What does indirect calorimetry measure to estimate energy expenditure?

• A. Gas exchange, specifically CO2 produced and O2 consumed.
• B. Muscle oxygen saturation.
• C. Blood lactate concentration.
• D. Core body temperature.

Answer: (A)

Gas exchange is used to estimate energy expenditure in indirect calorimetry. When the body metabolizes fuels, it consumes oxygen and produces carbon dioxide. By measuring how much O2 is taken in (VO2) and how much CO2 is released (VCO2) during rest or exercise, we can determine how many calories are being burned. The ratio of CO2 produced to O2 consumed (the respiratory exchange ratio) hints at which fuels are being used and, together with the measured gas fluxes, allows calculation of energy expenditure through standard conversion equations.

The other measures aren’t direct indicators of total energy use: muscle oxygen saturation reflects local tissue oxygenation, not whole-body energy expenditure; blood lactate shows glycolytic activity but not total calories burned; and core temperature is a secondary outcome influenced by many factors, not a direct measure of energy expenditure.