Hydrometry or total body water (TBW) by isotope dilution is a common method for the assessment of body composition at the molecular level.
The method is based on the principle that water is distributed in all parts of the body except body fat. Water is found exclusively within the fat free mass (FFM), which is approximately 73.2% water in adults. Water is the largest component of the human body and includes both intracellular fluid and extracellular fluid. At birth, the body contains approximately 80% water, but as the body matures, this proportion decreases to 50–60% in lean adults and to less than 40% in obese adults.
With measurement of TBW by isotope dilution, the amount of FFM can be estimated using hydration factors. Body fat mass (FM) is the difference between body weight and FFM.
Total body water (TBW) assessment is based on the principle of isotope dilution. Enrichment of the body water pool (see Figure 1) following a bolus dose of deuterium oxide (2H2O) allows the isotope dilution space to be calculated using the following equations:
F1 is the deuterium enrichment (concentration) of the dose
F2 is the deuterium enrichment (concentration) of the distribution space
N1 is the size of the deuterium dose
N2 is the size of the distribution space
N.B F1N1 = F2N2 is the isotope version of the principle of C1V1 = C2V2, where C is concentration and V is volume.
All that remains is for corrections due to non-aqueous exchange of the isotope to be made. That is, corrections for the loss of isotope outside of the TBW into fats and proteins predominantly.
TBW can theoretically be measured using water labelled with either of the stable isotopes deuterium (2H2O) or oxygen-18 (H218O) or the radio-active isotope tritium (3H2O). This is sometimes referred to as the ‘tracer’. Stable isotopes are preferred over radio-active isotopes due to participant acceptance and minimal risk; water labelled with deuterium is far more economical than 18O.
A known bolus dose of deuterium oxide is given orally to the participant. This mixes with the body water pool (see Figure 1). Biological fluids such as usually, plasma, saliva or urine are sampled.
Two methods to determine total body water
There are two methods used to determine TBW: 1) the intercept method and 2) the equilibration/plateau method.
Key instructions for participants
N.B Participants should not drink at least 30-min before a saliva sample is taken.
Main sources of errors
Figure 1 Estimating TBW by deuterium dilution. At baseline, the body water pool naturally contains a small amount of deuterium. After a known bolus dose of deuterium oxide is given orally to the participant, this mixes with and enriches the
body water pool.
Source: International Atomic Energy Agency (2010).
TBW can be measured in the field using the deuterium oxide dilution technique. An advantage of this technique is that it can be used to assess longitudinal changes in body composition before and after an intervention. In an ideal situation, this method should be used alongside other body composition measures as it is limited to estimating fat mass (FM) and fat free mass (FFM) (2-component (2C) model). Error rates have been estimated to be 1% for TBW and 0.5% for FFM.
TBW by isotope dilution is also used to derive the criterion method for overall body composition, the 4-component model together with other body composition methods (bone mass from whole body DEXA scan and body volume/density by hydrostatic underwater weighing or air displacement plethysmography).
The first step is to calculate TBW:
Once TBW has been calculated, FFM is simply TBW divided by the hydration factor:
FFM = TBW / hydration factor
FM = Weight - FFM
In adults, the hydration factor is assumed to be 73.2%. The actual values range from 67.4 to 77.5%. These variations can result in considerable error when calculating total body fat. For studies involving infants and/or children the hydration factor varies with age and sex.
An overview of the characteristics of hydrometry is outlined in Table 1.
Table 1 Characteristics of the hydrometry method.
|Number of participants
|Researcher burden of data collection
|Researcher burden of coding and data analysis
|Risk of reactivity bias
|Depends on blinding
|Risk of recall bias
|Risk of social desirability bias
|Risk of observer bias
|Suitability for field use
|Participant literacy required
Considerations relating to the use of hydrometry in specific populations are described in Table 2.
Table 2 Anthropometry by hydrometry in different populations.
|Suitable. More information is provided in the further considerations section.
|Infancy and lactation
|Getting an accurate dose can be difficult. Urine sampling easier. Because of the wide between-individual variation in hydration of FFM, this method may not be optimal for assessing total body fat in neonates. However, it is the most reliable method to assess TBW More information is provided in the further considerations section
|Toddlers and young children
|Suitable. This method is not supported in people with cardiac, renal disease and those with oedema and other fluid retention problems.
|Suitable. This method is not supported in people with cardiac, renal disease and those with oedema and other fluid retention problems. More information is provided in the further considerations section.
|Suitable. In athletes it is more accurate to use the back extrapolation technique due to the high water turnover resulting from high levels of exercise. This technique measures water turnover over a 2-week period as part of the doubly labelled technique of estimating energy expenditure. More information is provided in the further considerations section.
|Suitable. In obese participants due to a higher fat mass it may be sensible (and more economical) to dose on estimated total body water assuming that 65% of body weight is water.
Refer to section: practical considerations for objective anthropometry