shirreffs - 2003 - markers of hydration status
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ORIGINAL COMMUNICATION
Markers of hydration status
SM Shirreffs1*
1School of Sport and Exercise Sciences, Loughborough University, Leicestershire, UK
Many indices have been investigated to establish their potential as markers of hydration status. Body mass changes, bloodindices, urine indices and bioelectrical impedance analysis have been the most widely investigated. The current evidence andopinion tend to favour urine indices, and in particular urine osmolality, as the most promising marker available.
European Journal of Clinical Nutrition (2003) 57, Suppl 2, S6–S9. doi:10.1038/sj.ejcn.1601895
Keywords: hydration status; water balance; euhydration; hypohydration
Hydration status—some definitionsEuhydration is the state or situation of being in water
balance. However, although the dictionary definition is an
easy one, establishing the physiological definition is not so
simple. Hyperhydration is a state of being in positive water
balance (a water excess) and hypohydration the state
of being in negative water balance (a water deficit).
Dehydration is the process of losing water from the
body and rehydration the process of gaining body water.
Euhydration, however, is not a steady state, but rather is a
dynamic state in that we continually lose water from
the body and there may be a time delay before replacing it
or we may take in a slight excess and then lose this
(Greenleaf, 1992).
Water intake and lossThe routes of water loss from the body are the urinary system
via the kidney, the respiratory system via the lungs and
respiratory tract, via the skin, even when not visibly
sweating, and the gastrointestinal system as faeces or vomit.
The routes of water gain into the body are gastrointestinally
from food and drink consumption and due to metabolic
production. Many textbooks, both recent and older, state
water gain and loss figures for the average sedentary adult in
a moderate environment in the order of 2550 ml (McArdle
et al, 1996), 2600 ml (Astrand & Rodahl, 1986) and 2500 ml
(Diem, 1962). However, it is interesting to note that the
source of this data is never given.
Measurement of total body waterThe body water content of an individual can be measured or
estimated in a number of ways, but the current consensus
is that tracer methodology gives the best measure of total
body water. Deuterium oxide (D2O or 2H2O) is the most
commonly used tracer for this purpose and full details of the
methods and protocols, assumptions and limitations are well
discussed elsewhere (Schoeller, 1996). Briefly, the tracers
are distributed relatively rapidly in the body (in the order of
3–4 h for an oral dose) and correction can be made for
exchange with nonaqueous hydrogen. It is estimated that
total body water can be measured with a precision and
accuracy of 1–2%.
Assessing hydration statusHydration status has been attempted to be assessed in a
variety of situations for a number of years. In 1975, Grant
and Kubo divided the tests open to use in a clinical setting
into three categories: laboratory tests, objective noninvasive
measurements and subjective observations. The laboratory
tests were measures of serum osmolality and sodium
concentration, blood urea nitrogen, haematocrit and urine
osmolality. The objective, noninvasive measurements in-
cluded body mass, intake and output measurements, stool
number and consistency and ‘vital signs’, for example,
temperature, heart rate and respiratory rate. The subjective
observations were skin turgor, thirst and mucous membrane
moisture. This manuscript concluded that, although the
subjective measurements were least reliable, in terms of
consistency of measurement between measurers, they were
the simplest, fastest and most economical. The laboratory
tests were deemed to be the most accurate means to assess a
*Correspondence: SM Shirreffs, School of Sport and Exercise Sciences,
Loughborough University, Leicestershire LE11 3TU, UK.
E-mail: [email protected]
Guarantor: SM Shirreffs.
European Journal of Clinical Nutrition (2003) 57, Suppl 2, S6–S9& 2003 Nature Publishing Group All rights reserved 0954-3007/03 $25.00
www.nature.com/ejcn
patient’s hydration status. Since this manuscript was pub-
lished, there has been a large amount of research into some
of these measurements, observations and tests, and some of
the main ones, along with others, are discussed in the rest of
this paper.
