|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|Energy requirements in normal infants and children|
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).
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.