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

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