Professional Description:
Doublly-labelled water is a form of indirect calorimetry where the average metabolic rate is measured over a period of time. This process is done by administering a dose of doubly-labelled water (water in which both the hydrogen and the oxygen have been partly or completely replaced with an uncommon isotope), then tracking the loss of deuterium and O-18 in the subject, over time, through the use of regular sampling of heavy isotope concentrations in the body water (by sampling saliva, urine, or blood).