
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This large prospective cohort of adults living all over Thailand has been the first studied nationally for a
wide array of risk factors for incident hypertension. Overall, the 4-year incidence of hypertension from 2005
to 2009 was 3.5%, notably higher in men (5.2%) than women (2.1%). These incidence rates are drawn from
a cohort of adults on average younger than the Thai population so they are lower than corresponding rates of
the adult population. However, our focus was on risk factor associations and we noted that in both sexes,
age, obesity and co-morbidity with diabetes mellitus, high blood lipids and kidney disease strongly linked to
hypertension incidence. In males, instant food consumption, smoking and drinking alcohol had a moderate
adverse effect on hypertension risk while planned physical activity averaging 15 or more sessions per week
over the 4-year period was substantially protective, reducing risk by 22%. In females, having a partner was
associated with increased incidence of hypertension.
The age effect on hypertension incidence noted by us is consistent with findings of many other population
studies of hypertension including (for example) a longitudinal study in China [18] and cross-sectional
studies in Korea [19], Taiwan [20] and the USA [21]. The noticeably greater incidence of hypertension in
men than in women that we found in Thailand has also been reported from many parts of the world
including neighboring Malaysia [22] and distant USA [23] as well as in a recent global review [24].
In men and women of the Thai cohort, a progressively higher BMI was positively and significantly
associated with an increased risk of incident hypertension over the four years. This finding was similar to
that reported for other prospective cohorts in the US [25 26] and Finland [27]. Furthermore, after seven
years of follow-up in a US study [28], weight loss associated with a decreased risk of incident hypertension.
Among a cohort of US nurses, the risk of incident hypertension increased 5% for each kilogram addition of
weight [29], and rose 15% for each increase of one kilogram per meter squared of BMI in Taiwan [18].
These results suggest that the adverse effect of high BMI on risk of hypertension does not depend on
ethinicity or sex and is wide spread around the world.
In our cohort, we noted a higher risk of incident hypertension among those with diabetes mellitus. This
finding is consistent with longitudinal studies in the USA [30 31] which reported, respectively, that in
diabetic patients the risk of incident hypertension rose 41% and 56%. As well, prospective cohort studies in
Japan showed that diabetes [32] and impaired fasting blood glucose [33] were associated with an increased
risk of hypertension incidence. Supportive cross-sectional results emerge from studies in diverse settings
including Bangladesh and India [34], Barbados [35] and the USA [36]. A causal association between
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diabetes and hypertension is suspected and possible mechanisms include endothelial dysfunction and
vascular inflammation [37].
We also found that high blood lipids significantly associated with increased risk of incident hypertension.
This supports the results from previous longitudinal studies in men [38 39], women [40 41] and adults [32
42]. The risk of hypertension increased with an increase of total cholesterol, triglyceride and low-density
lipoprotein cholesterol levels and decreased with an increase of high-density lipoprotein cholesterol level
[38 39 41].The mechanism by which dyslipidemia provokes high blood pressure is uncertain but could
reflect blood vessel dysfunction and damaged endothelium [43].
Among men and women in the Thai cohort, kidney disease strongly associated with increased risk of
hypertension incidence. This result is supported by cross-sectional data from the Czech Republic [44] and
the USA [45]. Microalbuminuria is a biomarker for renal hypertension [45] and the risk increases with the
severity of kidney impairment, revealed by the serum creatinine level [46 47]. Glomerular damage is more
closely connected with hypertension than tubular or interstitial disease [47]. Chronic kidney disease
facilitates high blood pressure through many mechanisms including excessive intravascular volume,
increased endothelin production, activity of the renin-angiotensin system and sympathetic nervous system
and decreased vasodilators, nitric oxide production and endothelial function [48].
In men we found that cigarette smoking modestly associated with increased risk of developing hypertension.
This is consistent with many other reports such as longitudinal studies in Japanese [49] and American men
[50] and a cross-sectional study in French men [51]. In addition, smoking Jordanian men had significantly
higher blood pressure than their non-smoking counterparts [52]. However, we also found that smoking had
no influence on hypertension in women. This result for females is compatible with reports from Korea [53]
and the USA [54]. In contrast, one prospective US study [55] reported an increased risk of hypertension
among women. It is likely that men tend to smoke more cigarettes than women and the impact of smoking
on hypertension is a dose-response pattern [49].
Regular alcohol consumption was strongly associated with an increased risk of incident hypertension. This
result is supported by findings from longitudinal studies in Japan [56 57] and cross-sectional studies in
China [58] and Korea [53]. In Japanese men, alcohol consumption, particularly more than 200 grams per
week, significantly associated with an increase of blood pressure [59] while alcohol restriction was
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noticeably related to a decreased blood pressure [60]. Among our Thai cohort females, alcohol consumption
had no influence on hypertension; this result was supported by longitudinal data from Japan [56] and cross-
sectional data from Korea [53]. In American females the quantity of alcohol consumption per week had no
effect on systolic blood pressure [61]. Males usually consume more alcohol than females and the
hypertension risk is linear with a dose-response [57 62]. Our results suggest that regular alcohol
consumption, especially in males, is a significant risk factor for developing hypertension.
Marital status in males had no influence on hypertension which supports the results from more economically
developed countries, the USA and Korea [53 54]. However, in middle-income countries, results diverged:
married males, when compared to their unmarried counterparts, had a higher risk of hypertension in
Barbados [35] and a lower risk in Poland [63]. Married individuals had higher emotional and social support
and lower stress [63] and probability of being a current smoker [64]. We found incident hypertension was
relatively more frequent among married females, unlike the no-sex-link pattern in developed countries such
as the USA and Korea [53 54]. The influence of marrital status on hypertension varies from one country to
another partly due to the difference of socio-economy and social structure. For our Thai cohort, the pattern
in males was similar to that in developed countries and in females it was not.
Participant educational attainment did not protect against hypertension in either sex. This is similar to cross-
sectional results for adults in developed countries such as Korea [53] and the USA [36 65]. However, other
studies in various settings and with diverse designs reported educational attainment protects against
hypertension in adults; these included a prospective cohort in middle income Thailand [5], a cross-sectional
study in middle income Malaysia [22] and a cross-sectional study in high income USA [66]. Opposite
findings have been reported in low income settings in both Bangladesh and India [34] where education was
a risk factor for hypertension. Taken collectively, these data suggest that income modifies the protective
education-hypertension effect, inverting it when income is low and boosting it when income is high. Our
middle-income cohort could be in the middle stage of this education effect as it is transiting from the low-
income pattern.
We found no association of personal monthly income and risk of hypertension in either sex. This was similar
to previous studies in middle-income countries including a longitudinal study in Thailand [5] and a cross-
sectional study in Malaysia [67] and in cross-sectional studies in high-income countries such as in Korea
[53] and the USA [21 36]. However, income had an inverse association with risk of hypertension in another
study in high-income developed USA [65] while it had a direct positive association in a low-income
developing country such as Ghana [68]. The health transition in Thailand is likely to be in the middle stage,
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according to the concept of a tipping-point for the relationship between socio-economic status, body size and
hypertension risk factors. High income, large body size and high blood pressure is seen in developing
countries while in developed countries, high income is a protective factor [69-71]. Indeed, the Thai cohort
has already been shown to be at a transitional point for metabolic states because females had an inverse
relationship between income and BMI whereas males had a positive (traditional) relationship [14]. However,
now all Thais benefit from universal health insurance so access to health care is not dependent on their
income making disease associations more attenuated than before [72].
In our cohort, the reduction of risk of hypertension incidence in males but not in females by a longitudinal
measure of planned physical activity was comparable with a prospective study in middle-aged American
whites [73]. In another prospective study in middle-aged Finns, the hypertension protective effect of
vigorous physical activity or total energy expenditure from leisure-time physical activity was observed in
men but not in women [74]. Such physical activity among females may be less intense than among males so
having no effect on the risk of hypertension [74]. A large Finnish study [27] demonstrated that the incidence
of hypertension was inversely associated with the level of physical activity in a dose-response manner in
both sexes. In our cohort study, risk of obesity had an inverse association with quantified planned exercise in
both sexes but the effect was significantly stronger in males than in females [12]. As Thailand progresses
economically, more people migrate to urban areas and decrease their physical activity [7]. Therefore, in the
future, Thai people may encounter a higher risk of hypertension due to lower physical activity.
Instant food - mostly combinations of carbohydrate, fat and salt - associated with an increased risk of
incident hypertension in males but not in females. Other foods, fruit and vegetables had no impact on
hypertension. This may be as a result of men comsuming more instant foods, such as instant noodles, than
women. For example a study in Korea showed daily instant food consumption in men and women averages
23.7 and 11.1 grams respectively [75]. Other studies showed that carbohydrate [76 77], fruit and vegetables
[78 79] consumption had no influence on hypertension. Most students consume instant noodles which lead
to a higher intake of energy, fat, sodium, thiamine, and riboflavin but a lower intake of protein, calcium,
phosphorus [75]. Koreans favoring instant noodles have a salt intake more than three times higher than the
daily recommendation (6.4 vs 2 gms) [75]. Daily salt intake of Thai people has been reported to average
10.8gm, much higher than recommended (http://www.worldactiononsalt.com/worldaction/asia/75286.pdf).
High salt consumption was associated with an increased risk of developing hypertension in a longitudinal
study in Taiwan [80] and in cross-sectional studies in China [81 82] while other reports revealed that salt
restriction reduced blood pressure [83 84]. High plasma sodium, due to an overwhelmed capacity of the
kidney to excrete excessive salt consumption, leads to excessive intravascular volume and high blood
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pressure [85]. It seems reasonable to conclude that high salt intake from instant noodle consumption is likely
to be causally associated with an increased risk of hypertension in Thailand.
In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. Indeed, the study size along with its national representation are
important factors to consider when exploring study validity and utility. The influence of risk factors on
hypertension incidence is clearly demonstrated since this is a large longitudinal cohort covering a 4-year
period. As a consequence, independent risk factors for hypertension in Thais can be identified. Our cohort
represents Thais well in geography and socio-economy [9] so the results reflect the trend of health-risk
transition in the Thai population. STOU students, with better education attainment combined with their
representative incomes and geographical location, represent the next generation of Thais. It would be
reasonable to conclude that our 4-year follow-up results show the future hypertension trends for Thai people.
However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
But a study of the validity of self-reported weight and height in our cohort found that the accuracy was quite
acceptable [86]. As well, we have conducted a validation study of self-reported hypertension (Prasutr
Thawornchaisit, 2013, under review) in a random age-sex matched sub-sample of the cohort reporting
hypertension (n=240) or no hypertension (n=240). We found the sensitivity was high (82%) and that
negative reports were usually accurate (86%). In another report, we investigated the impact of the non-
responses to the 2009 follow up and found small effects with under-representation of young urban males;
this missing group would have a minor influence on our results and would not be expected to have a high
rate of incident hypertension [87]. The findings from our study are likely to be robust and represent well the
future trends of health-risk transition in middle-income Thailand and similar ASEAN countries.
In conclusion, older age, obesity and underlying morbidity due to diabetes, high blood lipids and kidney
disease were strongly associated with an increased risk of incident hypertension in both sexes while in
males, physical inactivity, instant food consumption, smoking and drinking alcohol had a moderate
association. Sedentary life-style and the consumption of friut and vegetables had no detected influence on
hypertension. The Thai health-risk transition is likely to be in the middle stage since education attainment
and income still have no net effect to protect against hypertension. Prevention should focus on obesity, high
blood lipids and high salt consumption and people should be encouraged to increase physical exercise and
consume low fat foods and less instant foods. A reduction of salt consumption should be a national policy by
limiting the concentration of salt in ready-to-eat and industrial foods. Smoking and drinking cessation
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should be promoted for males. Thailand should also consciously aim to take advantage of its culturally
determined low rate of drinking and smoking among women and ensure that there is no attempt to
undermine this situation and develop the market for the female half of the population.
Acknowledgements and Funding
This study was supported by the International Collaborative Research Grants Scheme with joint grants from
the Wellcome Trust UK (GR071587MA) and the Australian National Health and Medical Research Council
(268055), and by a global health grant from the NHMRC (585426). We thank the staff at Sukhothai
Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are
participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen
University for guiding us successfully through the complex data processing.
Conflict of interest
The authors declare that there are no conflicts of interest.
Ethical considerations
Ethical approval was obtained from Sukhothai Thammathirat Open University Research and Development
Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee
(protocol 2004344 and 2009570). Informed, written consent was obtained from all participants.
Author contribution: PT devised the hypertension study, analysed the data and wrote the paper. FDL and
CR assisted with planning the study, analysis and interpretation. AS and SS conceived and developed the
cohort, and helped plan, analyse and interpret this study. All authors approved the final manuscript.
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Table 1 Characteristics of 57558 participants normotensive at the 2005 Thai Cohort Study baseline
Male Female Difference
Factor
n % n % P-valuea
Demographic data
Participants 25320 44.0 32238 56.0 <0.0001 Age (y) mean 32.9 30.0 <0.0001
b
(SD) (8.5) (7.6) Age group <0.0001 ≤30 y 11293 44.6 19229 59.6 31-40 y 9177 36.2 9638 29.9 >40 y 4850 19.2 3371 10.5 Married/partnered <0.0001 No 11060 44.8 18851 60.1 Yes 13607 55.2 12513 39.9 Regions <0.0001 Bangkok 3488 13.9 6057 18.9 Central 5712 22.7 8267 25.8 North 5180 20.6 5818 18.1 North-east 6142 24.4 5760 18.0 East 1515 6.0 1922 6.0 South 3103 12.3 4254 13.3 Urbanization status
c <0.0001
Rural–rural (RR) 11512 46,1 13895 43.6 Rural–urban (RU) 7891 31.6 9546 29.9 Urban–rural (UR) 1078 4.3 1344 4.2 Urban–urban (UU) 4495 18.0 7114 22.3
Socioeconomic status
Education level <0.0001 High school 13366 52.9 13135 40.8 Diploma 5749 22.8 9791 30.4 University 6153 24.4 9231 28.7 Personal monthly income (baht)
d <0.0001 ≤7000 8025 32.3 13890 44.1 7001–10000 5790 23.3 7578 24.0 10001–20000 7549 30.4 7194 22.8 >20000 3452 13.9 2851 9.0 Household assets
e (baht)
d <0.001
Low 9852 39.1 12434 38.7 Medium 8065 32.0 9863 30.7 High 7298 28.9 9804 30.5 aChi-square test
b unpaired t test cLocation of residence (rural, R, or urban, U) at age 10-12 years and again in 2005. dAt the time of the survey in 2009, US$1 = 31 Thai baht eReplacement value in Thai baht: low ≤30,000, medium 30,001-60,000 and high >60,000
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Table 2 Hypertension incidence and association with risk factors in male and female participants.
Males Females
HT(n) Incia%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Participants 1306 5.2 (4.9-5.4) 690 2.1 (2.0-2.3)
Demography
Age group
≤30 y 279 2.5 (2.2-2.8) 1 215 1.1 (0.97-1.3) 1 31-40 y 484 5.3 (4.8-5.7) 1.66 (1.39-1.98) 245 2.5 (2.23-2.9) 1.93 (1.56-2.39) >40 y 543 11.2 (10.3-12.1) 3.13 (2.57-3.81) 230 6.8 (6.0-7.7) 4.2 (3.26-5.42) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Marital status (married/partnered)
No 368 3.3 (3.0-3.7) 1 303 1.6 (1.43-1.8) 1 Yes 896 6.6 (6.17-7.0) 0.97 (0.83-1.12) 368 2.9 (2.65-3.2) 1.22 (1.03-1.45)
Socio-economic status
Education level
High school 697 5.2 (4.8-5.6) 1 303 2.3 (2.05-2.57) 1 Diploma 276 4.8 (4.25-5.4) 1.0 (0.86-1.17) 180 1.8 (1.57-2.1) 0.81 (0.66-0.99) University 331 5.4 (4.8-5.9) 0.93 (0.8-1.1) 204 2.2 (1.9-2.51) 0.87 (0.71-1.07) P-trend 0.83 0.61 0.463 0.1 Personal monthly income (baht)c ≤7000 268 3.3 (2.95-3.7) 1 217 1.6 (1.36-1.8) 1 7001-10000 229 4.0 (3.45-4.5) 1.0 (0.81-1.2) 142 1.9 (1.57-2.2) 1.1 (0.87-1.37) 10001-20000 497 6.6 (6.0-7.14) 1.1 (0.91-1.31) 196 2.7 (2.35-3.1) 0.97 (0.77-1.23) >20000 289 8.4 (7.46-9.3) 1.0 (0.8-1.25) 122 4.3 (3.54-5.0) 1.04 (0.78-1.39) P-trend <0.0001 0.527 <0.0001 0.783 BMI classification
d
Underweight 359 2.9 (2.62-3.2) 0.46 (0.27-0.78) 60 0.85 (0.63-1.15) 0.9 (0.67-1.21) (BMI <18.5) Normal 16 1.1 (0.56-1.6) 1 258 1.4 (1.2-1.5) 1 (18.5 ≤ BMI < 23) Overweight 333 5.9 (5.3-6.54) 1.66 (1.41-1.95) 129 3.9 (3.3-4.62) 2.32 (1.8-2.91) (23 ≤ BMI < 25) Obese 575 10.4 (9.6-11.2) 2.82 (2.43-3.27) 231 7.4 (6.4-8.27) 4.14 (3.4-5.05) (BMI ≥25) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Continued….
