The doubly labeled water (DLW) technique allows measurement of energy expenditure in animals. The DLW method is based on the assumption that after injection of a dose of doubly labeled water, 2H218O, the two isotopes equilibrate with total body water and then are eliminated differentially from the body. Deuterium (2H) leaves the body as water (2H2O), while 18O leaves as water (H218O) and carbon dioxide (C18O2). Therefore, CO2 production can be calculated by subtracting 2H elimination from 18O elimination.
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