A life history perspective on size, A life history perspective on size, health and adult mortality in the health and adult mortality in the

GambiaGambia

Rebecca Sear

London School of Economics

Life history theory and demographyLife history theory and demography

• Life history is concerned with allocation of energy over the life course

• How do organisms solve problem of adaptively allocating energy between growth, body subsistence and repair, and reproduction?

• Demography a key component of LHT – both a constraint on, and an outcome of, life history decisions

Adult mortality and resource Adult mortality and resource availabilityavailability

• This study investigates relationship between adult mortality and resource availability

• Measures of resources availability

– Height

– BMI

– Haemoglobin (Hb)

HeightHeight

• Height: measure of past investment in growth

– Previous research:

• Human studies (height):

– height usually found to be negatively correlated with mortality

– but recent evidence suggests mortality higher in tall people for certain causes

• Animal studies (size):

– caloric restriction increases longevity

– costs of fast growth

BMI & HbBMI & Hb

• BMI & Hb: measures of current body condition

– Mortality should be inversely related to both BMI and Hb but

• What is the shape of the relationship?

• Does this relationship differ between women and men?

– Previous (human) studies suggest a non-linear relationship between BMI and mortality

AnalysisAnalysis

• Analysis of adult (21+ years) mortality by:– Height

– BMI

– Haemoglobin level

• Investigating both whether there is a relationship, and the shape of the relationship

• For both women and men

DataData

• Data collected 1950-74

• 4 rural villages

• Demographic data collected continuously

• Anthropometric surveys conducted (at least) annually

Adult anthropometricsAdult anthropometricsWomenWomen MenMen

Height (cm)

Mean 158 168 Range 122-178 128-197 N (measurements) 13,290 9,680BMI

Mean 20.7 20.4 Range 12-40 13-30 % underweight 16 13 % overweight 4 1 N (measurements) 11,598 9,674Hb (g/dl)

Mean 11.8 13.5 Range 2-17 3-20 % anaemic 48 33 N (measurements) 10,986 9,653

Height and ageHeight and age

154

156

158

160

162

164

166

168

20 25 30 35 40 45 50 55 60 65 70 75

Age

Hei

gh

t

Men

Women

BMI and ageBMI and age

18

18.5

19

19.5

20

20.5

21

21.5

22

20 25 30 35 40 45 50 55 60 65 70 75

Age

BM

I

Women

Men

Hb and ageHb and age

10

10.5

11

11.5

12

12.5

13

13.5

14

20 25 30 35 40 45 50 55 60 65 70 75

Age

Hb Women

Men

Adult mortalityAdult mortality

Age (years)

9080706050403020

Pro

po

rtio

n s

urv

ivin

g

1.0

.8

.6

.4

.2

0.0

Women

Men

MethodsMethods

• Discrete-time event-history analysis on probability of adult death (21+ years)

• Separate models for women and men

• Anthropometric variables entered as time dependent

• Controlled for birth cohort

ResultsResultsWomenWomen MenMen

Variable Estimate (SE)

Constant 84.3 (37.0)* 4.81 (4.64)

Age -0.02 (0.03) 0.07 (0.01)*

Height

Height2

-0.95 (0.47)*

0.003 (0.001)*

0.005 (0.01)

BMI

BMI2

-0.86 (0.27)*

0.02 (0.01)*

-0.91 (0.39)*

0.02 (0.01)*

Hb

Hb*age

-0.68 (0.14)*

0.01 (0.002)*

-0.23 (0.03)*

No. deaths 182 172

No. survivors 823 683

Height and mortalityHeight and mortality

0

0.005

0.01

0.015

0.02

0.025

135 140 145 150 155 160 165 170 175

Height (cm)

Pro

bab

ilit

y o

f d

yin

g

WomenMen

BMI and mortalityBMI and mortality

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

BMI

Pro

bab

ilit

y o

f d

yin

g

WomenMen

Hb and mortalityHb and mortality

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Haemoglobin

Pro

bab

ilit

y o

f d

yin

g

WomenMen

Conclusions IConclusions I

• Height:– matters for female mortality – though relationship not linear

– Why not for men?

• Measures of current body condition significantly correlated with mortality for both sexes– High BMI increases mortality (costs of fat storage?), but low

BMI more important

– Hb important for both sexes, especially women (maternal mortality?)

Conclusions IIConclusions II

• Clearly, there is variation in adult mortality by resource availability

• Though these relationships tend not to be linear• Current health/body condition matters for both sexes,

height only for women– growing tall has costs for women (or linked to greater

reproductive effort of tall women?), but apparently not for men?

• Influence of genetic factors?