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: s.shirreffs@lboro.ac.uk

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.

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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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