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Table 2 (continued...)
Males Females HT(n) Inci
a%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Physical activities
Weighted total sessions exercise-related physical activity (average between 2005 and 2009) 0-7 ses/we 198 6.3 (5.5-7.1) 1 162 2.2 (1.8-2.6) 1 8-14 ses/w 589 5.2 (4.8-5.6) 0.9 (0.76-1.07) 352 2.1 (1.9-2.3) 1.0 (0.82-1.23)
≥15 ses/w 380 4.4 (4.0-5.0) 0.78 (0.65-0.95) 119 2.2 (1.8-2.6) 1.0 (0.77-1.3)
P-trend <0.0001 <0.029 0.905 1.0
Underlying diseases
Diabetes mellitus
No 1251 5.0 (4.7-5.27) 1 674 2.1 (1.9-2.26) 1 Yes 55 18.3 (13.9-22.7) 1.81 (1.29-2.52) 16 10.7 (5.7-15.8) 2.54 (1.41-4.56)
High lipids
No 983 4.4 (4.13-4.67) 1 576 1.9 (1.76-2.1) 1 Yes 323 10.8 (9.7-11.9) 1.4 (1.21-1.63) 114 5.4 (4.47-6.4) 1.38 (1.09-1.75)
Kidney diseases
No 1248 5.1 (4.8-5.33) 1 659 2.1 (1.9-2.26) 1 Yes 58 9.6 (7.2-11.9) 1.62 (1.19-2.2) 31 3.9 (2.52-5.2) 1.77 (1.19-2.63)
Personal behaviours
Smoking status Never 506 4.1 (3.7-4.4) 1 617 2.1 (1.9-2.23) 1 Ex-smoker 496 6.6 (6.0-7.13) 1.15 (1.0-1.32) 37 3.2 (2.2-4.22) 1.28 (0.88-1.87) Cur-smokerf 276 5.8 (5.16-6.5) 1.22 (1.04-1.44) 8 3.1 (0.97-5.2) 1.54 (0.74-3.2) P-trend <0.0001 <0.032 <0.008 0.23
Drinking status
Never 105 3.9 (3.2-4.7) 1 268 2.1 (1.8-2.32) 1 Ex-drinker 155 5.7 (4.85-6.6) 0.98 (0.74-1.3) 50 2.2 (1.6-2.85) 1.14 (0.82-1.59) Occ- drinkerg 821 4.8 (4.45-5.1) 1.0 (0.8-1.25) 352 2.1 (1.9-2.36) 1.14 (0.96-1.36) Reg- drinker
h 212 8.6 (7.5-9.7) 1.61 (1.23-2.11) 9 4.7 (1.66-7.7) 1.83 (0.86-3.9)
P-trend <0.0001 <0.0001 0.471 0.254
Food consumption habit
Instant food
<1 time/m 335 6.3 (5.6-6.95) 1 168 2.8(2.36-3.2) 1 1-3 times/m 485 5.1 (4.63-5.5) 1.02 (0.87-1.19) 273 2.1 (1.84-2.3) 0.95 (0.77-1.17) 1-2 times/wk 280 4.4 (3.9-4.9) 1.09 (0.91-1.30) 153 1.9 (1.6-2.2) 1.04 (0.81-1.33) 3-6 times/wk 165 5.0 (4.3-5.75) 1.36 (1.1-1.69) 73 1.8 (1.4-2.22) 1.14 (0.84-1.55) ≥1 times/d 32 5.5 (3.6-7.33) 1.67 (1.11-2.52) 18 2.3 (1.3-3.35) 1.48 (0.86-2.53) P-trend <0.002 <0.008 <0.002 0.403 aIncidence of Hypertension
bAdjusted relative risks were calculated from multi-variate logistic regression models of hypertension adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification eSessions / week fCurrent-smoker gOccasional drinker hRegular drinker
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71. Sobal J, Stunkard AJ. Socioeconomic status and obesity: a review of the literature. Psychological
bulletin 1989;105(2):260-75.
72. Yiengprugsawan V, Seubsman SA, Lim LL, et al. Used and foregone health services among a cohort of
87,134 adult open university students residing throughout Thailand. The Southeast Asian journal of
tropical medicine and public health 2009;40(6):1347-58.
73. Pereira MA, Folsom AR, McGovern PG, et al. Physical activity and incident hypertension in black and
white adults: the Atherosclerosis Risk in Communities Study. Prev Med 1999;28(3):304-12.
74. Haapanen N, Miilunpalo S, Vuori I, et al. Association of leisure time physical activity with the risk of
coronary heart disease, hypertension and diabetes in middle-aged men and women. Int J Epidemiol
1997;26(4):739-47.
75. Park J, Lee JS, Jang YA, et al. A comparison of food and nutrient intake between instant noodle
consumers and non-instant noodle consumers in Korean adults. Nutr Res Pract 2011;5(5):443-9.
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76. Jolly SE, Eilat-Adar S, Wang H, et al. Sex-specific associations of nutrition with hypertension and
systolic blood pressure in Alaska Natives findings from the GOCADAN study. Int J Circumpolar
Health 2011;70(3):254-65.
77. Alonso A, Beunza JJ, Bes-Rastrollo M, et al. Vegetable protein and fiber from cereal are inversely
associated with the risk of hypertension in a Spanish cohort. Arch Med Res 2006;37(6):778-86.
78. Steffen LM, Kroenke CH, Yu X, et al. Associations of plant food, dairy product, and meat intakes with
15-y incidence of elevated blood pressure in young black and white adults: the Coronary Artery Risk
Development in Young Adults (CARDIA) Study. Am J Clin Nutr 2005;82(6):1169-77; quiz 363-4.
79. Schulze MB, Hoffmann K, Kroke A, et al. Risk of hypertension among women in the EPIC-Potsdam
Study: comparison of relative risk estimates for exploratory and hypothesis-oriented dietary patterns.
Am J Epidemiol 2003;158(4):365-73.
80. Chien KL, Hsu HC, Chen PC, et al. Urinary sodium and potassium excretion and risk of hypertension in
Chinese: report from a community-based cohort study in Taiwan. J Hypertens 2008;26(9):1750-6.
81. Zhang L, Qin LQ, Liu AP, et al. Prevalence of risk factors for cardiovascular disease and their
associations with diet and physical activity in suburban Beijing, China. J Epidemiol 2010;20(3):237-
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82. Sun Z, Zheng L, Xu C, et al. Prevalence of prehypertension, hypertension and, associated risk factors in
Mongolian and Han Chinese populations in Northeast China. Int J Cardiol 2008;128(2):250-4.
83. He FJ, MacGregor GA. Effect of modest salt reduction on blood pressure: a meta-analysis of
randomized trials. Implications for public health. J Hum Hypertens 2002;16(11):761-70.
84. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the
Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research
Group. N Engl J Med 2001;344(1):3-10.
85. He FJ, MacGregor GA. Reducing population salt intake worldwide: from evidence to implementation.
Prog Cardiovasc Dis 2010;52(5):363-82.
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86. Lim LL, Seubsman SA, Sleigh A. Validity of self-reported weight, height, and body mass index among
university students in Thailand: Implications for population studies of obesity in developing
countries. Population health metrics 2009;7:1-15.
87. Thinkhamrop K, Seubsman S, Sleigh A, et al. Reasons for Non-Response to Mailed Questionnaires –
Experience of a Large Cohort Study in Thailand. KKU Jph Research 2011;4(3):73-87.
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Health-risk factors and the incidence of hypertension: 4-year
prospective findings from a national cohort of 60,569 Thai open
university students
Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University of
Queensland, 288 Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of
Medicine, Lerdsin Hospital, Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected]
Telephone: 662-585-8015
Fax: 662-912-0121
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc
1, Christopher M Reid,
PhD2, Sam-ang Seubsman, PhD
3, Adrian Sleigh, MD
4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia, 2 School of Public Health
and Preventive Medicine, Monash University, Melbourne, Australia, 3 School of Human
Ecology, Sukhothai Thammathirat Open University, Nonthaburi, Thailand, 4 National Centre for
Epidemiology and Population Health, ANU College of Medicine, Biology and Environment, The
Australian National University, Canberra, Australia.
*Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang
Pandee, Suttinan Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman,
Boonchai Somboonsook, Nintita Sripaiboonkij, Pathumvadee Somsamai, Prasutr
Thawornchaisit, Duangkae Vilainerun, Wanee Wimonwattanaphan, Cha-aim Pachanee, Arunrat
Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie
Dellora, Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim,
Anthony McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara
Yiengprugsawan, Susan Jordan, Janneke Berecki-Gisolf, Rod McClure.
Running Head: Health-risks and hypertension incidence in the Thai Cohort Study
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Key words: hypertension; health-risks; cohort; Thailand; socioeconomic
Word Count: 4483
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STROBE 2007 (v4) Statement—Checklist of items that should be included in reports of cohort studies
Section/Topic Item
# Recommendation Reported on page #
Title and abstract 1 (a) Indicate the study’s design with a commonly used term in the title or the abstract 1
(b) Provide in the abstract an informative and balanced summary of what was done and what was found 2
Introduction
Background/rationale 2 Explain the scientific background and rationale for the investigation being reported 4
Objectives 3 State specific objectives, including any prespecified hypotheses 4
Methods
Study design 4 Present key elements of study design early in the paper 4
Setting 5 Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data
collection
5
Participants 6 (a) Give the eligibility criteria, and the sources and methods of selection of participants. Describe methods of follow-up 4
(b) For matched studies, give matching criteria and number of exposed and unexposed 5-6
Variables 7 Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if
applicable
5
Data sources/
measurement
8* For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe
comparability of assessment methods if there is more than one group
5-6
Bias 9 Describe any efforts to address potential sources of bias 12
Study size 10 Explain how the study size was arrived at 4-5
Quantitative variables 11 Explain how quantitative variables were handled in the analyses. If applicable, describe which groupings were chosen and
why
5-6
Statistical methods 12 (a) Describe all statistical methods, including those used to control for confounding 6
(b) Describe any methods used to examine subgroups and interactions 6
(c) Explain how missing data were addressed 12
(d) If applicable, explain how loss to follow-up was addressed 12
(e) Describe any sensitivity analyses 12
Results 7
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Participants 13* (a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, examined for eligibility, confirmed
eligible, included in the study, completing follow-up, and analysed
4-5
(b) Give reasons for non-participation at each stage 12
(c) Consider use of a flow diagram
Descriptive data 14* (a) Give characteristics of study participants (eg demographic, clinical, social) and information on exposures and potential
confounders
5
(b) Indicate number of participants with missing data for each variable of interest
(c) Summarise follow-up time (eg, average and total amount) 5
Outcome data 15* Report numbers of outcome events or summary measures over time 7
Main results 16 (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (eg, 95% confidence
interval). Make clear which confounders were adjusted for and why they were included
7
(b) Report category boundaries when continuous variables were categorized 7
(c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period 7
Other analyses 17 Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses 7
Discussion
Key results 18 Summarise key results with reference to study objectives 7
Limitations
Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from
similar studies, and other relevant evidence
12
Generalisability 21 Discuss the generalisability (external validity) of the study results 12
Other information
Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on
which the present article is based
13
*Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed groups in cohort and cross-sectional studies.
Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent reporting. The STROBE
checklist is best used in conjunction with this article (freely available on the Web sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at
http://www.annals.org/, and Epidemiology at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org.
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1
Health-risk factors and the incidence of hypertension: 4-year prospective
findings from a national cohort of 60,569 Thai open university students
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc
1, Christopher M
Reid, PhD2, Sam-ang Seubsman, PhD
3, Adrian Sleigh, MD
4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia; 2 School of Public Health and
Preventive Medicine, Monash University, Melbourne, Australia; 3 School of Human Ecology, Sukhothai
Thammathirat Open University, Nonthaburi, Thailand; 4 National Centre for Epidemiology and Population
Health, ANU College of Medicine, Biology and Environment, The Australian National University,
Canberra, Australia.
*Thai Cohort Study Team
Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan
Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita
Sripaiboonkij, Pathumvadee Somsamai, Prasutr Thawornchaisit, Duangkae Vilainerun, Wanee
Wimonwattanaphan, Cha-aim Pachanee, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin
Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora,
Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim, Anthony
McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara Yiengprugsawan, Susan Jordan,
Janneke Berecki-Gisolf, Rod McClure.
Corresponding author: Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University
of Queensland, 288 Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of Medicine,
Lerdsin Hospital, Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected] Telephone: 66-2-585-8015 Fax: 66-2-912-0121
Running Head: Health-risks and hypertension incidence in the Thai Cohort Study
Key words: hypertension; health-risks; cohort; Thailand; socioeconomic; body mass index; smoking;
drinking; physical activity
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Abstract
Objective
This study evaluates the impact of a number of demographic, biological, behavioural and lifestyle health-
risk factors on the incidence of hypertension in Thailand over a 4-year period.
Design
A 4-year prospective study of health-risk factors and their effects on incidence of hypertension in a national
Thai Cohort Study (TCS) from 2005 to 2009.
Setting
As Thailand is transitioning from a developing to a middle-income developed country, chronic diseases
(particularly cardiovascular disease) have emerged as major health issues. Hypertension is a major risk
factor for heart attack and stroke and cross-sectional studies have indicated that the prevalence is increasing.
Study participants
Sukhothai Thammathirat Open University students (57,558) who participated in both 2005 and 2009
questionnaire surveys and who were normotensive in 2005 were included in the analysis.
Measures
Adjusted relative risks associating each risk factor and incidence of hypertension by sex, after controlling for
confounders such as age, socio-economic status, body mass index (BMI) and underlying diseases.
Results
The overall 4-year incidence of hypertension was 3.5% with the rate in men being remarkably higher than
that in women (5.2% vs. 2.1%). In both sexes, hypertension associated with age, higher BMI and co-
morbidities but did not associate with income and education. In men, hypertension associated with physical
inactivity, smoking, alcohol and fast food intake. In women, hypertension was related to having a partner.
Conclusion
In both men and women, hypertension strongly associated with age, obesity and co-morbidities while it had
no association with socio-economic factors. The cohort pattern of socio-economy and hypertension reflects
that the health-risk transition in Thais is likely to be at the middle stage. Diet and lifestyle factors associate
with incidence of hypertension in Thais and may be amenable targets for hypertension control programs.
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Introduction
Hypertension is a global public health challenge due to its high prevalence and the concomitant increase in
risk of stroke and cardiovascular diseases [1]. In 2007, according to the World Health Organization,
cardiovascular diseases caused 33.7% of all deaths in the world, while other chronic diseases were
responsible for 26.5% [2]. Globally, it has been estimated that premature deaths resulting from hypertension
annually are approximately 7.1 million, which account for 64 million disability-adjusted life years (DALYs)
[3]. Hypertension ranks third, after underweight and unsafe sex, of the six major risk factors contributing to
the global disease burden[3].
In Thailand, hypertension is the third major risk factor of the national disease burden and causes 600,000
DALY losses per year [4]. The prevalence of hypertension has been estimated from a number of cross-
sectional studies among adults and has been increasing over the past decades from 20% in 1985 to 22% in
2004 [4-6]. The recent Thai National Health Examination Survey (NHES) in 2009 reported the prevalence
of hypertension in adults was 21.5% [4]. The emergence of hypertension has been the result of rapid
socioeconomic development which has resulted in one third of Thais migrating to urban areas [7].
Urbanization has been associated with increased obesity as people become less physically active and adopt
sedentary life styles such as spending more time watching television, sitting and using computers [7 8].
These factors, in association with Thailand’s ageing population [9], underpin the importance of controlling
hypertension in Thai community.
There is limited longitudinal research on health risk factors and hypertension incidence in Thailand [5]. The
purpose of this study is to address that knowledge gap. We prospectively examined the influence of an array
of demographic, biological, health-risk and lifestyle factors on the incidence of hypertension during 4 years
of follow-up in the Thai Cohort Study (TCS). This is part of an overarching research program investigating
Thailand’s health-risk transition from high maternal-child and infectious disease mortality to increased
longevity and emerging chronic disease. The large national cohort of Thai adults reported on here has been
followed since 2005 for risk factor trends and associated disease outcomes, including hypertension.
Methods
Study population
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The first TCS evaluation was conducted in 2005 by sending twenty-page health-risk questionnaires to the
200,000 distance-learning students enrolled at Sukhothai Thammathirat Open University (STOU) and
87,134 (44%) completed them and signed the consent forms. The cohort has been shown to be socio-
economically and geographically well representative of the Thai population [10]. However, they resided
more in urban areas (51.8% vs. 31.1%) and on average were younger and better educated than most Thai
adults. Details of the baseline TCS methodology have been published [10]. This questionnaire focused on
socio-demography, habitation, work, health services, injury, sedentary habits, physical activity, transport,
underlying diseases, family history, personal behaviours, BMI, and consumption of foods, vegetables and
fruit. A 4-year follow-up was carried out in 2009. Overall, 71% of the cohort responded generating
longitudinal data on 60,569 participants.
