Measurement of energy expenditure in free-living humans by using doubly labeled water.

The doubly labeled water method is a form of indirect calorimetry that has been developed only recently to the stage of application to human studies. The method measures integral CO2 production for up to 3 wk from the difference in elimination rates of deuterium and 18O from labeled body water. Validations against near-continuous respiratory gas exchange have demonstrated that the method is accurate and has a precision of 2-8% depending on the isotope dose and the length of the elimination period. Although the method has been validated, there is still some debate on refinements of the kinetic model that may lead to improved accuracy and precision. Because the method only requires periodic sampling of body fluids, it is non-restrictive and ideally suited to use in free-living subjects. Recent applications of the method have included obesity research, determination of energy requirements in both developing and developed countries and studies of human growth.