Body massAcute changes in body mass over a short time period
can frequently be assumed to be due to body water loss or
gain; 1 ml of water has a mass of 1 g (Lentner, 1981) and
therefore changes in body mass can be used to quantify
water gain or loss. Over a short time period, no other
body component will be lost at such a rate, making this
assumption possible.
Throughout the exercise literature, changes in body mass
over a period of exercise have been used as the main method
of quantifying body water losses or gains due to sweating and
drinking. Indeed, this method is frequently used as the
method to which other methods are compared. Respiratory
water loss and water exchange due to substrate oxidation are
sometimes calculated and used to correct the sweat loss
values, but this is not always done (Mitchell et al, 1972).
Examples of such types of calculations are shown in Table 1.
Blood indicesCollection of a blood sample for subsequent analysis has
been both investigated and used as a hydration status marker.
Measurement of haemoglobin concentration and haemato-
crit has the potential to be used as a marker of hydration status
or change in hydration status, provided a reliable baseline can
be established. In this regard, standardisation of posture for a
time prior to blood collection is necessary to distinguish
between postural changes in blood volume, and therefore in
haemoglobin concentration and haematocrit, which occur
(Harrison, 1985) and change due to water loss or gain.
Plasma or serum sodium concentration and osmolality
will increase when the water loss inducing dehydration
is hypotonic with respect to plasma. An increase in these
concentrations would be expected, therefore, in many
cases of hypohydration, including water loss by sweat
secretion, urine production or diarrhoea. However, in
subjects studied by Francesconi et al (1987), who lost more
than 3% of their body mass mainly through sweating, no
change in haematocrit or serum osmolality was found,
although as described below certain urine parameters did
show changes. Similar findings to this were reported by
Armstrong et al (1994, 1998). This perhaps suggests that
plasma volume is defended in an attempt to maintain
cardiovascular stability, and so plasma variables will not be
affected by hypohydration until a certain degree of body
water loss has occurred.
Plasma testosterone, adrenaline and cortisol concentra-
tions were reported by Hoffman et al (1994) not to be
influenced by hypohydration to the extent of a body mass
loss of up to 5.1% induced by exercise in the heat. In
contrast, however, plasma noradrenaline concentration did
respond to the hydration changes, which means that it may
be possible to use this as a marker of hydration status, at least
when induced by exercise in the heat.
Urine indicesCollection of a urine sample for subsequent analysis has also
been investigated and used as a hydration status marker.
Measurement of urine osmolality has recently been an
extensively studied parameter as a possible hydration status
marker. In studies of fluid restriction, urine osmolality has
increased to values greater than 900 mosm/kg for the first
urine of the day passed in individuals dehydrated by 1.9% of
their body mass, as determined by body mass changes
(Shirreffs & Maughan, 1998). Armstrong et al (1994) have
determined that measures of urine osmolality can be used
interchangeably with urine-specific gravity, opening this as
another potential marker.
Urine colour is determined by the amount of urochrome
present in it (Diem, 1962). When large volumes of urine are
excreted, the urine is dilute and the solutes are excreted in a
large volume. This generally gives the urine a very pale
colour. When small volumes of urine are excreted, the urine
is concentrated and the solutes are excreted in a small
volume. This generally gives the urine a dark colour.
Armstrong et al (1998) have investigated the relationship
Table 1 Examples of hydration status calculations
Exercise
Pre-exerciseBody massa
(kg)
Post-exerciseBody massa
(kg)
Total body massloss or gaind
(ml or g)
Drinks consumedduring exerciseb
(ml)
Urine excretedduring exercisec
(ml)Sweat volume
(ml)Hydration statusd
(%)
60 min Running 70.00 68.00 �2000 0 200 1800 �2.93 h Walking 70.00 70.00 0 500 400 100 0.02 h Cycling 70.00 70.20 þ200 1000 0 800 þ0.3
aBody mass measured nude with dry skin.bDrinks consumed any time between the two body mass measurements.cUrine emptied from the bladder any time between the two body mass measurements.dþ¼water gain, �¼water loss, 0¼no change in water balance.