Incident Hypertension
Incident hypertension was defined as being normotensive in 2005 and self-reported doctor diagnosed
hypertensive in 2009. Hypertension incidence was the dependent variable for analyses and the person-time
denominator included all individuals at risk - those 57,558 participants with longitudinal data who were
negative for hypertension in 2005, the beginning of the 4-year study period.
Risk Factors
Hypertension was defined as describe above, underlying diseases diagnosed by a doctor were also reported
including diabetes mellitus, kidney diseases and high blood lipids, all of which were investigated as
potential risk factors of hypertension.
Participants were grouped into three age categories: younger (≤30 years), middle (31-40 years) and older
(>40 years). Urbanization status was based on rural (R) or urban (U) residence, first when aged 10-12 years
old and again in 2005, producing four groups: lifelong ruralites (RR), urbanizers (RU), de-urbanizers (UR)
and urbanites (UU). Personal monthly income was divided into four categories: ≤7000 baht, 7001–10000
baht, 10001–20000 baht and >20000 baht. In 2005, one US dollar was equivalent to 42 baht so most of
participants had quite low incomes. Household assets were classified into three categories: low (≤30,000
baht), medium (30,001-60,000) and high (>60,000).
Sedentariness was assessed by screen time (hours/day spent before TV or computer) and (separately) sitting
time (hours per day spent sitting for any purpose). Incidental exercise was measured by the frequency of
housework or gardening, categorised as follows: ≤3 times per month; 1-2 times per week; 3-4 times per
week; most days. We measured planned physical activity at the beginning and end of the 4-year follow up
period. On both occasions, cohort members reported the number of sessions per week of strenuous and
moderate exercise for at least 20 minutes, and of walking for at least 10 minutes. We derived an “overall
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measure” of planned physical activity calculated separately at both the baseline and the 4-year mark. The
measure was weighted as follows: (2 × strenuous + 1 × moderate + 1 × walking) sessions per week. This
weighting system is based on the recommendation of the International Physical Activity Questionnaire and
the Active Australia Survey as used in other analyses of the cohort data [11 12]. Finally, for each individual,
the “overall measures” of weekly exercise for 2005 and 2009 were added and then averaged by dividing by
2, creating a longitudinal measure of planned physical activity (LPPA).
Smoking status was classified as never, ex-smoker or current-smoker. Alcohol exposure was classified by
four categories: never, ex-drinker, occasional drinker or current-drinker. Body mass index (BMI) was
calculated from self-reported height in metres and weight in kilograms as weight/ height2. It was divided
into 4 categories as follows according to Asian criteria:[8 13-16] underweight (BMI <18.5), normal (18.5 to
<23), overweight (23 to <25), and obese (≥25). Foods that could potentially influence blood pressure (deep
fried, instant, roast or smoked, soybean products and soft drinks) were assessed for consumption frequency
based on a five-point Likert scale ranging from less than once a month to once or more a day. Western-style
fast food exposure was noted on a three-point scale from less than once to more than 3 times per month.
Fruit and vegetable consumption were recorded as standard serves eaten per day.
Statistical analyses
All analyses were performed using SPSS software. The incidence of hypertension and its 95% confidence
interval (CI) were calculated for each value of each categorical variable in both male and female participants
and the influence on incidence by each variable was evaluated by Chi-square test. For statistical inference,
all p-values were 2 tailed and significance was set at 5%.
Relative risks in a large study of an uncommon disease (incidence less than 10%) can be accurately
estimated as odds ratios [17]. Accordingly, for each risk variable, the relative risk (RR) and 95% confidence
interval were estimated using logistic regression to calculate the bivariate odds ratio for hypertension.
Adjusted RRs (ARRs) were estimated by calculating multivariate logistic regression odds ratios. The ARRs
were controlled for confounding by age, marital status, SES, BMI, underlying diseases and personal
behaviours (cigarette smoking and alcohol drinking). A variable was included in a multivariate model if
bivariate analysis had indicated a statistically significant association with incidence of hypertension. Some
variables were included because earlier analyses reported elsewhere had shown significant or substantial
association with hypertension.
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Results
Baseline Characteristics
Baseline characteristics of study participants self-reporting as normotensive in 2005 are shown in Table 1.
The highest proportion of males resided in the North-eastern regions and Central while there were more
females in the Central and Bangkok area. More than half of the male and female participants lived in urban
areas. Males had a higher monthly income than females whereas females had a higher educational
attainment than males. The distribution of household assets for males and females was similar and over half
of them were in medium and high categories. The study population had a similar socio-demography, income
and location of residence to the general Thai population [9].
Incidence and risks of hypertension
Overall, the incidence of self-reported doctor diagnosed hypertension in Thai adults was 3.5% and the rate in
males was more than twice the corresponding rate in females (5.2% vs. 2.1%) (Table 2). In males and
females, the incidence of hypertension increased with increasing age. Marriage or having a partner increased
the risk of hypertension in females. Educational attainment and personal income had no influence on the risk
of hypertension in both sexes. In males and females, the risk of hypertension increased with an increased
BMI. The risk of hypertension increased in both males and females who had diabetes mellitus, high blood
lipids and kidney disease.
Planned physical activity, averaged across the 4-year follow-up, was protective for males. In males, higher
LPPA (≥15 sessions / week) associated with lower incidence of hypertension; however, this association was
not observed in females. In males, current-smokers had a higher risk of developing hypertension than non-
smokers while in females, smoking (which was rare) had no apparent hypertension effect. Among males,
regular drinking increased the risk of incident hypertension; patterns of relative risks were similar for
females but, reflecting the low frequency of female drinking, did not reach statistical significance. In males,
the risk of hypertension had a direct association with the frequency of instant food consumption but there
was no such association detected in females.
Over the 4-year period, a variety of factors that could have been risks for hypertension over a longer period
were found not to be in this setting (data not shown). Thus, in males and females, factors that had no
statistically significant influence on the longitudinal 4-year risk of hypertension incidence were urbanization
status, sedentary lifestyle (non-participation in housework or gardening, high levels of television and
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computer watching, sleeping and sitting time), and food consumption habits that included deep fried food
and soft drink, Western fast food, roasted or smoked foods, fruit, vegetables and soybean products.
Discussion
This large prospective cohort of adults living all over Thailand has been the first studied nationally for a
wide array of risk factors for incident hypertension. Overall, the 4-year incidence of hypertension from 2005
to 2009 was 3.5%, notably higher in men (5.2%) than women (2.1%). These incidence rates are drawn from
a cohort of adults on average younger than the Thai population so they are lower than corresponding rates of
the adult population. However, our focus was on risk factor associations and we noted that in both sexes,
age, obesity and co-morbidity with diabetes mellitus, high blood lipids and kidney disease strongly linked to
hypertension incidence. In males, instant food consumption, smoking and drinking alcohol had a moderate
adverse effect on hypertension risk while planned physical activity averaging 15 or more sessions per week
over the 4-year period was substantially protective, reducing risk by 22%. In females, having a partner was
associated with increased incidence of hypertension.
The age effect on hypertension incidence noted by us is consistent with findings of many other population
studies of hypertension including (for example) a longitudinal study in China [18] and cross-sectional
studies in Korea [19], Taiwan [20] and the USA [21]. The noticeably greater incidence of hypertension in
men than in women that we found in Thailand has also been reported from many parts of the world
including neighboring Malaysia [22] and distant USA [23] as well as in a recent global review [24].
In men and women of the Thai cohort, a progressively higher BMI was positively and significantly
associated with an increased risk of incident hypertension over the four years. This finding was similar to
that reported for other prospective cohorts in the US [25 26] and Finland [27]. Furthermore, after seven
years of follow-up in a US study [28], weight loss associated with a decreased risk of incident hypertension.
Among a cohort of US nurses, the risk of incident hypertension increased 5% for each kilogram addition of
weight [29], and rose 15% for each increase of one kilogram per meter squared of BMI in Taiwan [18].
These results suggest that the adverse effect of high BMI on risk of hypertension does not depend on
ethinicity or sex and is wide spread around the world.
In our cohort, we noted a higher risk of incident hypertension among those with diabetes mellitus. This
finding is consistent with longitudinal studies in the USA [30 31] which reported, respectively, that in
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diabetic patients the risk of incident hypertension rose 41% and 56%. As well, prospective cohort studies in
Japan showed that diabetes [32] and impaired fasting blood glucose [33] were associated with an increased
risk of hypertension incidence. Supportive cross-sectional results emerge from studies in diverse settings
including Bangladesh and India [34], Barbados [35] and the USA [36]. A causal association between
diabetes and hypertension is suspected and possible mechanisms include endothelial dysfunction and
vascular inflammation [37].
We also found that high blood lipids significantly associated with increased risk of incident hypertension.
This supports the results from previous longitudinal studies in men [38 39], women [40 41] and adults [32
42]. The risk of hypertension increased with an increase of total cholesterol, triglyceride and low-density
lipoprotein cholesterol levels and decreased with an increase of high-density lipoprotein cholesterol level
[38 39 41].The mechanism by which dyslipidemia provokes high blood pressure is uncertain but could
reflect blood vessel dysfunction and damaged endothelium [43].
Among men and women in the Thai cohort, kidney disease strongly associated with increased risk of
hypertension incidence. This result is supported by cross-sectional data from the Czech Republic [44] and
the USA [45]. Microalbuminuria is a biomarker for renal hypertension [45] and the risk increases with the
severity of kidney impairment, revealed by the serum creatinine level [46 47]. Glomerular damage is more
closely connected with hypertension than tubular or interstitial disease [47]. Chronic kidney disease
facilitates high blood pressure through many mechanisms including excessive intravascular volume,
increased endothelin production, activity of the renin-angiotensin system and sympathetic nervous system
and decreased vasodilators, nitric oxide production and endothelial function [48].
In men we found that cigarette smoking modestly associated with increased risk of developing hypertension.
This is consistent with many other reports such as longitudinal studies in Japanese [49] and American men
[50] and a cross-sectional study in French men [51]. In addition, smoking Jordanian men had significantly
higher blood pressure than their non-smoking counterparts [52]. However, we also found that smoking had
no influence on hypertension in women. This result for females is compatible with reports from Korea [53]
and the USA [54]. In contrast, one prospective US study [55] reported an increased risk of hypertension
among women. It is likely that men tend to smoke more cigarettes than women and the impact of smoking
on hypertension is a dose-response pattern [49].
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Regular alcohol consumption was strongly associated with an increased risk of incident hypertension. This
result is supported by findings from longitudinal studies in Japan [56 57] and cross-sectional studies in
China [58] and Korea [53]. In Japanese men, alcohol consumption, particularly more than 200 grams per
week, significantly associated with an increase of blood pressure [59] while alcohol restriction was
noticeably related to a decreased blood pressure [60]. Among our Thai cohort females, alcohol consumption
had no influence on hypertension; this result was supported by longitudinal data from Japan [56] and cross-
sectional data from Korea [53]. In American females the quantity of alcohol consumption per week had no
effect on systolic blood pressure [61]. Males usually consume more alcohol than females and the
hypertension risk is linear with a dose-response [57 62]. Our results suggest that regular alcohol
consumption, especially in males, is a significant risk factor for developing hypertension.
Marital status in males had no influence on hypertension which supports the results from more economically
developed countries, the USA and Korea [53 54]. However, in middle-income countries, results diverged:
married males, when compared to their unmarried counterparts, had a higher risk of hypertension in
Barbados [35] and a lower risk in Poland [63]. Married individuals had higher emotional and social support
and lower stress [63] and probability of being a current smoker [64]. We found incident hypertension was
relatively more frequent among married females, unlike the no-sex-link pattern in developed countries such
as the USA and Korea [53 54]. The influence of marrital status on hypertension varies from one country to
another partly due to the difference of socio-economy and social structure. For our Thai cohort, the pattern
in males was similar to that in developed countries and in females it was not.
Participant educational attainment did not protect against hypertension in either sex. This is similar to cross-
sectional results for adults in developed countries such as Korea [53] and the USA [36 65]. However, other
studies in various settings and with diverse designs reported educational attainment protects against
hypertension in adults; these included a prospective cohort in middle income Thailand [5], a cross-sectional
study in middle income Malaysia [22] and a cross-sectional study in high income USA [66]. Opposite
findings have been reported in low income settings in both Bangladesh and India [34] where education was
a risk factor for hypertension. Taken collectively, these data suggest that income modifies the protective
education-hypertension effect, inverting it when income is low and boosting it when income is high. Our
middle-income cohort could be in the middle stage of this education effect as it is transiting from the low-
income pattern.
We found no association of personal monthly income and risk of hypertension in either sex. This was similar
to previous studies in middle-income countries including a longitudinal study in Thailand [5] and a cross-
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sectional study in Malaysia [67] and in cross-sectional studies in high-income countries such as in Korea
[53] and the USA [21 36]. However, income had an inverse association with risk of hypertension in another
study in high-income developed USA [65] while it had a direct positive association in a low-income
developing country such as Ghana [68]. The health transition in Thailand is likely to be in the middle stage,
according to the concept of a tipping-point for the relationship between socio-economic status, body size and
hypertension risk factors. High income, large body size and high blood pressure is seen in developing
countries while in developed countries, high income is a protective factor [69-71]. Indeed, the Thai cohort
has already been shown to be at a transitional point for metabolic states because females had an inverse
relationship between income and BMI whereas males had a positive (traditional) relationship [14]. However,
now all Thais benefit from universal health insurance so access to health care is not dependent on their
income making disease associations more attenuated than before [72].
In our cohort, the reduction of risk of hypertension incidence in males but not in females by a longitudinal
measure of planned physical activity was comparable with a prospective study in middle-aged American
whites [73]. In another prospective study in middle-aged Finns, the hypertension protective effect of
vigorous physical activity or total energy expenditure from leisure-time physical activity was observed in
men but not in women [74]. Such physical activity among females may be less intense than among males so
having no effect on the risk of hypertension [74]. A large Finnish study [27] demonstrated that the incidence
of hypertension was inversely associated with the level of physical activity in a dose-response manner in
both sexes. In our cohort study, risk of obesity had an inverse association with quantified planned exercise in
both sexes but the effect was significantly stronger in males than in females [12]. As Thailand progresses
economically, more people migrate to urban areas and decrease their physical activity [7]. Therefore, in the
future, Thai people may encounter a higher risk of hypertension due to lower physical activity.
Instant food - mostly combinations of carbohydrate, fat and salt - associated with an increased risk of
incident hypertension in males but not in females. Other foods, fruit and vegetables had no impact on
hypertension. This may be as a result of men comsuming more instant foods, such as instant noodles, than
women. For example a study in Korea showed daily instant food consumption in men and women averages
23.7 and 11.1 grams respectively [75]. Other studies showed that carbohydrate [76 77], fruit and vegetables
[78 79] consumption had no influence on hypertension. Most students consume instant noodles which lead
to a higher intake of energy, fat, sodium, thiamine, and riboflavin but a lower intake of protein, calcium,
phosphorus [75]. Koreans favoring instant noodles have a salt intake more than three times higher than the
daily recommendation (6.4 vs 2 gms) [75]. Daily salt intake of Thai people has been reported to average
10.8gm, much higher than recommended (http://www.worldactiononsalt.com/worldaction/asia/75286.pdf).
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High salt consumption was associated with an increased risk of developing hypertension in a longitudinal
study in Taiwan [80] and in cross-sectional studies in China [81 82] while other reports revealed that salt
restriction reduced blood pressure [83 84]. High plasma sodium, due to an overwhelmed capacity of the
kidney to excrete excessive salt consumption, leads to excessive intravascular volume and high blood
pressure [85]. It seems reasonable to conclude that high salt intake from instant noodle consumption is likely
to be causally associated with an increased risk of hypertension in Thailand.
In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. Indeed, the study size along with its national representation are
important factors to consider when exploring study validity and utility. The influence of risk factors on
hypertension incidence is clearly demonstrated since this is a large longitudinal cohort covering a 4-year
period. As a consequence, independent risk factors for hypertension in Thais can be identified. Our cohort
represents Thais well in geography and socio-economy [9] so the results reflect the trend of health-risk
transition in the Thai population. STOU students, with better education attainment combined with their
representative incomes and geographical location, represent the next generation of Thais. It would be
reasonable to conclude that our 4-year follow-up results show the future hypertension trends for Thai people.
However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
But a study of the validity of self-reported weight and height in our cohort found that the accuracy was quite
acceptable [86]. As well, we have conducted a validation study of self-reported hypertension (Prasutr
Thawornchaisit, 2013, under review) in a random age-sex matched sub-sample of the cohort reporting
hypertension (n=240) or no hypertension (n=240). We found the sensitivity was high (82%) and that
negative reports were usually accurate (86%). In another report, we investigated the impact of the non-
responses to the 2009 follow up and found small effects with under-representation of young urban males;
this missing group would have a minor influence on our results and would not be expected to have a high
rate of incident hypertension [87]. The findings from our study are likely to be robust and represent well the
future trends of health-risk transition in middle-income Thailand and similar ASEAN countries.
