AQA A-Level PE Energy Systems Practice Exam 2026 - Free PE Energy Systems Practice Questions and Study Guide

Indirect calorimetry estimates energy expenditure by tracking how the body uses oxygen and produces carbon dioxide during metabolism. When nutrients are oxidized, oxygen is consumed and carbon dioxide is generated, so the rates of oxygen uptake (VO2) and carbon dioxide production (VCO2) together reflect how much energy is being produced. Measuring both allows you to apply established equations that convert gas exchange into energy expenditure, and the ratio of CO2 produced to O2 consumed (the respiratory exchange ratio) provides insight into which fuels are being burned. If you only measure one gas, you’d miss important information about substrate use and would struggle to estimate energy expenditure accurately. Nitrogen balance relates to protein metabolism and isn’t used for the overall energy estimate in standard indirect calorimetry.