It is time to abandon single-value oxygen uptake energy equivalents.

Physiologists commonly use single-value energy equivalents (e.g., 20.1 kJ/LO 2 and 20.9 kJ/LO 2 ) to convert oxygen uptake (V̇o 2 ) to energy, but doing so ignores how the substrate oxidation ratio (carbohydrate:fat) changes across aerobic intensities. Using either 20.1 kJ/LO 2 or 20.9 kJ/LO 2 can incur systematic errors of up to 7%. In most circumstances, the best approach for estimating energy expenditure is to measure both V̇o 2 and V̇co 2 and use accurate, species-appropriate stoichiometry. However, there are circumstances when V̇co 2 measurements may be unreliable. In those circumstances, we recommend that the research report or compare only V̇o 2 . NEW & NOTEWORTHY We quantify that the common practice of using single-value oxygen uptake energy equivalents for exercising subjects can incur systematic errors of up to 7%. We argue that such errors can be greatly reduced if researchers measure both V̇o 2 and V̇co 2 and adopt appropriate stoichiometry equations.