In conclusion, older age, obesity and underlying morbidity due to diabetes, high blood lipids and kidney
disease were strongly associated with an increased risk of incident hypertension in both sexes while in
males, physical inactivity, instant food consumption, smoking and drinking alcohol had a moderate
association. Sedentary life-style and the consumption of friut and vegetables had no detected influence on
hypertension. The Thai health-risk transition is likely to be in the middle stage since education attainment
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and income still have no net effect to protect against hypertension. Prevention should focus on obesity, high
blood lipids and high salt consumption and people should be encouraged to increase physical exercise and
consume low fat foods and less instant foods. A reduction of salt consumption should be a national policy by
limiting the concentration of salt in ready-to-eat and industrial foods. Smoking and drinking cessation
should be promoted for males. Thailand should also consciously aim to take advantage of its culturally
determined low rate of drinking and smoking among women and ensure that there is no attempt to
undermine this situation and develop the market for the female half of the population.
Acknowledgements and Funding
This study was supported by the International Collaborative Research Grants Scheme with joint grants from
the Wellcome Trust UK (GR071587MA) and the Australian National Health and Medical Research Council
(268055), and by a global health grant from the NHMRC (585426). We thank the staff at Sukhothai
Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are
participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen
University for guiding us successfully through the complex data processing.
Conflict of interest
The authors declare that there are no conflicts of interest.
Ethical considerations
Ethical approval was obtained from Sukhothai Thammathirat Open University Research and Development
Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee
(protocol 2004344 and 2009570). Informed, written consent was obtained from all participants.
Author contribution: PT devised the hypertension study, analysed the data and wrote the paper. FDL and
CR assisted with planning the study, analysis and interpretation. AS and SS conceived and developed the
cohort, and helped plan, analyse and interpret this study. All authors approved the final manuscript.
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Table 1 Characteristics of 57558 participants normotensive at the 2005 Thai Cohort Study baseline
Male Female Difference
Factor
n % n % P-valuea
Demographic data
Participants 25320 44.0 32238 56.0 <0.0001 Age (y) mean 32.9 30.0 <0.0001
b
(SD) (8.5) (7.6) Age group <0.0001 ≤30 y 11293 44.6 19229 59.6 31-40 y 9177 36.2 9638 29.9 >40 y 4850 19.2 3371 10.5 Married/partnered <0.0001 No 11060 44.8 18851 60.1 Yes 13607 55.2 12513 39.9 Regions <0.0001 Bangkok 3488 13.9 6057 18.9 Central 5712 22.7 8267 25.8 North 5180 20.6 5818 18.1 North-east 6142 24.4 5760 18.0 East 1515 6.0 1922 6.0 South 3103 12.3 4254 13.3 Urbanization status
c <0.0001
Rural–rural (RR) 11512 46,1 13895 43.6 Rural–urban (RU) 7891 31.6 9546 29.9 Urban–rural (UR) 1078 4.3 1344 4.2 Urban–urban (UU) 4495 18.0 7114 22.3
Socioeconomic status
Education level <0.0001 High school 13366 52.9 13135 40.8 Diploma 5749 22.8 9791 30.4 University 6153 24.4 9231 28.7 Personal monthly income (baht)
d <0.0001 ≤7000 8025 32.3 13890 44.1 7001–10000 5790 23.3 7578 24.0 10001–20000 7549 30.4 7194 22.8 >20000 3452 13.9 2851 9.0 Household assets
e (baht)
d <0.001
Low 9852 39.1 12434 38.7 Medium 8065 32.0 9863 30.7 High 7298 28.9 9804 30.5 aChi-square test
b unpaired t test cLocation of residence (rural, R, or urban, U) at age 10-12 years and again in 2005. dAt the time of the survey in 2009, US$1 = 31 Thai baht eReplacement value in Thai baht: low ≤30,000, medium 30,001-60,000 and high >60,000
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Table 2 Hypertension incidence and association with risk factors in male and female participants.
Males Females HT(n) Incia%(95% CI) ARRb(95% CI) HT(n) Incia%(95% CI) ARRb(95% CI) Participants 1306 5.2 (4.9-5.4) 690 2.1 (2.0-2.3)
Demography
Age group
≤30 y 279 2.5 (2.2-2.8) 1 215 1.1 (0.97-1.3) 1 31-40 y 484 5.3 (4.8-5.7) 1.66 (1.39-1.98) 245 2.5 (2.23-2.9) 1.93 (1.56-2.39) >40 y 543 11.2 (10.3-12.1) 3.13 (2.57-3.81) 230 6.8 (6.0-7.7) 4.2 (3.26-5.42) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Marital status (married/partnered)
No 368 3.3 (3.0-3.7) 1 303 1.6 (1.43-1.8) 1 Yes 896 6.6 (6.17-7.0) 0.97 (0.83-1.12) 368 2.9 (2.65-3.2) 1.22 (1.03-1.45)
Socio-economic status
Education level
High school 697 5.2 (4.8-5.6) 1 303 2.3 (2.05-2.57) 1 Diploma 276 4.8 (4.25-5.4) 1.0 (0.86-1.17) 180 1.8 (1.57-2.1) 0.81 (0.66-0.99) University 331 5.4 (4.8-5.9) 0.93 (0.8-1.1) 204 2.2 (1.9-2.51) 0.87 (0.71-1.07) P-trend 0.83 0.61 0.463 0.1 Personal monthly income (baht)c ≤7000 268 3.3 (2.95-3.7) 1 217 1.6 (1.36-1.8) 1 7001-10000 229 4.0 (3.45-4.5) 1.0 (0.81-1.2) 142 1.9 (1.57-2.2) 1.1 (0.87-1.37) 10001-20000 497 6.6 (6.0-7.14) 1.1 (0.91-1.31) 196 2.7 (2.35-3.1) 0.97 (0.77-1.23) >20000 289 8.4 (7.46-9.3) 1.0 (0.8-1.25) 122 4.3 (3.54-5.0) 1.04 (0.78-1.39) P-trend <0.0001 0.527 <0.0001 0.783
BMI classificationd
Underweight 359 2.9 (2.62-3.2) 0.46 (0.27-0.78) 60 0.85 (0.63-1.15) 0.9 (0.67-1.21) (BMI <18.5) Normal 16 1.1 (0.56-1.6) 1 258 1.4 (1.2-1.5) 1 (18.5 ≤ BMI < 23) Overweight 333 5.9 (5.3-6.54) 1.66 (1.41-1.95) 129 3.9 (3.3-4.62) 2.32 (1.8-2.91) (23 ≤ BMI < 25) Obese 575 10.4 (9.6-11.2) 2.82 (2.43-3.27) 231 7.4 (6.4-8.27) 4.14 (3.4-5.05) (BMI ≥25) P-trend <0.0001 <0.0001 <0.0001 <0.0001
aIncidence of Hypertension
bAdjusted relative risks (ARR) were calculated from multi-variate logistic regression models of hypertension
adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification
Continued….
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Table 2 (continued...)
Males Females HT(n) Inci
a%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Physical activities
Weighted total sessions exercise-related physical activity (average between 2005 and 2009) 0-7 ses/wc 198 6.3 (5.5-7.1) 1 162 2.2 (1.8-2.6) 1 8-14 ses/w 589 5.2 (4.8-5.6) 0.9 (0.76-1.07) 352 2.1 (1.9-2.3) 1.0 (0.82-1.23)
≥15 ses/w 380 4.4 (4.0-5.0) 0.78 (0.65-0.95) 119 2.2 (1.8-2.6) 1.0 (0.77-1.3)
P-trend <0.0001 <0.029 0.905 1.0
Underlying diseases
Diabetes mellitus
No 1251 5.0 (4.7-5.27) 1 674 2.1 (1.9-2.26) 1 Yes 55 18.3 (13.9-22.7) 1.81 (1.29-2.52) 16 10.7 (5.7-15.8) 2.54 (1.41-4.56)
High lipids
No 983 4.4 (4.13-4.67) 1 576 1.9 (1.76-2.1) 1 Yes 323 10.8 (9.7-11.9) 1.4 (1.21-1.63) 114 5.4 (4.47-6.4) 1.38 (1.09-1.75)
Kidney diseases
No 1248 5.1 (4.8-5.33) 1 659 2.1 (1.9-2.26) 1 Yes 58 9.6 (7.2-11.9) 1.62 (1.19-2.2) 31 3.9 (2.52-5.2) 1.77 (1.19-2.63)
Personal behaviours
Smoking status
Never 506 4.1 (3.7-4.4) 1 617 2.1 (1.9-2.23) 1 Ex-smoker 496 6.6 (6.0-7.13) 1.15 (1.0-1.32) 37 3.2 (2.2-4.22) 1.28 (0.88-1.87) Cur-smokerd 276 5.8 (5.16-6.5) 1.22 (1.04-1.44) 8 3.1 (0.97-5.2) 1.54 (0.74-3.2) P-trend <0.0001 <0.032 <0.008 0.23
Drinking status
Never 105 3.9 (3.2-4.7) 1 268 2.1 (1.8-2.32) 1 Ex-drinker 155 5.7 (4.85-6.6) 0.98 (0.74-1.3) 50 2.2 (1.6-2.85) 1.14 (0.82-1.59) Occ- drinkere 821 4.8 (4.45-5.1) 1.0 (0.8-1.25) 352 2.1 (1.9-2.36) 1.14 (0.96-1.36) Reg- drinker
f 212 8.6 (7.5-9.7) 1.61 (1.23-2.11) 9 4.7 (1.66-7.7) 1.83 (0.86-3.9)
P-trend <0.0001 <0.0001 0.471 0.254
Food consumption habit
Instant food
<1 time/m 335 6.3 (5.6-6.95) 1 168 2.8(2.36-3.2) 1 1-3 times/m 485 5.1 (4.63-5.5) 1.02 (0.87-1.19) 273 2.1 (1.84-2.3) 0.95 (0.77-1.17) 1-2 times/wk 280 4.4 (3.9-4.9) 1.09 (0.91-1.30) 153 1.9 (1.6-2.2) 1.04 (0.81-1.33) 3-6 times/wk 165 5.0 (4.3-5.75) 1.36 (1.1-1.69) 73 1.8 (1.4-2.22) 1.14 (0.84-1.55) ≥1 times/d 32 5.5 (3.6-7.33) 1.67 (1.11-2.52) 18 2.3 (1.3-3.35) 1.48 (0.86-2.53) P-trend <0.002 <0.008 <0.002 0.403 aIncidence of Hypertension
bAdjusted relative risks were calculated from multi-variate logistic regression models of hypertension adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification eSessions / week fCurrent-smoker gOccasional drinker hRegular drinker
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43 doi: JST.JSTAGE/jea/JE20090119 [pii][published Online First: Epub Date]|.
82. Sun Z, Zheng L, Xu C, et al. Prevalence of prehypertension, hypertension and, associated risk factors in
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10.1016/j.ijcard.2007.08.127[published Online First: Epub Date]|.
83. He FJ, MacGregor GA. Effect of modest salt reduction on blood pressure: a meta-analysis of
randomized trials. Implications for public health. J Hum Hypertens 2002;16(11):761-70 doi:
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84. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the
Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research
Group. N Engl J Med 2001;344(1):3-10 doi: 10.1056/NEJM200101043440101[published Online
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85. He FJ, MacGregor GA. Reducing population salt intake worldwide: from evidence to implementation.
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10.1016/j.pcad.2009.12.006[published Online First: Epub Date]|.
86. Lim LL, Seubsman SA, Sleigh A. Validity of self-reported weight, height, and body mass index among
university students in Thailand: Implications for population studies of obesity in developing
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87. Thinkhamrop K, Seubsman S, Sleigh A, et al. Reasons for Non-Response to Mailed Questionnaires –
Experience of a Large Cohort Study in Thailand. KKU Jph Research 2011;4(3):73-87
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Health-risk factors and the incidence of hypertension: 4-year prospective findings from a national cohort of 60,569
Thai open university students
Journal: BMJ Open
Manuscript ID: bmjopen-2013-002826.R2
Article Type: Research
Date Submitted by the Author: 31-May-2013
Complete List of Authors: thawornchaisit, prasutr; Faculty of Health Sciences, School of Medicine; Lerdsin Hospital, Medicine de Looze, Ferdinandus; Faculty of Health Sciences, School of Medicine
Reid, Christopher; Monash University, School of Public Health and Preventive Medicine Seubsman, Sam-ang; Sukhothai Thammathirat Open University, School of Human Ecology Sleigh, Adrian; The Australian National University, National Centre for Epidemiology and Population Health
<b>Primary Subject Heading</b>:
Epidemiology
Secondary Subject Heading: Cardiovascular medicine, Public health
Keywords: Hypertension < CARDIOLOGY, General diabetes < DIABETES & ENDOCRINOLOGY, Risk management < HEALTH SERVICES ADMINISTRATION & MANAGEMENT, Adult nephrology < NEPHROLOGY
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Health-risk factors and the incidence of hypertension: 4-year prospective
findings from a national cohort of 60,569 Thai open university students
Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University of Queensland, 288
Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of Medicine, Lerdsin Hospital,
Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected]
Telephone: 662-585-8015
Fax: 662-912-0121
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc1, Christopher M Reid, PhD2, Sam-
ang Seubsman, PhD3, Adrian Sleigh, MD4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia, 2 School of Public Health and
Preventive Medicine, Monash University, Melbourne, Australia, 3 School of Human Ecology, Sukhothai
Thammathirat Open University, Nonthaburi, Thailand, 4 National Centre for Epidemiology and Population
Health, ANU College of Medicine, Biology and Environment, The Australian National University,
Canberra, Australia.
*Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan
Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita
Sripaiboonkij, Pathumvadee Somsamai, Prasutr Thawornchaisit, Duangkae Vilainerun, Wanee
Wimonwattanaphan, Cha-aim Pachanee, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin
Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora,
Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim, Anthony
McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara Yiengprugsawan, Susan Jordan,
Janneke Berecki-Gisolf, Rod McClure.
Running Head: Health-risks and hypertension incidence in the Thai Cohort Study
Key words: hypertension; health-risks; cohort; Thailand; socioeconomic
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Abstract
Objective
This study evaluates the impact of a number of demographic, biological, behavioural and lifestyle health-
risk factors on the incidence of hypertension in Thailand over a 4-year period.
Design
A 4-year prospective study of health-risk factors and their effects on incidence of hypertension in a national
Thai Cohort Study (TCS) from 2005 to 2009
Setting
As Thailand is transitioning from a developing to a middle-income developed country, chronic diseases
(particularly cardiovascular disease) have emerged as major health issues. Hypertension is a major risk
factor for heart attack and stroke and cross-sectional studies have indicated that the prevalence is increasing.
Study participants
Sukhothai Thammathirat Open University students (57,558) who participated in both 2005 and 2009
questionnaire surveys and who were normotensive in 2005 were included in the analysis.
Measures
Adjusted relative risks associating each risk factor and incidence of hypertension by sex, after controlling for
confounders such as age, socio-economic status, body mass index (BMI) and underlying diseases.
Results
The overall 4-year incidence of hypertension was 3.5% with the rate in men being remarkably higher than
that in women (5.2% vs. 2.1%). In both sexes, hypertension associated with age, higher BMI and co-
morbidities but did not associate with income and education. In men, hypertension associated with physical
inactivity, smoking, alcohol and fast food intake. In women, hypertension was related to having a partner.
Conclusion
In both men and women, hypertension strongly associated with age, obesity and co-morbidities while it had
no association with socio-economic factors. The cohort pattern of socio-economy and hypertension reflects
that the health-risk transition in Thais is likely to be at the middle stage. Diet and lifestyle factors associate
with incidence of hypertension in Thais and may be amenable targets for hypertension control programs.
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Article summary
Article focus
In a large Thai Cohort Study, we measured the effect of exposure to health-risk factors recorded in 2005 on
the incidence rate of hypertension over the following 4 years. This is part of a study of the health-risk
transition underway in Thailand. We investigated socio-demographic factors, co-morbidity, body size,
physical activity and health behaviour. Cohort members are Open University students, live nationwide, are
well educated and of modest means, representing the next generation of adult Thais.
Key Messages
Overall, the incidence of self-reported hypertension in Thai adults was 3.5% with the rate for males much
higher than females (5.2% vs. 2.1%). In both sexes hypertension incidence was associated with age, higher
body mass index, kidney disease, diabetes and high lipids, but not with socioeconomic status.
Marriage or partnering increased risk of hypertension in females but not in males. Planned physical activity,
averaged across the 4-year follow-up, protected males: ≥15 sessions /week associated with lower incidence
of hypertension. However, this was not observed in females.
In males, current-smokers had a higher risk of developing hypertension than non-smokers while in females
smoking (which was rare) had no apparent hypertension effect. Among males, regular drinking increased the
risk of incident hypertension.
Strengths
In Thailand, this cohort constitutes the largest longitudinal study of health-risk factors and hypertension
development. As a consequence, many independent risk factors for hypertension in Thais can be identified.
Our cohort represents Thais well in geography and socio-economy so the results reflect trends of the health-
risk transition in the population.
Limitations
Our study relies on self-report. There may be recall and reporting errors. Some risk factors may take more
than four years to produce hypertension and will manifest as the cohort ages.
