A method of indirectly estimating energy expenditure. The subject ingests a known volume of water labelled with two isotopes (2H218O). The deuterium (2H2) and oxygen (18O) diffuse throughout the body's water, and their disappearance rate from the body fluid (e.g. in blood, urine, or saliva) is measured. When a subject is loaded with 2H218O, the decrease in 18O is a measure for H2O output plus CO2 outputs, and the decrease in 2H2 is a measure of H2O output alone. Therefore, CO2 output can be obtained by the difference. CO2 output is converted to energy expenditure using the energy equivalent of CO2, calculated using additional information on the substrate mixture respired (see respiratory quotient).
Subjects: Sports and Exercise Medicine.