Many indices have been investigated to establish their potential as markers of hydration status. Body mass changes, blood indices, urine indices and bioelectrical impedance analysis have been the most widely investigated. The current evidence and opinion tend to favour urine indices, and in particular urine osmolality, as the most promising marker available.
Hydration status—some definitions
Euhydration 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 loss
The 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 water
The 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 status
Hydration 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 included 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 patient's hydration status. Since this manuscript was published, 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.
Acute 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.
Collection of a blood sample for subsequent analysis has been both investigated and used as a hydration status marker.
Measurement of haemoglobin concentration and haematocrit 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 concentrations 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.
Collection 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 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 analysis
Bioelectrical impedance analysis (BIA) has been widely investigated as a tool for assessing body composition. It has the potential to assess hydration status by the determination of body water and its cellular divisions if a multifrequency 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 including… 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.
Hydration status has also been investigated by a number of less commonly investigated parameters. For example, alterations 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).
The 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 physiological definition of euhydration, which is not as simple as giving the dictionary definition.
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Shirreffs, S. Markers of hydration status. Eur J Clin Nutr 57, S6–S9 (2003). https://doi.org/10.1038/sj.ejcn.1601895
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