Introduction
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Hypertension is a global public health challenge due to its high prevalence and the concomitant increase in
risk of stroke and cardiovascular diseases [1]. In 2007, according to the World Health Organization,
cardiovascular diseases caused 33.7% of all deaths in the world, while other chronic diseases were
responsible for 26.5% [2]. Globally, it has been estimated that premature deaths resulting from hypertension
annually are approximately 7.1 million, which account for 64 million disability-adjusted life years (DALYs)
[3]. Hypertension ranks third, after underweight and unsafe sex, of the six major risk factors contributing to
the global disease burden[3].
In Thailand, hypertension is the third major risk factor of the national disease burden and causes 600,000
DALY losses per year [4]. The prevalence of hypertension has been estimated from a number of cross-
sectional studies among adults and has been increasing over the past decades from 20% in 1985 to 22% in
2004 [4-6]. The recent Thai National Health Examination Survey (NHES) in 2009 reported the prevalence
of hypertension in adults was 21.5% [4]. The emergence of hypertension has been the result of rapid
socioeconomic development which has resulted in one third of Thais migrating to urban areas [7].
Urbanization has been associated with increased obesity as people become less physically active and adopt
sedentary life styles such as spending more time watching television, sitting and using computers [7 8].
These factors, in association with Thailand’s ageing population [9], underpin the importance of controlling
hypertension in Thai community.
There is limited longitudinal research on health risk factors and hypertension incidence in Thailand [5]. The
purpose of this study is to address that knowledge gap. We prospectively examined the influence of an array
of demographic, biological, health-risk and lifestyle factors on the incidence of hypertension during 4 years
of follow-up in the Thai Cohort Study (TCS). This is part of an overarching research program investigating
Thailand’s health-risk transition from high maternal-child and infectious disease mortality to increased
longevity and emerging chronic disease. The large national cohort of Thai adults reported on here has been
followed since 2005 for risk factor trends and associated disease outcomes, including hypertension.
Methods
Study population
The first TCS evaluation was conducted in 2005 by sending twenty-page health-risk questionnaires to the
200,000 distance-learning students enrolled at Sukhothai Thammathirat Open University (STOU) and
87,134 (44%) completed them and signed the consent forms. The cohort has been shown to be socio-
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economically and geographically well representative of the Thai population [10]. However, they resided
more in urban areas (51.8% vs. 31.1%) and on average were younger and better educated than most Thai
adults. Details of the baseline TCS methodology have been published [10]. This questionnaire focused on
socio-demography, habitation, work, health services, injury, sedentary habits, physical activity, transport,
underlying diseases, family history, personal behaviours, BMI, and consumption of foods, vegetables and
fruit. A 4-year follow-up was carried out in 2009. Overall, 71% of the cohort responded generating
longitudinal data on 60,569 participants.
Incident Hypertension
Incident hypertension was defined as being normotensive in 2005 and self-reported doctor diagnosed
hypertensive in 2009. Hypertension incidence was the dependent variable for analyses and the person-time
denominator included all individuals at risk - those 57,558 participants with longitudinal data who were
negative for hypertension in 2005, the beginning of the 4-year study period.
Risk Factors
Hypertension was defined as describe above, underlying diseases diagnosed by a doctor were also reported
including diabetes mellitus, kidney diseases and high blood lipids, all of which were investigated as
potential risk factors of hypertension.
Participants were grouped into three age categories: younger (≤30 years), middle (31-40 years) and older
(>40 years). Urbanization status was based on rural (R) or urban (U) residence, first when aged 10-12 years
old and again in 2005, producing four groups: lifelong ruralites (RR), urbanizers (RU), de-urbanizers (UR)
and urbanites (UU). Personal monthly income was divided into four categories: ≤7000 baht, 7001–10000
baht, 10001–20000 baht and >20000 baht. In 2005, one US dollar was equivalent to 42 baht so most of
participants had quite low incomes. Household assets were classified into three categories: low (≤30,000
baht), medium (30,001-60,000 baht) and high (>60,000 baht).
Sedentariness was assessed by screen time (hours/day spent before TV or computer) and (separately) sitting
time (hours per day spent sitting for any purpose). Incidental exercise was measured by the frequency of
housework or gardening, categorised as follows: ≤3 times per month; 1-2 times per week; 3-4 times per
week; most days. We measured planned physical activity at the beginning and end of the 4-year follow up
period. On both occasions, cohort members reported the number of sessions per week of strenuous and
moderate exercise for at least 20 minutes, and of walking for at least 10 minutes. We derived an “overall
measure” of planned physical activity calculated separately at both the baseline and the 4-year mark. The
measure was weighted as follows: (2 × strenuous + 1 × moderate + 1 × walking) sessions per week. This
weighting system is based on the recommendation of the International Physical Activity Questionnaire and
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the Active Australia Survey as used in other analyses of the cohort data [11 12]. Finally, for each individual,
the “overall measures” of weekly exercise for 2005 and 2009 were added and then averaged by dividing by
2, creating a longitudinal measure of planned physical activity (LPPA).
Smoking status was classified as never, ex-smoker or current-smoker. Alcohol exposure was classified by
four categories: never, ex-drinker, occasional drinker or current-drinker. Body mass index (BMI) was
calculated from self-reported height in metres and weight in kilograms as weight/height2. It was divided into
4 categories as follows according to Asian criteria:[8 13-16] underweight (BMI <18.5), normal (18.5 to
<23), overweight (23 to <25), and obese (≥25). Foods that could potentially influence blood pressure (deep
fried, instant, roast or smoked, soybean products and soft drinks) were assessed for consumption frequency
based on a five-point Likert scale ranging from less than once a month to once or more a day. Western-style
fast food exposure was noted on a three-point scale from less than once to more than 3 times per month.
Fruit and vegetable consumption were recorded as standard serves eaten per day.
Statistical analyses
All analyses were performed using SPSS software. The incidence of hypertension and its 95% confidence
interval (CI) were calculated for each value of each categorical variable in both male and female participants
and the influence on incidence by each variable was evaluated by Chi-square test. For statistical inference,
all p-values were 2 tailed and significance was set at 5%.
Relative risks in a large study of an uncommon disease (incidence less than 10%) can be accurately
estimated as odds ratios [17]. Accordingly, for each risk variable, the relative risk (RR) and 95% confidence
interval were estimated using logistic regression to calculate the bivariate odds ratio for hypertension.
Adjusted RRs (ARRs) were estimated by calculating multivariate logistic regression odds ratios. The ARRs
were controlled for confounding by age, marital status, SES, BMI, underlying diseases and personal
behaviours (cigarette smoking and alcohol drinking). A variable was included in a multivariate model if
bivariate analysis had indicated a statistically significant association with incidence of hypertension. Some
variables were included because earlier analyses reported elsewhere had shown significant or substantial
association with hypertension.
Results
Baseline Characteristics
Baseline characteristics of study participants self-reporting as normotensive in 2005 are shown in Table 1.
The highest proportion of males resided in the North-eastern regions and Central while there were more
females in the Central and Bangkok area. More than half of the male and female participants lived in urban
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areas. Males had a higher monthly income than females whereas females had a higher educational
attainment than males. The distribution of household assets for males and females was similar and over half
of them were in medium and high categories. The study population had a similar socio-demography, income
and location of residence to the general Thai population [9].
Incidence and risks of hypertension
Overall, the incidence of self-reported doctor diagnosed hypertension in Thai adults was 3.5% and the rate in
males was more than twice the corresponding rate in females (5.2% vs. 2.1%) (Table 2). In males and
females, the incidence of hypertension increased with increasing age. Marriage or having a partner increased
the risk of hypertension in females. However, this was not noticed in male. Educational attainment and
personal income had no influence on the risk of hypertension in both sexes. In males and females, the risk of
hypertension increased with an increased BMI. The risk of hypertension increased in both males and females
who had diabetes mellitus, high blood lipids and kidney disease.
Planned physical activity, averaged across the 4-year follow-up, was protective for males. In males, higher
LPPA (≥15 sessions / week) associated with lower incidence of hypertension; however, this association was
not observed in females. In males, current-smokers had a higher risk of developing hypertension than non-
smokers while in females, smoking (which was rare) had no apparent hypertension effect. Among males,
regular drinking increased the risk of incident hypertension; patterns of relative risks were similar for
females but, reflecting the low frequency of female drinking, did not reach statistical significance. In males,
the risk of hypertension had a direct association with the frequency of instant food consumption but there
was no such association detected in females.
Over the 4-year period, a variety of factors that could have been risks for hypertension over a longer period
were found not to be in this setting (data not shown). Thus, in males and females, factors that had no
statistically significant influence on the longitudinal 4-year risk of hypertension incidence were urbanization
status, sedentary lifestyle (non-participation in housework or gardening, high levels of television and
computer watching, sleeping and sitting time), and food consumption habits that included deep fried food
and soft drink, Western fast food, roasted or smoked foods, fruit, vegetables and soybean products.
Discussion
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This large prospective cohort of adults living all over Thailand has been the first studied nationally for a
wide array of risk factors for incident hypertension. Overall, the 4-year incidence of hypertension from 2005
to 2009 was 3.5%, notably higher in men (5.2%) than women (2.1%). These incidence rates are drawn from
a cohort of adults on average younger than the Thai population so they are lower than corresponding rates of
the adult population. However, our focus was on risk factor associations and we noted that in both sexes,
age, obesity and co-morbidity with diabetes mellitus, high blood lipids and kidney disease strongly linked to
hypertension incidence. In males, instant food consumption, smoking and drinking alcohol had a moderate
adverse effect on hypertension risk while planned physical activity averaging 15 or more sessions per week
over the 4-year period was substantially protective, reducing risk by 22%. In females, having a partner was
associated with increased incidence of hypertension.
The age effect on hypertension incidence noted by us is consistent with findings of many other population
studies of hypertension including (for example) a longitudinal study in China [18] and cross-sectional
studies in Korea [19], Taiwan [20] and the USA [21]. The noticeably greater incidence of hypertension in
men than in women that we found in Thailand has also been reported from many parts of the world
including neighboring Malaysia [22] and distant USA [23] as well as in a recent global review [24].
In men and women of the Thai cohort, a progressively higher BMI was positively and significantly
associated with an increased risk of incident hypertension over the four years. This finding was similar to
that reported for other prospective cohorts in the US [25 26] and Finland [27]. Furthermore, after seven
years of follow-up in a US study [28], weight loss associated with a decreased risk of incident hypertension.
Among a cohort of US nurses, the risk of incident hypertension increased 5% for each kilogram addition of
weight [29], and rose 15% for each increase of one kilogram per meter squared of BMI in Taiwan [18].
These results suggest that the adverse effect of high BMI on risk of hypertension does not depend on
ethinicity or sex and is wide spread around the world.
In our cohort, we noted a higher risk of incident hypertension among those with diabetes mellitus. This
finding is consistent with longitudinal studies in the USA [30 31] which reported, respectively, that in
diabetic patients the risk of incident hypertension rose 41% and 56%. As well, prospective cohort studies in
Japan showed that diabetes [32] and impaired fasting blood glucose [33] were associated with an increased
risk of hypertension incidence. Supportive cross-sectional results emerge from studies in diverse settings
including Bangladesh and India [34], Barbados [35] and the USA [36]. A causal association between
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diabetes and hypertension is suspected and possible mechanisms include endothelial dysfunction and
vascular inflammation [37].
We also found that high blood lipids significantly associated with increased risk of incident hypertension.
This supports the results from previous longitudinal studies in men [38 39], women [40 41] and adults [32
42]. The risk of hypertension increased with an increase of total cholesterol, triglyceride and low-density
lipoprotein cholesterol levels and decreased with an increase of high-density lipoprotein cholesterol level
[38 39 41].The mechanism by which dyslipidemia provokes high blood pressure is uncertain but could
reflect blood vessel dysfunction and damaged endothelium [43].
Among men and women in the Thai cohort, kidney disease strongly associated with increased risk of
hypertension incidence. This result is supported by cross-sectional data from the Czech Republic [44] and
the USA [45]. Microalbuminuria is a biomarker for renal hypertension [45] and the risk increases with the
severity of kidney impairment, revealed by the serum creatinine level [46 47]. Glomerular damage is more
closely connected with hypertension than tubular or interstitial disease [47]. Chronic kidney disease
facilitates high blood pressure through many mechanisms including excessive intravascular volume,
increased endothelin production, activity of the renin-angiotensin system and sympathetic nervous system
and decreased vasodilators, nitric oxide production and endothelial function [48].
In men we found that cigarette smoking modestly associated with increased risk of developing hypertension.
This is consistent with many other reports such as longitudinal studies in Japanese [49] and American men
[50] and a cross-sectional study in French men [51]. In addition, smoking Jordanian men had significantly
higher blood pressure than their non-smoking counterparts [52]. However, we also found that smoking had
no influence on hypertension in women. This result for females is compatible with reports from Korea [53]
and the USA [54]. In contrast, one prospective US study [55] reported an increased risk of hypertension
among women. It is likely that men tend to smoke more cigarettes than women and the impact of smoking
on hypertension is a dose-response pattern [49].
Regular alcohol consumption was strongly associated with an increased risk of incident hypertension. This
result is supported by findings from longitudinal studies in Japan [56 57] and cross-sectional studies in
China [58] and Korea [53]. In Japanese men, alcohol consumption, particularly more than 200 grams per
week, significantly associated with an increase of blood pressure [59] while alcohol restriction was
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noticeably related to a decreased blood pressure [60]. Among our Thai cohort females, alcohol consumption
had no influence on hypertension; this result was supported by longitudinal data from Japan [56] and cross-
sectional data from Korea [53]. In American females the quantity of alcohol consumption per week had no
effect on systolic blood pressure [61]. Males usually consume more alcohol than females and the
hypertension risk is linear with a dose-response [57 62]. Our results suggest that regular alcohol
consumption, especially in males, is a significant risk factor for developing hypertension.
Marital status in males had no influence on hypertension which supports the results from more economically
developed countries, the USA and Korea [53 54]. However, in middle-income countries, results diverged:
married males, when compared to their unmarried counterparts, had a higher risk of hypertension in
Barbados [35] and a lower risk in Poland [63]. Married individuals had higher emotional and social support
and lower stress [63] and probability of being a current smoker [64]. We found incident hypertension was
relatively more frequent among married females, unlike the no-sex-link pattern in developed countries such
as the USA and Korea [53 54]. The influence of marrital status on hypertension varies from one country to
another partly due to the difference of socio-economy and social structure. For our Thai cohort, the pattern
in males was similar to that in developed countries and in females it was not.
Participant educational attainment did not protect against hypertension in either sex. This is similar to cross-
sectional results for adults in developed countries such as Korea [53] and the USA [36 65]. However, other
studies in various settings and with diverse designs reported educational attainment protects against
hypertension in adults; these included a prospective cohort in middle income Thailand [5], a cross-sectional
study in middle income Malaysia [22] and a cross-sectional study in high income USA [66]. Opposite
findings have been reported in low income settings in both Bangladesh and India [34] where education was
a risk factor for hypertension. Taken collectively, these data suggest that income modifies the protective
education-hypertension effect, inverting it when income is low and boosting it when income is high. Our
middle-income cohort could be in the middle stage of this education effect as it is transiting from the low-
income pattern.
We found no association of personal monthly income and risk of hypertension in either sex. This was similar
to previous studies in middle-income countries including a longitudinal study in Thailand [5] and a cross-
sectional study in Malaysia [67] and in cross-sectional studies in high-income countries such as in Korea
[53] and the USA [21 36]. However, income had an inverse association with risk of hypertension in another
study in high-income developed USA [65] while it had a direct positive association in a low-income
developing country such as Ghana [68]. The health transition in Thailand is likely to be in the middle stage,
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according to the concept of a tipping-point for the relationship between socio-economic status, body size and
hypertension risk factors. High income, large body size and high blood pressure is seen in developing
countries while in developed countries, high income is a protective factor [69-71]. Indeed, the Thai cohort
has already been shown to be at a transitional point for metabolic states because females had an inverse
relationship between income and BMI whereas males had a positive (traditional) relationship [14]. However,
now all Thais benefit from universal health insurance so access to health care is not dependent on their
income making disease associations more attenuated than before [72].
In our cohort, the reduction of risk of hypertension incidence in males but not in females by a longitudinal
measure of planned physical activity was comparable with a prospective study in middle-aged American
whites [73]. In another prospective study in middle-aged Finns, the hypertension protective effect of
vigorous physical activity or total energy expenditure from leisure-time physical activity was observed in
men but not in women [74]. Such physical activity among females may be less intense than among males so
having no effect on the risk of hypertension [74]. A large Finnish study [27] demonstrated that the incidence
of hypertension was inversely associated with the level of physical activity in a dose-response manner in
both sexes. In our cohort study, risk of obesity had an inverse association with quantified planned exercise in
both sexes but the effect was significantly stronger in males than in females [12]. As Thailand progresses
economically, more people migrate to urban areas and decrease their physical activity [7]. Therefore, in the
future, Thai people may encounter a higher risk of hypertension due to lower physical activity.
