Stable isotopes of hydrogen and oxygen have been recently used to measure energy expenditure in free-living humans. The doubly labeled water method using these isotopes is a form of indirect calorimetry which has been extensively validated in animals and humans. The method is completely safe, requires only periodic sampling of body fluids, and is ideally suited for measurement of energy expenditure in free-living or hospitalized patients.
After the administration of doubly labeled water (D218O), the labeled hydrogen (D2) would be eliminated as water (D2O), corresponding to water output, whereas the oxygen isotope would be eliminated as water (H218O) and as expired carbon dioxide (C18O2). By measuring the difference between the elimination rates of labeled oxygen and hydrogen, the carbon dioxide production rate can be calculated. The carbon dioxide production rate is converted into energy expenditure by knowing the respiratory quotient of the food ingested during the observation period.
The advantage of this technique is that sample collection only involves urine collection at timed intervals following initial dosing. These samples are then analyzed using isotope ratio mass spectrometry (IRMS) to determine the stable isotope remaining in the body.
Reference:
Measuring Energy Expenditure Using the Doubly Labeled Water Method Technical Paper 905.
