Physical activity assessment – Doubly labelled water

Doubly labelled water (DLW) measures total energy expenditure (TEE) by observing the differential rates of elimination of a bolus dose of the stable isotope tracers ²H (deuterium) and ¹⁸O. The tracers used are non-radioactive and occur naturally in all waters (including drinking waters), and therefore completely safe to use in any population. The method has been used in adults, children and infants to measure TEE, in many diverse investigations including the energy costs of clinical conditions, and the energy utilisation of people participating in intensive physical activities under extreme conditions. It has also been used widely to validate other methods of assessment of dietary and physical activity. Indeed the application of DLW led to seminal work in the identification of widespread under-reporting in dietary assessment. In physical activity measurement, DLW has been used to validate various methods which estimate energy expenditure e.g. questionnaires, diaries, logs and accelerometers; the combination of DLW and indirect calorimetry provides a robust method of measuring the energy expenditure due to physical activity.

The theoretical considerstions and assumptions which underpin the method are complex and comprehensive reviews of these are listed in the reference section. The DLW method was originally developed by Lifson et al (1955) with many refinements since. Schoeller (1983) was the first to use the method in humans when the cost of ¹⁸O reduced sufficiently to make this a viable albeit expensive measurement for research. In subsequent years advances in measurement technology and the falling bcost of the isotopes has made the method more accessible and it is now used routinely.

The fundamental basis of the DLW technique is that whilst the hydrogen label is lost only as water, the oxygen is lost as both water and carbon dioxide; transference of oxygen between water and carbon dioxide is the consequence of rapid exchange promoted by carbonic anhydrase. Therefore the difference between the turnover of the two labels can be used as a measure of the production of carbon dioxide.

In practice the subject is asked to drink a known dose of water enriched in ²H and ¹⁸O. Samples of blood, saliva or urine, from which the isotopic composition of the body water can be determined, are collected over the next 5-14days. From the isotope disappearance curves four parameters are deduced, the two pool sizes of hydrogen and oxygen and the fractional rate constants of elimination for each of these species, and these are combined to give an estimate of CO₂ production.

Translation of CO₂ production into into TEE
In contrast with the use of oxygen uptake to calculate energy expeniture, where the relationship between the quantity of gas utilised and the energy liberated is largely independent of the substrate being oxidised. The nature of the metabolic fuel being used plays an important role in the quantity of CO₂ liberated for unit energy expended. This relationship, which is charcterized by the respiratory quotient, RQ, must be taken into account when deducing total energy expenditure from DLW experiments. Approximate expressions which assume normal ratios of fat, carbohydrate and protein being metabolised have been derived, but for the most accurate work individual RQ values can be obtained from the analysis of food diaries.