Instant food - mostly combinations of carbohydrate, fat and salt - associated with an increased risk of
incident hypertension in males but not in females. Other foods, fruit and vegetables had no impact on
hypertension. This may be as a result of men comsuming more instant foods, such as instant noodles, than
women. For example a study in Korea showed daily instant food consumption in men and women averages
23.7 and 11.1 grams respectively [75]. Other studies showed that carbohydrate [76 77], fruit and vegetables
[78 79] consumption had no influence on hypertension. Most students consume instant noodles which lead
to a higher intake of energy, fat, sodium, thiamine, and riboflavin but a lower intake of protein, calcium,
phosphorus [75]. Koreans favoring instant noodles have a salt intake more than three times higher than the
daily recommendation (6.4 vs 2 gms) [75]. Daily salt intake of Thai people has been reported to average
10.8gm, much higher than recommended (http://www.worldactiononsalt.com/worldaction/asia/75286.pdf).
High salt consumption was associated with an increased risk of developing hypertension in a longitudinal
study in Taiwan [80] and in cross-sectional studies in China [81 82] while other reports revealed that salt
restriction reduced blood pressure [83 84]. High plasma sodium, due to an overwhelmed capacity of the
kidney to excrete excessive salt consumption, leads to excessive intravascular volume and high blood
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pressure [85]. It seems reasonable to conclude that high salt intake from instant noodle consumption is likely
to be causally associated with an increased risk of hypertension in Thailand.
In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. Indeed, the study size along with its national representation are
important factors to consider when exploring study validity and utility. The influence of risk factors on
hypertension incidence is clearly demonstrated since this is a large longitudinal cohort covering a 4-year
period. As a consequence, independent risk factors for hypertension in Thais can be identified. With the high
initial non-response rate and participants drawn from only one university distance-learning students enrolled
at Sukhothai Thammathirat Open University may not be representative of the Thai people or next generation
of Thais. However, our nationwide cohort represents STOU students well and also represents the Thai
population well in geography and socio-economy [9] so the results to some extent reflect the trend of health-
risk transition in the Thai population. Indeed, STOU students, with better education attainment combined
with their representative incomes and geographical location, are likely to represent the next generation of
Thais. It would be reasonable to conclude that our 4-year follow-up results may indicate future hypertension
trends for Thai people.
However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
But a study of the validity of self-reported weight and height in our cohort found that the accuracy was quite
acceptable [86]. As well, we have conducted a validation study of self-reported hypertension (Prasutr
Thawornchaisit, 2013, under review) in a random age-sex matched sub-sample of the cohort reporting
hypertension (n=240) or no hypertension (n=240). We found the sensitivity was high (82%) and that
negative reports were usually accurate (86%). In another report, we investigated the impact of the non-
responses to the 2009 follow up and found small effects with under-representation of young urban males;
this missing group would have a minor influence on our results and would not be expected to have a high
rate of incident hypertension [87]. The findings from our study are likely to be robust and represent well the
future trends of health-risk transition in middle-income Thailand and similar ASEAN countries.
In conclusion, older age, obesity and underlying morbidity due to diabetes, high blood lipids and kidney
disease were strongly associated with an increased risk of incident hypertension in both sexes while in
males, physical inactivity, instant food consumption, smoking and drinking alcohol had a moderate
association. Sedentary life-style and the consumption of friut and vegetables had no detected influence on
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hypertension. The Thai health-risk transition is likely to be in the middle stage since education attainment
and income still have no net effect to protect against hypertension. Prevention should focus on obesity, high
blood lipids and high salt consumption and people should be encouraged to increase physical exercise and
consume low fat foods and less instant foods. A reduction of salt consumption should be a national policy by
limiting the concentration of salt in ready-to-eat and industrial foods. Smoking and drinking cessation
should be promoted for males. Thailand should also consciously aim to take advantage of its culturally
determined low rate of drinking and smoking among women and ensure that there is no attempt to
undermine this situation and develop the market for the female half of the population.
Acknowledgements and Funding
This study was supported by the International Collaborative Research Grants Scheme with joint grants from
the Wellcome Trust UK (GR071587MA) and the Australian National Health and Medical Research Council
(268055), and by a global health grant from the NHMRC (585426). We thank the staff at Sukhothai
Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are
participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen
University for guiding us successfully through the complex data processing.
Conflict of interest
The authors declare that there are no conflicts of interest.
Ethical considerations
Ethical approval was obtained from Sukhothai Thammathirat Open University Research and Development
Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee
(protocol 2004344 and 2009570). Informed, written consent was obtained from all participants.
Author contribution: PT devised the hypertension study, analysed the data and wrote the paper. FDL and
CR assisted with planning the study, analysis and interpretation. AS and SS conceived and developed the
cohort, and helped plan, analyse and interpret this study. All authors approved the final manuscript.
Data sharing: No additional data available.
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Table 1 Characteristics of 57558 participants normotensive at the 2005 Thai Cohort Study baseline
Male Female Difference
Factor
n % n % P-valuea
Demographic data
Participants 25320 44.0 32238 56.0 <0.0001 Age (y) mean 32.9 30.0 <0.0001
b
(SD) (8.5) (7.6) Age group <0.0001 ≤30 y 11293 44.6 19229 59.6 31-40 y 9177 36.2 9638 29.9 >40 y 4850 19.2 3371 10.5 Married/partnered <0.0001 No 11060 44.8 18851 60.1 Yes 13607 55.2 12513 39.9 Regions <0.0001 Bangkok 3488 13.9 6057 18.9 Central 5712 22.7 8267 25.8 North 5180 20.6 5818 18.1 North-east 6142 24.4 5760 18.0 East 1515 6.0 1922 6.0 South 3103 12.3 4254 13.3 Urbanization status
c <0.0001
Rural–rural (RR) 11512 46,1 13895 43.6 Rural–urban (RU) 7891 31.6 9546 29.9 Urban–rural (UR) 1078 4.3 1344 4.2 Urban–urban (UU) 4495 18.0 7114 22.3
Socioeconomic status
Education level <0.0001 High school 13366 52.9 13135 40.8 Diploma 5749 22.8 9791 30.4 University 6153 24.4 9231 28.7 Personal monthly income (baht)
d <0.0001 ≤7000 8025 32.3 13890 44.1 7001–10000 5790 23.3 7578 24.0 10001–20000 7549 30.4 7194 22.8 >20000 3452 13.9 2851 9.0 Household assets
e (baht)
d <0.001
Low 9852 39.1 12434 38.7 Medium 8065 32.0 9863 30.7 High 7298 28.9 9804 30.5 aChi-square test
b unpaired t test cLocation of residence (rural, R, or urban, U) at age 10-12 years and again in 2005. dAt the time of the survey in 2009, US$1 = 31 Thai baht eReplacement value in Thai baht: low ≤30,000, medium 30,001-60,000 and high >60,000
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Table 2 Hypertension incidence and association with risk factors in male and female participants.
Males Females
HT(n) Incia%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Participants 1306 5.2 (4.9-5.4) 690 2.1 (2.0-2.3)
Demography
Age group
≤30 y 279 2.5 (2.2-2.8) 1 215 1.1 (0.97-1.3) 1 31-40 y 484 5.3 (4.8-5.7) 1.66 (1.39-1.98) 245 2.5 (2.23-2.9) 1.93 (1.56-2.39) >40 y 543 11.2 (10.3-12.1) 3.13 (2.57-3.81) 230 6.8 (6.0-7.7) 4.2 (3.26-5.42) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Marital status (married/partnered)
No 368 3.3 (3.0-3.7) 1 303 1.6 (1.43-1.8) 1 Yes 896 6.6 (6.17-7.0) 0.97 (0.83-1.12) 368 2.9 (2.65-3.2) 1.22 (1.03-1.45)
Socio-economic status
Education level
High school 697 5.2 (4.8-5.6) 1 303 2.3 (2.05-2.57) 1 Diploma 276 4.8 (4.25-5.4) 1.0 (0.86-1.17) 180 1.8 (1.57-2.1) 0.81 (0.66-0.99) University 331 5.4 (4.8-5.9) 0.93 (0.8-1.1) 204 2.2 (1.9-2.51) 0.87 (0.71-1.07) P-trend 0.83 0.61 0.463 0.1 Personal monthly income (baht)c ≤7000 268 3.3 (2.95-3.7) 1 217 1.6 (1.36-1.8) 1 7001-10000 229 4.0 (3.45-4.5) 1.0 (0.81-1.2) 142 1.9 (1.57-2.2) 1.1 (0.87-1.37) 10001-20000 497 6.6 (6.0-7.14) 1.1 (0.91-1.31) 196 2.7 (2.35-3.1) 0.97 (0.77-1.23) >20000 289 8.4 (7.46-9.3) 1.0 (0.8-1.25) 122 4.3 (3.54-5.0) 1.04 (0.78-1.39) P-trend <0.0001 0.527 <0.0001 0.783 BMI classification
d
Underweight 359 2.9 (2.62-3.2) 0.46 (0.27-0.78) 60 0.85 (0.63-1.15) 0.9 (0.67-1.21) (BMI <18.5) Normal 16 1.1 (0.56-1.6) 1 258 1.4 (1.2-1.5) 1 (18.5 ≤ BMI < 23) Overweight 333 5.9 (5.3-6.54) 1.66 (1.41-1.95) 129 3.9 (3.3-4.62) 2.32 (1.8-2.91) (23 ≤ BMI < 25) Obese 575 10.4 (9.6-11.2) 2.82 (2.43-3.27) 231 7.4 (6.4-8.27) 4.14 (3.4-5.05) (BMI ≥25) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Continued….
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Table 2 (continued...)
Males Females HT(n) Inci
a%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Physical activities
Weighted total sessions exercise-related physical activity (average between 2005 and 2009) 0-7 ses/we 198 6.3 (5.5-7.1) 1 162 2.2 (1.8-2.6) 1 8-14 ses/w 589 5.2 (4.8-5.6) 0.9 (0.76-1.07) 352 2.1 (1.9-2.3) 1.0 (0.82-1.23)
≥15 ses/w 380 4.4 (4.0-5.0) 0.78 (0.65-0.95) 119 2.2 (1.8-2.6) 1.0 (0.77-1.3)
P-trend <0.0001 <0.029 0.905 1.0
Underlying diseases
Diabetes mellitus
No 1251 5.0 (4.7-5.27) 1 674 2.1 (1.9-2.26) 1 Yes 55 18.3 (13.9-22.7) 1.81 (1.29-2.52) 16 10.7 (5.7-15.8) 2.54 (1.41-4.56)
High lipids
No 983 4.4 (4.13-4.67) 1 576 1.9 (1.76-2.1) 1 Yes 323 10.8 (9.7-11.9) 1.4 (1.21-1.63) 114 5.4 (4.47-6.4) 1.38 (1.09-1.75)
Kidney diseases
No 1248 5.1 (4.8-5.33) 1 659 2.1 (1.9-2.26) 1 Yes 58 9.6 (7.2-11.9) 1.62 (1.19-2.2) 31 3.9 (2.52-5.2) 1.77 (1.19-2.63)
Personal behaviours
Smoking status Never 506 4.1 (3.7-4.4) 1 617 2.1 (1.9-2.23) 1 Ex-smoker 496 6.6 (6.0-7.13) 1.15 (1.0-1.32) 37 3.2 (2.2-4.22) 1.28 (0.88-1.87) Cur-smokerf 276 5.8 (5.16-6.5) 1.22 (1.04-1.44) 8 3.1 (0.97-5.2) 1.54 (0.74-3.2) P-trend <0.0001 <0.032 <0.008 0.23
Drinking status
Never 105 3.9 (3.2-4.7) 1 268 2.1 (1.8-2.32) 1 Ex-drinker 155 5.7 (4.85-6.6) 0.98 (0.74-1.3) 50 2.2 (1.6-2.85) 1.14 (0.82-1.59) Occ- drinkerg 821 4.8 (4.45-5.1) 1.0 (0.8-1.25) 352 2.1 (1.9-2.36) 1.14 (0.96-1.36) Reg- drinker
h 212 8.6 (7.5-9.7) 1.61 (1.23-2.11) 9 4.7 (1.66-7.7) 1.83 (0.86-3.9)
P-trend <0.0001 <0.0001 0.471 0.254
Food consumption habit
Instant food
<1 time/m 335 6.3 (5.6-6.95) 1 168 2.8(2.36-3.2) 1 1-3 times/m 485 5.1 (4.63-5.5) 1.02 (0.87-1.19) 273 2.1 (1.84-2.3) 0.95 (0.77-1.17) 1-2 times/wk 280 4.4 (3.9-4.9) 1.09 (0.91-1.30) 153 1.9 (1.6-2.2) 1.04 (0.81-1.33) 3-6 times/wk 165 5.0 (4.3-5.75) 1.36 (1.1-1.69) 73 1.8 (1.4-2.22) 1.14 (0.84-1.55) ≥1 times/d 32 5.5 (3.6-7.33) 1.67 (1.11-2.52) 18 2.3 (1.3-3.35) 1.48 (0.86-2.53) P-trend <0.002 <0.008 <0.002 0.403 aIncidence of Hypertension
bAdjusted relative risks were calculated from multi-variate logistic regression models of hypertension adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification eSessions / week fCurrent-smoker gOccasional drinker hRegular drinker
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65. Kaplan MS, Huguet N, Feeny DH, et al. Self-reported hypertension prevalence and income among older
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74. Haapanen N, Miilunpalo S, Vuori I, et al. Association of leisure time physical activity with the risk of
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75. Park J, Lee JS, Jang YA, et al. A comparison of food and nutrient intake between instant noodle
consumers and non-instant noodle consumers in Korean adults. Nutr Res Pract 2011;5(5):443-9.
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76. Jolly SE, Eilat-Adar S, Wang H, et al. Sex-specific associations of nutrition with hypertension and
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Health 2011;70(3):254-65.
77. Alonso A, Beunza JJ, Bes-Rastrollo M, et al. Vegetable protein and fiber from cereal are inversely
associated with the risk of hypertension in a Spanish cohort. Arch Med Res 2006;37(6):778-86.
78. Steffen LM, Kroenke CH, Yu X, et al. Associations of plant food, dairy product, and meat intakes with
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79. Schulze MB, Hoffmann K, Kroke A, et al. Risk of hypertension among women in the EPIC-Potsdam
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84. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the
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85. He FJ, MacGregor GA. Reducing population salt intake worldwide: from evidence to implementation.
Prog Cardiovasc Dis 2010;52(5):363-82.
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86. Lim LL, Seubsman SA, Sleigh A. Validity of self-reported weight, height, and body mass index among
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Experience of a Large Cohort Study in Thailand. KKU Jph Research 2011;4(3):73-87.
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STROBE 2007 (v4) Statement—Checklist of items that should be included in reports of cohort studies
Section/Topic Item
# Recommendation Reported on page #
Title and abstract 1 (a) Indicate the study’s design with a commonly used term in the title or the abstract 1
(b) Provide in the abstract an informative and balanced summary of what was done and what was found 2
Introduction
Background/rationale 2 Explain the scientific background and rationale for the investigation being reported 4
Objectives 3 State specific objectives, including any prespecified hypotheses 4
Methods
Study design 4 Present key elements of study design early in the paper 4
Setting 5 Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data
collection
5
Participants 6 (a) Give the eligibility criteria, and the sources and methods of selection of participants. Describe methods of follow-up 4
(b) For matched studies, give matching criteria and number of exposed and unexposed 5-6
Variables 7 Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if
applicable
5
Data sources/
measurement
8* For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe
comparability of assessment methods if there is more than one group
5-6
Bias 9 Describe any efforts to address potential sources of bias 12
Study size 10 Explain how the study size was arrived at 4-5
Quantitative variables 11 Explain how quantitative variables were handled in the analyses. If applicable, describe which groupings were chosen and
why
5-6
Statistical methods 12 (a) Describe all statistical methods, including those used to control for confounding 6
(b) Describe any methods used to examine subgroups and interactions 6
(c) Explain how missing data were addressed 12
(d) If applicable, explain how loss to follow-up was addressed 12
(e) Describe any sensitivity analyses 12
Results 7
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Participants 13* (a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, examined for eligibility, confirmed
eligible, included in the study, completing follow-up, and analysed
4-5
(b) Give reasons for non-participation at each stage 12
(c) Consider use of a flow diagram
Descriptive data 14* (a) Give characteristics of study participants (eg demographic, clinical, social) and information on exposures and potential
confounders
5
(b) Indicate number of participants with missing data for each variable of interest
(c) Summarise follow-up time (eg, average and total amount) 5
Outcome data 15* Report numbers of outcome events or summary measures over time 7
Main results 16 (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (eg, 95% confidence
interval). Make clear which confounders were adjusted for and why they were included
7
(b) Report category boundaries when continuous variables were categorized 7
(c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period 7
Other analyses 17 Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses 7
Discussion
Key results 18 Summarise key results with reference to study objectives 7
Limitations
Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from
similar studies, and other relevant evidence
12
Generalisability 21 Discuss the generalisability (external validity) of the study results 12
Other information
Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on
which the present article is based
13
*Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed groups in cohort and cross-sectional studies.
Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent reporting. The STROBE
checklist is best used in conjunction with this article (freely available on the Web sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at
http://www.annals.org/, and Epidemiology at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org.
