Doubly labeled water (DLW) is a water in which both the hydrogen and the oxygen have been replaced with known concentration of deuterium and oxygen-18, for tracing study.
Nowadays, applying doubly-labeled water procedure as a noninvasive method to evaluate energy expenditure, body composition and water flux in humans and animals is extended greatly. In this method, following the decline of the enrichment of oxygen-18 and deuterium content of body water after initial labeling of the body water pool is monitored through a pre-determined period to evaluate the energy expenditure of body. During body activities, water lost its oxygen-18 content in the form of water and carbon dioxide (CO2), while deuterium is lost just as water. The difference in loss from the body shows the CO2 production during the period. Carbon dioxide production is rooted in the oxidation of fat, carbohydrate and protein. So, it is served as an index of energy expenditure.
As an example, doubly labeled water method starts with a baseline plasma, saliva or urine collection to determine pre-dose values for the hydrogen and oxygen isotopes. Then, the subjects are given a single oral of double labeled water (2H218O) with known concentration of deuterium and oxygen-18. In general, adults are given a dose of water including 0.15 g H218O/kg body weight, and 0.06 g D2O/kg body weight, but, children are given a dose of water with higher isotopic content per kilogram, because, the water turnover rates is faster in their body. Additional sample will be collected through a regular period of time, usually between 4 and 21 days. The isotope concentrations of sample analyzed via mass spectrometry or laser spectroscopy.