Dual isotopically labelled water, containing the stable isotopes deuterium and 18O (i.e. 2H218O). Widely used as a means of estimating energy expenditure over a period of 1–3 weeks. After drinking the labelled water, the labelled oxygen is lost from the body more rapidly than the deuterium, since 2H is lost only in water, while 18O is lost in both water and carbon dioxide because of the rapid equilibrium between carbon dioxide and bicarbonate. Determination of the relative amounts of both isotopes in urine or saliva thus permits estimation of total carbon dioxide production, and hence oxygen consumption and energy expenditure.
Subjects: Medicine and Health.