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1
Health-risk factors and the incidence of hypertension: 4-year prospective
findings from a national cohort of 60,569 Thai open university students
Prasutr Thawornchaisit, MD, MPH1, Ferdinandus de Looze, MBBS, MSc
1, Christopher M
Reid, PhD2, Sam-ang Seubsman, PhD
3, Adrian Sleigh, MD
4, Thai Cohort Study Team*
1 School of Medicine, University of Queensland, Brisbane, Australia; 2 School of Public Health and
Preventive Medicine, Monash University, Melbourne, Australia; 3 School of Human Ecology, Sukhothai
Thammathirat Open University, Nonthaburi, Thailand; 4 National Centre for Epidemiology and Population
Health, ANU College of Medicine, Biology and Environment, The Australian National University,
Canberra, Australia.
*Thai Cohort Study Team
Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan
Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita
Sripaiboonkij, Pathumvadee Somsamai, Prasutr Thawornchaisit, Duangkae Vilainerun, Wanee
Wimonwattanaphan, Cha-aim Pachanee, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa, Wimalin
Rimpeekool.
Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora,
Jane Dixon, Sharon Friel, David Harley, Matthew Kelly, Tord Kjellstrom, Lynette Lim, Anthony
McMichael, Tanya Mark, Adrian Sleigh, Lyndall Strazdins, Vasoontara Yiengprugsawan, Susan Jordan,
Janneke Berecki-Gisolf, Rod McClure.
Corresponding author: Prasutr Thawornchaisit, School of Medicine, Faculty of Health Science, University
of Queensland, 288 Herston Road, Herston, Brisbane, QLD, 4006 Australia and Department of Medicine,
Lerdsin Hospital, Silom road, Bangrak, Bangkok, 10500 Thailand
Email: [email protected] Telephone: 66-2-585-8015 Fax: 66-2-912-0121
Running Head: Health-risks and hypertension incidence in the Thai Cohort Study
Key words: hypertension; health-risks; cohort; Thailand; socioeconomic; body mass index; smoking;
drinking; physical activity
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Abstract
Objective
This study evaluates the impact of a number of demographic, biological, behavioural and lifestyle health-
risk factors on the incidence of hypertension in Thailand over a 4-year period.
Design
A 4-year prospective study of health-risk factors and their effects on incidence of hypertension in a national
Thai Cohort Study (TCS) from 2005 to 2009.
Setting
As Thailand is transitioning from a developing to a middle-income developed country, chronic diseases
(particularly cardiovascular disease) have emerged as major health issues. Hypertension is a major risk
factor for heart attack and stroke and cross-sectional studies have indicated that the prevalence is increasing.
Study participants
Sukhothai Thammathirat Open University students (57,558) who participated in both 2005 and 2009
questionnaire surveys and who were normotensive in 2005 were included in the analysis.
Measures
Adjusted relative risks associating each risk factor and incidence of hypertension by sex, after controlling for
confounders such as age, socio-economic status, body mass index (BMI) and underlying diseases.
Results
The overall 4-year incidence of hypertension was 3.5% with the rate in men being remarkably higher than
that in women (5.2% vs. 2.1%). In both sexes, hypertension associated with age, higher BMI and co-
morbidities but did not associate with income and education. In men, hypertension associated with physical
inactivity, smoking, alcohol and fast food intake. In women, hypertension was related to having a partner.
Conclusion
In both men and women, hypertension strongly associated with age, obesity and co-morbidities while it had
no association with socio-economic factors. The cohort pattern of socio-economy and hypertension reflects
that the health-risk transition in Thais is likely to be at the middle stage. Diet and lifestyle factors associate
with incidence of hypertension in Thais and may be amenable targets for hypertension control programs.
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Introduction
Hypertension is a global public health challenge due to its high prevalence and the concomitant increase in
risk of stroke and cardiovascular diseases [1]. In 2007, according to the World Health Organization,
cardiovascular diseases caused 33.7% of all deaths in the world, while other chronic diseases were
responsible for 26.5% [2]. Globally, it has been estimated that premature deaths resulting from hypertension
annually are approximately 7.1 million, which account for 64 million disability-adjusted life years (DALYs)
[3]. Hypertension ranks third, after underweight and unsafe sex, of the six major risk factors contributing to
the global disease burden[3].
In Thailand, hypertension is the third major risk factor of the national disease burden and causes 600,000
DALY losses per year [4]. The prevalence of hypertension has been estimated from a number of cross-
sectional studies among adults and has been increasing over the past decades from 20% in 1985 to 22% in
2004 [4-6]. The recent Thai National Health Examination Survey (NHES) in 2009 reported the prevalence
of hypertension in adults was 21.5% [4]. The emergence of hypertension has been the result of rapid
socioeconomic development which has resulted in one third of Thais migrating to urban areas [7].
Urbanization has been associated with increased obesity as people become less physically active and adopt
sedentary life styles such as spending more time watching television, sitting and using computers [7 8].
These factors, in association with Thailand’s ageing population [9], underpin the importance of controlling
hypertension in Thai community.
There is limited longitudinal research on health risk factors and hypertension incidence in Thailand [5]. The
purpose of this study is to address that knowledge gap. We prospectively examined the influence of an array
of demographic, biological, health-risk and lifestyle factors on the incidence of hypertension during 4 years
of follow-up in the Thai Cohort Study (TCS). This is part of an overarching research program investigating
Thailand’s health-risk transition from high maternal-child and infectious disease mortality to increased
longevity and emerging chronic disease. The large national cohort of Thai adults reported on here has been
followed since 2005 for risk factor trends and associated disease outcomes, including hypertension.
Methods
Study population
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The first TCS evaluation was conducted in 2005 by sending twenty-page health-risk questionnaires to the
200,000 distance-learning students enrolled at Sukhothai Thammathirat Open University (STOU) and
87,134 (44%) completed them and signed the consent forms. The cohort has been shown to be socio-
economically and geographically well representative of the Thai population [10]. However, they resided
more in urban areas (51.8% vs. 31.1%) and on average were younger and better educated than most Thai
adults. Details of the baseline TCS methodology have been published [10]. This questionnaire focused on
socio-demography, habitation, work, health services, injury, sedentary habits, physical activity, transport,
underlying diseases, family history, personal behaviours, BMI, and consumption of foods, vegetables and
fruit. A 4-year follow-up was carried out in 2009. Overall, 71% of the cohort responded generating
longitudinal data on 60,569 participants.
Incident Hypertension
Incident hypertension was defined as being normotensive in 2005 and self-reported doctor diagnosed
hypertensive in 2009. Hypertension incidence was the dependent variable for analyses and the person-time
denominator included all individuals at risk - those 57,558 participants with longitudinal data who were
negative for hypertension in 2005, the beginning of the 4-year study period.
Risk Factors
Hypertension was defined as describe above, underlying diseases diagnosed by a doctor were also reported
including diabetes mellitus, kidney diseases and high blood lipids, all of which were investigated as
potential risk factors of hypertension.
Participants were grouped into three age categories: younger (≤30 years), middle (31-40 years) and older
(>40 years). Urbanization status was based on rural (R) or urban (U) residence, first when aged 10-12 years
old and again in 2005, producing four groups: lifelong ruralites (RR), urbanizers (RU), de-urbanizers (UR)
and urbanites (UU). Personal monthly income was divided into four categories: ≤7000 baht, 7001–10000
baht, 10001–20000 baht and >20000 baht. In 2005, one US dollar was equivalent to 42 baht so most of
participants had quite low incomes. Household assets were classified into three categories: low (≤30,000
baht), medium (30,001-60,000) and high (>60,000).
Sedentariness was assessed by screen time (hours/day spent before TV or computer) and (separately) sitting
time (hours per day spent sitting for any purpose). Incidental exercise was measured by the frequency of
housework or gardening, categorised as follows: ≤3 times per month; 1-2 times per week; 3-4 times per
week; most days. We measured planned physical activity at the beginning and end of the 4-year follow up
period. On both occasions, cohort members reported the number of sessions per week of strenuous and
moderate exercise for at least 20 minutes, and of walking for at least 10 minutes. We derived an “overall
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measure” of planned physical activity calculated separately at both the baseline and the 4-year mark. The
measure was weighted as follows: (2 × strenuous + 1 × moderate + 1 × walking) sessions per week. This
weighting system is based on the recommendation of the International Physical Activity Questionnaire and
the Active Australia Survey as used in other analyses of the cohort data [11 12]. Finally, for each individual,
the “overall measures” of weekly exercise for 2005 and 2009 were added and then averaged by dividing by
2, creating a longitudinal measure of planned physical activity (LPPA).
Smoking status was classified as never, ex-smoker or current-smoker. Alcohol exposure was classified by
four categories: never, ex-drinker, occasional drinker or current-drinker. Body mass index (BMI) was
calculated from self-reported height in metres and weight in kilograms as weight/ height2. It was divided
into 4 categories as follows according to Asian criteria:[8 13-16] underweight (BMI <18.5), normal (18.5 to
<23), overweight (23 to <25), and obese (≥25). Foods that could potentially influence blood pressure (deep
fried, instant, roast or smoked, soybean products and soft drinks) were assessed for consumption frequency
based on a five-point Likert scale ranging from less than once a month to once or more a day. Western-style
fast food exposure was noted on a three-point scale from less than once to more than 3 times per month.
Fruit and vegetable consumption were recorded as standard serves eaten per day.
Statistical analyses
All analyses were performed using SPSS software. The incidence of hypertension and its 95% confidence
interval (CI) were calculated for each value of each categorical variable in both male and female participants
and the influence on incidence by each variable was evaluated by Chi-square test. For statistical inference,
all p-values were 2 tailed and significance was set at 5%.
Relative risks in a large study of an uncommon disease (incidence less than 10%) can be accurately
estimated as odds ratios [17]. Accordingly, for each risk variable, the relative risk (RR) and 95% confidence
interval were estimated using logistic regression to calculate the bivariate odds ratio for hypertension.
Adjusted RRs (ARRs) were estimated by calculating multivariate logistic regression odds ratios. The ARRs
were controlled for confounding by age, marital status, SES, BMI, underlying diseases and personal
behaviours (cigarette smoking and alcohol drinking). A variable was included in a multivariate model if
bivariate analysis had indicated a statistically significant association with incidence of hypertension. Some
variables were included because earlier analyses reported elsewhere had shown significant or substantial
association with hypertension.
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Results
Baseline Characteristics
Baseline characteristics of study participants self-reporting as normotensive in 2005 are shown in Table 1.
The highest proportion of males resided in the North-eastern regions and Central while there were more
females in the Central and Bangkok area. More than half of the male and female participants lived in urban
areas. Males had a higher monthly income than females whereas females had a higher educational
attainment than males. The distribution of household assets for males and females was similar and over half
of them were in medium and high categories. The study population had a similar socio-demography, income
and location of residence to the general Thai population [9].
Incidence and risks of hypertension
Overall, the incidence of self-reported doctor diagnosed hypertension in Thai adults was 3.5% and the rate in
males was more than twice the corresponding rate in females (5.2% vs. 2.1%) (Table 2). In males and
females, the incidence of hypertension increased with increasing age. Marriage or having a partner increased
the risk of hypertension in females. Educational attainment and personal income had no influence on the risk
of hypertension in both sexes. In males and females, the risk of hypertension increased with an increased
BMI. The risk of hypertension increased in both males and females who had diabetes mellitus, high blood
lipids and kidney disease.
Planned physical activity, averaged across the 4-year follow-up, was protective for males. In males, higher
LPPA (≥15 sessions / week) associated with lower incidence of hypertension; however, this association was
not observed in females. In males, current-smokers had a higher risk of developing hypertension than non-
smokers while in females, smoking (which was rare) had no apparent hypertension effect. Among males,
regular drinking increased the risk of incident hypertension; patterns of relative risks were similar for
females but, reflecting the low frequency of female drinking, did not reach statistical significance. In males,
the risk of hypertension had a direct association with the frequency of instant food consumption but there
was no such association detected in females.
Over the 4-year period, a variety of factors that could have been risks for hypertension over a longer period
were found not to be in this setting (data not shown). Thus, in males and females, factors that had no
statistically significant influence on the longitudinal 4-year risk of hypertension incidence were urbanization
status, sedentary lifestyle (non-participation in housework or gardening, high levels of television and
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computer watching, sleeping and sitting time), and food consumption habits that included deep fried food
and soft drink, Western fast food, roasted or smoked foods, fruit, vegetables and soybean products.
Discussion
This large prospective cohort of adults living all over Thailand has been the first studied nationally for a
wide array of risk factors for incident hypertension. Overall, the 4-year incidence of hypertension from 2005
to 2009 was 3.5%, notably higher in men (5.2%) than women (2.1%). These incidence rates are drawn from
a cohort of adults on average younger than the Thai population so they are lower than corresponding rates of
the adult population. However, our focus was on risk factor associations and we noted that in both sexes,
age, obesity and co-morbidity with diabetes mellitus, high blood lipids and kidney disease strongly linked to
hypertension incidence. In males, instant food consumption, smoking and drinking alcohol had a moderate
adverse effect on hypertension risk while planned physical activity averaging 15 or more sessions per week
over the 4-year period was substantially protective, reducing risk by 22%. In females, having a partner was
associated with increased incidence of hypertension.
The age effect on hypertension incidence noted by us is consistent with findings of many other population
studies of hypertension including (for example) a longitudinal study in China [18] and cross-sectional
studies in Korea [19], Taiwan [20] and the USA [21]. The noticeably greater incidence of hypertension in
men than in women that we found in Thailand has also been reported from many parts of the world
including neighboring Malaysia [22] and distant USA [23] as well as in a recent global review [24].
In men and women of the Thai cohort, a progressively higher BMI was positively and significantly
associated with an increased risk of incident hypertension over the four years. This finding was similar to
that reported for other prospective cohorts in the US [25 26] and Finland [27]. Furthermore, after seven
years of follow-up in a US study [28], weight loss associated with a decreased risk of incident hypertension.
Among a cohort of US nurses, the risk of incident hypertension increased 5% for each kilogram addition of
weight [29], and rose 15% for each increase of one kilogram per meter squared of BMI in Taiwan [18].
These results suggest that the adverse effect of high BMI on risk of hypertension does not depend on
ethinicity or sex and is wide spread around the world.
In our cohort, we noted a higher risk of incident hypertension among those with diabetes mellitus. This
finding is consistent with longitudinal studies in the USA [30 31] which reported, respectively, that in
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diabetic patients the risk of incident hypertension rose 41% and 56%. As well, prospective cohort studies in
Japan showed that diabetes [32] and impaired fasting blood glucose [33] were associated with an increased
risk of hypertension incidence. Supportive cross-sectional results emerge from studies in diverse settings
including Bangladesh and India [34], Barbados [35] and the USA [36]. A causal association between
diabetes and hypertension is suspected and possible mechanisms include endothelial dysfunction and
vascular inflammation [37].
We also found that high blood lipids significantly associated with increased risk of incident hypertension.
This supports the results from previous longitudinal studies in men [38 39], women [40 41] and adults [32
42]. The risk of hypertension increased with an increase of total cholesterol, triglyceride and low-density
lipoprotein cholesterol levels and decreased with an increase of high-density lipoprotein cholesterol level
[38 39 41].The mechanism by which dyslipidemia provokes high blood pressure is uncertain but could
reflect blood vessel dysfunction and damaged endothelium [43].
Among men and women in the Thai cohort, kidney disease strongly associated with increased risk of
hypertension incidence. This result is supported by cross-sectional data from the Czech Republic [44] and
the USA [45]. Microalbuminuria is a biomarker for renal hypertension [45] and the risk increases with the
severity of kidney impairment, revealed by the serum creatinine level [46 47]. Glomerular damage is more
closely connected with hypertension than tubular or interstitial disease [47]. Chronic kidney disease
facilitates high blood pressure through many mechanisms including excessive intravascular volume,
increased endothelin production, activity of the renin-angiotensin system and sympathetic nervous system
and decreased vasodilators, nitric oxide production and endothelial function [48].
In men we found that cigarette smoking modestly associated with increased risk of developing hypertension.
This is consistent with many other reports such as longitudinal studies in Japanese [49] and American men
[50] and a cross-sectional study in French men [51]. In addition, smoking Jordanian men had significantly
higher blood pressure than their non-smoking counterparts [52]. However, we also found that smoking had
no influence on hypertension in women. This result for females is compatible with reports from Korea [53]
and the USA [54]. In contrast, one prospective US study [55] reported an increased risk of hypertension
among women. It is likely that men tend to smoke more cigarettes than women and the impact of smoking
on hypertension is a dose-response pattern [49].
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Regular alcohol consumption was strongly associated with an increased risk of incident hypertension. This
result is supported by findings from longitudinal studies in Japan [56 57] and cross-sectional studies in
China [58] and Korea [53]. In Japanese men, alcohol consumption, particularly more than 200 grams per
week, significantly associated with an increase of blood pressure [59] while alcohol restriction was
noticeably related to a decreased blood pressure [60]. Among our Thai cohort females, alcohol consumption
had no influence on hypertension; this result was supported by longitudinal data from Japan [56] and cross-
sectional data from Korea [53]. In American females the quantity of alcohol consumption per week had no
effect on systolic blood pressure [61]. Males usually consume more alcohol than females and the
hypertension risk is linear with a dose-response [57 62]. Our results suggest that regular alcohol
consumption, especially in males, is a significant risk factor for developing hypertension.
