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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