How does indirect calorimetry calculate resting energy expenditure (REE) and respiratory quotient (RQ)?

• A. Measuring whole body oxygen (VO2) and carbon dioxide (VCO2) gas exchange using the abbreviated Weir equation
• B. Measuring blood gas
• C. Only VO2
• D. Using a simple body weight estimate

Answer: (A)

Indirect calorimetry estimates resting energy expenditure by capturing the body’s gas exchange at rest—specifically how much oxygen is consumed and how much carbon dioxide is produced. From these measurements, the respiratory quotient is determined as the ratio of carbon dioxide output to oxygen uptake (VCO2/VO2), which indicates the mix of fuels being burned. Those same gas-exchange values are entered into the Weir equation (often the abbreviated form used in practice) to convert VO2 and VCO2 into total energy expenditure per day, giving the resting energy expenditure. This method provides both REE and RQ from one measurement, reflecting actual metabolic activity rather than reliance on body weight or blood gas data. Measuring blood gas, using VO2 alone, or estimating from weight would not yield the same direct assessment of energy expenditure or substrate use.