Marital status in males had no influence on hypertension which supports the results from more economically
developed countries, the USA and Korea [53 54]. However, in middle-income countries, results diverged:
married males, when compared to their unmarried counterparts, had a higher risk of hypertension in
Barbados [35] and a lower risk in Poland [63]. Married individuals had higher emotional and social support
and lower stress [63] and probability of being a current smoker [64]. We found incident hypertension was
relatively more frequent among married females, unlike the no-sex-link pattern in developed countries such
as the USA and Korea [53 54]. The influence of marrital status on hypertension varies from one country to
another partly due to the difference of socio-economy and social structure. For our Thai cohort, the pattern
in males was similar to that in developed countries and in females it was not.
Participant educational attainment did not protect against hypertension in either sex. This is similar to cross-
sectional results for adults in developed countries such as Korea [53] and the USA [36 65]. However, other
studies in various settings and with diverse designs reported educational attainment protects against
hypertension in adults; these included a prospective cohort in middle income Thailand [5], a cross-sectional
study in middle income Malaysia [22] and a cross-sectional study in high income USA [66]. Opposite
findings have been reported in low income settings in both Bangladesh and India [34] where education was
a risk factor for hypertension. Taken collectively, these data suggest that income modifies the protective
education-hypertension effect, inverting it when income is low and boosting it when income is high. Our
middle-income cohort could be in the middle stage of this education effect as it is transiting from the low-
income pattern.
We found no association of personal monthly income and risk of hypertension in either sex. This was similar
to previous studies in middle-income countries including a longitudinal study in Thailand [5] and a cross-
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sectional study in Malaysia [67] and in cross-sectional studies in high-income countries such as in Korea
[53] and the USA [21 36]. However, income had an inverse association with risk of hypertension in another
study in high-income developed USA [65] while it had a direct positive association in a low-income
developing country such as Ghana [68]. The health transition in Thailand is likely to be in the middle stage,
according to the concept of a tipping-point for the relationship between socio-economic status, body size and
hypertension risk factors. High income, large body size and high blood pressure is seen in developing
countries while in developed countries, high income is a protective factor [69-71]. Indeed, the Thai cohort
has already been shown to be at a transitional point for metabolic states because females had an inverse
relationship between income and BMI whereas males had a positive (traditional) relationship [14]. However,
now all Thais benefit from universal health insurance so access to health care is not dependent on their
income making disease associations more attenuated than before [72].
In our cohort, the reduction of risk of hypertension incidence in males but not in females by a longitudinal
measure of planned physical activity was comparable with a prospective study in middle-aged American
whites [73]. In another prospective study in middle-aged Finns, the hypertension protective effect of
vigorous physical activity or total energy expenditure from leisure-time physical activity was observed in
men but not in women [74]. Such physical activity among females may be less intense than among males so
having no effect on the risk of hypertension [74]. A large Finnish study [27] demonstrated that the incidence
of hypertension was inversely associated with the level of physical activity in a dose-response manner in
both sexes. In our cohort study, risk of obesity had an inverse association with quantified planned exercise in
both sexes but the effect was significantly stronger in males than in females [12]. As Thailand progresses
economically, more people migrate to urban areas and decrease their physical activity [7]. Therefore, in the
future, Thai people may encounter a higher risk of hypertension due to lower physical activity.
Instant food - mostly combinations of carbohydrate, fat and salt - associated with an increased risk of
incident hypertension in males but not in females. Other foods, fruit and vegetables had no impact on
hypertension. This may be as a result of men comsuming more instant foods, such as instant noodles, than
women. For example a study in Korea showed daily instant food consumption in men and women averages
23.7 and 11.1 grams respectively [75]. Other studies showed that carbohydrate [76 77], fruit and vegetables
[78 79] consumption had no influence on hypertension. Most students consume instant noodles which lead
to a higher intake of energy, fat, sodium, thiamine, and riboflavin but a lower intake of protein, calcium,
phosphorus [75]. Koreans favoring instant noodles have a salt intake more than three times higher than the
daily recommendation (6.4 vs 2 gms) [75]. Daily salt intake of Thai people has been reported to average
10.8gm, much higher than recommended (http://www.worldactiononsalt.com/worldaction/asia/75286.pdf).
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High salt consumption was associated with an increased risk of developing hypertension in a longitudinal
study in Taiwan [80] and in cross-sectional studies in China [81 82] while other reports revealed that salt
restriction reduced blood pressure [83 84]. High plasma sodium, due to an overwhelmed capacity of the
kidney to excrete excessive salt consumption, leads to excessive intravascular volume and high blood
pressure [85]. It seems reasonable to conclude that high salt intake from instant noodle consumption is likely
to be causally associated with an increased risk of hypertension in Thailand.
In Thailand, this cohort constitutes the largest study to examine the longitudinal effect of most health-risk
factors on hypertension development. Indeed, the study size along with its national representation are
important factors to consider when exploring study validity and utility. The influence of risk factors on
hypertension incidence is clearly demonstrated since this is a large longitudinal cohort covering a 4-year
period. As a consequence, independent risk factors for hypertension in Thais can be identified. Our cohort
represents Thais well in geography and socio-economy [9] so the results reflect the trend of health-risk
transition in the Thai population. STOU students, with better education attainment combined with their
representative incomes and geographical location, represent the next generation of Thais. It would be
reasonable to conclude that our 4-year follow-up results show the future hypertension trends for Thai people.
However, our study has limitations since it relied on self-report. There may be recall and reporting errors.
But a study of the validity of self-reported weight and height in our cohort found that the accuracy was quite
acceptable [86]. As well, we have conducted a validation study of self-reported hypertension (Prasutr
Thawornchaisit, 2013, under review) in a random age-sex matched sub-sample of the cohort reporting
hypertension (n=240) or no hypertension (n=240). We found the sensitivity was high (82%) and that
negative reports were usually accurate (86%). In another report, we investigated the impact of the non-
responses to the 2009 follow up and found small effects with under-representation of young urban males;
this missing group would have a minor influence on our results and would not be expected to have a high
rate of incident hypertension [87]. The findings from our study are likely to be robust and represent well the
future trends of health-risk transition in middle-income Thailand and similar ASEAN countries.
In conclusion, older age, obesity and underlying morbidity due to diabetes, high blood lipids and kidney
disease were strongly associated with an increased risk of incident hypertension in both sexes while in
males, physical inactivity, instant food consumption, smoking and drinking alcohol had a moderate
association. Sedentary life-style and the consumption of friut and vegetables had no detected influence on
hypertension. The Thai health-risk transition is likely to be in the middle stage since education attainment
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and income still have no net effect to protect against hypertension. Prevention should focus on obesity, high
blood lipids and high salt consumption and people should be encouraged to increase physical exercise and
consume low fat foods and less instant foods. A reduction of salt consumption should be a national policy by
limiting the concentration of salt in ready-to-eat and industrial foods. Smoking and drinking cessation
should be promoted for males. Thailand should also consciously aim to take advantage of its culturally
determined low rate of drinking and smoking among women and ensure that there is no attempt to
undermine this situation and develop the market for the female half of the population.
Acknowledgements and Funding
This study was supported by the International Collaborative Research Grants Scheme with joint grants from
the Wellcome Trust UK (GR071587MA) and the Australian National Health and Medical Research Council
(268055), and by a global health grant from the NHMRC (585426). We thank the staff at Sukhothai
Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are
participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen
University for guiding us successfully through the complex data processing.
Conflict of interest
The authors declare that there are no conflicts of interest.
Ethical considerations
Ethical approval was obtained from Sukhothai Thammathirat Open University Research and Development
Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee
(protocol 2004344 and 2009570). Informed, written consent was obtained from all participants.
Author contribution: PT devised the hypertension study, analysed the data and wrote the paper. FDL and
CR assisted with planning the study, analysis and interpretation. AS and SS conceived and developed the
cohort, and helped plan, analyse and interpret this study. All authors approved the final manuscript.
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Table 1 Characteristics of 57558 participants normotensive at the 2005 Thai Cohort Study baseline
Male Female Difference
Factor
n % n % P-valuea
Demographic data
Participants 25320 44.0 32238 56.0 <0.0001 Age (y) mean 32.9 30.0 <0.0001
b
(SD) (8.5) (7.6) Age group <0.0001 ≤30 y 11293 44.6 19229 59.6 31-40 y 9177 36.2 9638 29.9 >40 y 4850 19.2 3371 10.5 Married/partnered <0.0001 No 11060 44.8 18851 60.1 Yes 13607 55.2 12513 39.9 Regions <0.0001 Bangkok 3488 13.9 6057 18.9 Central 5712 22.7 8267 25.8 North 5180 20.6 5818 18.1 North-east 6142 24.4 5760 18.0 East 1515 6.0 1922 6.0 South 3103 12.3 4254 13.3 Urbanization status
c <0.0001
Rural–rural (RR) 11512 46,1 13895 43.6 Rural–urban (RU) 7891 31.6 9546 29.9 Urban–rural (UR) 1078 4.3 1344 4.2 Urban–urban (UU) 4495 18.0 7114 22.3
Socioeconomic status
Education level <0.0001 High school 13366 52.9 13135 40.8 Diploma 5749 22.8 9791 30.4 University 6153 24.4 9231 28.7 Personal monthly income (baht)
d <0.0001 ≤7000 8025 32.3 13890 44.1 7001–10000 5790 23.3 7578 24.0 10001–20000 7549 30.4 7194 22.8 >20000 3452 13.9 2851 9.0 Household assets
e (baht)
d <0.001
Low 9852 39.1 12434 38.7 Medium 8065 32.0 9863 30.7 High 7298 28.9 9804 30.5 aChi-square test
b unpaired t test cLocation of residence (rural, R, or urban, U) at age 10-12 years and again in 2005. dAt the time of the survey in 2009, US$1 = 31 Thai baht eReplacement value in Thai baht: low ≤30,000, medium 30,001-60,000 and high >60,000
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Table 2 Hypertension incidence and association with risk factors in male and female participants.
Males Females HT(n) Incia%(95% CI) ARRb(95% CI) HT(n) Incia%(95% CI) ARRb(95% CI) Participants 1306 5.2 (4.9-5.4) 690 2.1 (2.0-2.3)
Demography
Age group
≤30 y 279 2.5 (2.2-2.8) 1 215 1.1 (0.97-1.3) 1 31-40 y 484 5.3 (4.8-5.7) 1.66 (1.39-1.98) 245 2.5 (2.23-2.9) 1.93 (1.56-2.39) >40 y 543 11.2 (10.3-12.1) 3.13 (2.57-3.81) 230 6.8 (6.0-7.7) 4.2 (3.26-5.42) P-trend <0.0001 <0.0001 <0.0001 <0.0001
Marital status (married/partnered)
No 368 3.3 (3.0-3.7) 1 303 1.6 (1.43-1.8) 1 Yes 896 6.6 (6.17-7.0) 0.97 (0.83-1.12) 368 2.9 (2.65-3.2) 1.22 (1.03-1.45)
Socio-economic status
Education level
High school 697 5.2 (4.8-5.6) 1 303 2.3 (2.05-2.57) 1 Diploma 276 4.8 (4.25-5.4) 1.0 (0.86-1.17) 180 1.8 (1.57-2.1) 0.81 (0.66-0.99) University 331 5.4 (4.8-5.9) 0.93 (0.8-1.1) 204 2.2 (1.9-2.51) 0.87 (0.71-1.07) P-trend 0.83 0.61 0.463 0.1 Personal monthly income (baht)c ≤7000 268 3.3 (2.95-3.7) 1 217 1.6 (1.36-1.8) 1 7001-10000 229 4.0 (3.45-4.5) 1.0 (0.81-1.2) 142 1.9 (1.57-2.2) 1.1 (0.87-1.37) 10001-20000 497 6.6 (6.0-7.14) 1.1 (0.91-1.31) 196 2.7 (2.35-3.1) 0.97 (0.77-1.23) >20000 289 8.4 (7.46-9.3) 1.0 (0.8-1.25) 122 4.3 (3.54-5.0) 1.04 (0.78-1.39) P-trend <0.0001 0.527 <0.0001 0.783
BMI classificationd
Underweight 359 2.9 (2.62-3.2) 0.46 (0.27-0.78) 60 0.85 (0.63-1.15) 0.9 (0.67-1.21) (BMI <18.5) Normal 16 1.1 (0.56-1.6) 1 258 1.4 (1.2-1.5) 1 (18.5 ≤ BMI < 23) Overweight 333 5.9 (5.3-6.54) 1.66 (1.41-1.95) 129 3.9 (3.3-4.62) 2.32 (1.8-2.91) (23 ≤ BMI < 25) Obese 575 10.4 (9.6-11.2) 2.82 (2.43-3.27) 231 7.4 (6.4-8.27) 4.14 (3.4-5.05) (BMI ≥25) P-trend <0.0001 <0.0001 <0.0001 <0.0001
aIncidence of Hypertension
bAdjusted relative risks (ARR) were calculated from multi-variate logistic regression models of hypertension
adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification
Continued….
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Table 2 (continued...)
Males Females HT(n) Inci
a%(95% CI) ARR
b(95% CI) HT(n) Inci
a%(95% CI) ARR
b(95% CI)
Physical activities
Weighted total sessions exercise-related physical activity (average between 2005 and 2009) 0-7 ses/wc 198 6.3 (5.5-7.1) 1 162 2.2 (1.8-2.6) 1 8-14 ses/w 589 5.2 (4.8-5.6) 0.9 (0.76-1.07) 352 2.1 (1.9-2.3) 1.0 (0.82-1.23)
≥15 ses/w 380 4.4 (4.0-5.0) 0.78 (0.65-0.95) 119 2.2 (1.8-2.6) 1.0 (0.77-1.3)
P-trend <0.0001 <0.029 0.905 1.0
Underlying diseases
Diabetes mellitus
No 1251 5.0 (4.7-5.27) 1 674 2.1 (1.9-2.26) 1 Yes 55 18.3 (13.9-22.7) 1.81 (1.29-2.52) 16 10.7 (5.7-15.8) 2.54 (1.41-4.56)
High lipids
No 983 4.4 (4.13-4.67) 1 576 1.9 (1.76-2.1) 1 Yes 323 10.8 (9.7-11.9) 1.4 (1.21-1.63) 114 5.4 (4.47-6.4) 1.38 (1.09-1.75)
Kidney diseases
No 1248 5.1 (4.8-5.33) 1 659 2.1 (1.9-2.26) 1 Yes 58 9.6 (7.2-11.9) 1.62 (1.19-2.2) 31 3.9 (2.52-5.2) 1.77 (1.19-2.63)
Personal behaviours
Smoking status
Never 506 4.1 (3.7-4.4) 1 617 2.1 (1.9-2.23) 1 Ex-smoker 496 6.6 (6.0-7.13) 1.15 (1.0-1.32) 37 3.2 (2.2-4.22) 1.28 (0.88-1.87) Cur-smokerd 276 5.8 (5.16-6.5) 1.22 (1.04-1.44) 8 3.1 (0.97-5.2) 1.54 (0.74-3.2) P-trend <0.0001 <0.032 <0.008 0.23
Drinking status
Never 105 3.9 (3.2-4.7) 1 268 2.1 (1.8-2.32) 1 Ex-drinker 155 5.7 (4.85-6.6) 0.98 (0.74-1.3) 50 2.2 (1.6-2.85) 1.14 (0.82-1.59) Occ- drinkere 821 4.8 (4.45-5.1) 1.0 (0.8-1.25) 352 2.1 (1.9-2.36) 1.14 (0.96-1.36) Reg- drinker
f 212 8.6 (7.5-9.7) 1.61 (1.23-2.11) 9 4.7 (1.66-7.7) 1.83 (0.86-3.9)
P-trend <0.0001 <0.0001 0.471 0.254
Food consumption habit
Instant food
<1 time/m 335 6.3 (5.6-6.95) 1 168 2.8(2.36-3.2) 1 1-3 times/m 485 5.1 (4.63-5.5) 1.02 (0.87-1.19) 273 2.1 (1.84-2.3) 0.95 (0.77-1.17) 1-2 times/wk 280 4.4 (3.9-4.9) 1.09 (0.91-1.30) 153 1.9 (1.6-2.2) 1.04 (0.81-1.33) 3-6 times/wk 165 5.0 (4.3-5.75) 1.36 (1.1-1.69) 73 1.8 (1.4-2.22) 1.14 (0.84-1.55) ≥1 times/d 32 5.5 (3.6-7.33) 1.67 (1.11-2.52) 18 2.3 (1.3-3.35) 1.48 (0.86-2.53) P-trend <0.002 <0.008 <0.002 0.403 aIncidence of Hypertension
bAdjusted relative risks were calculated from multi-variate logistic regression models of hypertension adjusted for age, marital status, education, income, BMI category, underlying diseases, personal behaviours. cAt the time of the survey in 2005, US$1 = 42 Thai baht dAsian standard BMI classification eSessions / week fCurrent-smoker gOccasional drinker hRegular drinker
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