4. The doubly-labelled water method

The doubly-labelled water method was originally developed by LIFSON and coworkers (1955) for use in small mammals and was later applied to man (SCHOELLER and VAN SANTEN, 1982; COWARD et al., 1984; KLEIN et al., 1984). The principle of the method is that two stable isotopes of water (H₂¹⁸O and ²H₂O) are administered simultaneously, and their initial enrichment and subsequent disappearance rates in a body fluid (e.g., urine or saliva) are monitored by isotope ratio mass spectrometry. The initial increase in enrichment of either isotope reflects body-water pool size (from the principle of dilution) and permits an estimation of body composition.

Subsequently, the disappearance rate of ²H₂O reflects water output (and hence water intake); that of ²H₂¹⁸O reflects water output plus CO₂ production, because ¹⁸O is free to interchange between water and CO₂ through the action of carbonic anhydrase (see Figure 1). The difference between the two disappearance rates is therefore a measure of CO₂ production rate. With additional knowledge of the subject's respiratory quotient (measured or derived from previous studies on a similar population, or calculated from food intake), oxygen consumption and hence energy expenditure can be calculated using a standard equation (WEIR, 1949).

Figure 1. Principle of the doubly-labelled water method.

k = experimentally-determined rate constant;
r = production rate

In addition, the technique also allows water intake and hence milk intake to be assessed, and total body-water pool size, derived from the method, gives an estimate of body composition from which energy storage can be calculated.