Physiologists commonly use single-value energy equivalents (e.g., 20.1 kJ/LO2 and 20.9 kJ/LO2) to convert oxygen uptake (Vo2) to energy, but doing so ignores how the substrate oxidation ratio (carbohydrate:fat) changes across aerobic intensities. Using either 20.1 kJ/LO2 or 20.9 kJ/LO2 can incur systematic errors of up to 7%. In most circumstances, the best approach for estimating energy expenditure is to measure both Vo2 and Vco2 and use accurate, species-appropriate stoichiometry. However, there are circumstances when Vco2 measurements may be unreliable. In those circumstances, we recommend that the research report or compare only Vo2.
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