*Geek Box: Doubly-Labelled Water
Doubly-labelled water has been utilised in human studies since the early 1980’s, as a reliable method for assessing total energy expenditure in free-living conditions. Total energy expenditure is made up of three components: basal metabolic rate [influenced by factors like age, gender, fat mass and lean body mass], thermic effect of feeding [heat generated during digestive processes], and physical activity. The DLW method relies on measurement of CO2 production to convert that measurement to total energy expenditure (9). It does this by ‘labelling’ water, which is H2O, with uncommon isotopes – deuterium [2H] and 18O – which will ultimately be excreted from the body: the difference between the rate of elimination of 2H and 18O from the body provides an estimate of the production rate of CO2. DLW is consumed orally, making it very effective for ‘in the field’ research, and providing estimates of human energy expenditure in the real-world (compared to a lab). Because it is consumed orally, and is safe, DLW can be used in all populations, including infants, and can provide a measurement over long periods of time. However, the method has not been validated against low-carbohydrate diets, which result in changes in body water stores.