Markers of hydration statusSM Shirreffs
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European Journal of Clinical Nutrition
between urine colour and specific gravity and conductivity.
Using a scale of eight colours (Armstrong, 2000), it was
concluded that a linear relationship existed between urine
colour and both specific gravity and osmolality of the urine,
and that urine colour could therefore be used in athletic or
industrial settings to estimate hydration status when a high
precision may not be needed.
Urine indices of hydration status perhaps have their
limitation in identifying changes in hydration status during
periods of rapid body fluid turnover, as in subjects studied
who lost approximately 5% of their body mass with, on
average, 62 min of exercise in the heat, then rehydrating by
replacing this lost fluid (Popowski et al, 2001). In these
subjects, in comparison to measures of plasma osmolality
which increased and decreased in an almost linear fashion,
urine osmolality and specific gravity were found to be less
sensitive and demonstrated a delayed response, lagging
behind the plasma osmolality changes.
Bioelectrical impedance analysisBioelectrical impedance analysis (BIA) has been widely
investigated as a tool for assessing body composition. It
has the potential to assess hydration status by the determi-
nation of body water and its cellular divisions if a multi-
frequency device is used. In multifrequency BIA, a current is
applied at different frequencies and the higher conductivity
of water compared to the other compartments is used
to determine its volume. The National Institute of Health
technology assessment statement (National Institute of
Health, 1994) concluded that ‘BIA provides a reliable estimate
of total body water under most conditions.’ It carried on to
state that ‘BIA values are affected by numerous variables
includingy hydration status’ and that ‘Reliable BIA requires
standardisation and control of these variables.’ Subsequent
work in this area has generally highlighted the limitations of
the technique. For example, Asselin et al (1998) concluded
that with acute dehydration and rehydration of 2–3% of body
mass, standard equations failed to predict changes in total
body water, as determined by changes in body mass. Saunders
et al (1998) reported that small body water changes were
reported as body fat changes in an athletic population, and
Berneis and Keller (2000) after inducing extracellular volume
and tonicity alterations by infusion and drinking concluded
that BIA may not be reliable.
Other markersHydration status has also been investigated by a number of
less commonly investigated parameters. For example, altera-
tions in the response of pulse rate and systolic blood pressure
to postural change have been demonstrated in clinical
settings of dehydration and rehydration (Johnson et al,
1995). The diameter of the inferior cava vein, measured with
the subject lying supine, has been used with patients
undergoing peritoneal dialysis (Cheriex et al, 1989).
ConclusionsThe body water content of a person is most appropriately
determined using tracer methodology with the use of
deuterium oxide. The determination of a person’s hydration
status has received increasing attention over the past 10
years, much of it influenced by body water losses that can
occur in a relatively short period of time with physical
activity. Blood-borne parameters and urinary markers have
been widely studied areas, with a substantial amount of
research into the use of BIA also being undertaken. In most
cases, acute changes in body mass are used to signify the
body water losses or gains to which comparisons are made.
However, an arbitrary decision or definition of euhydration
must be made before a person is assigned to being in a state
of hypohydration or hyperhydration, and this perhaps
remains a major issue to be resolved.
The choice of hydration status marker will ultimately be
determined by the sensitivity and accuracy with which
hydration status needs to be established, the technical and
time requirements and the expense of the method. However,
consideration must also be given to other conditions or
complicating factors that may impact on the parameter of
measurement.
From the studies reviewed above, it seems fair to conclude
that urinary measures are more sensitive than the other
methods, but they may have a time lag over the short term.
It must also be remembered that classification of the state of
hypohydration or hyperhydration depends on the physiolo-
gical definition of euhydration, which is not as simple as
giving the dictionary definition.
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