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The ratio is determined by comparing exhaled gases to room air. Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (carbohydrate or fat) is being metabolized to supply the body with energy. This estimation is only valid if metabolism is in a steady state.
RER is about 0.8 at rest with a modern diet. This value, however, can exceed 1 during intense exercise, as CO2 production by the working muscles becomes greater and more of the inhaled O2 gets used rather than being expelled. During moderate or higher intensity aerobic exercise and anaerobic exercise, using RER for estimating RQ loses accuracy because of factors including bicarbonate buffering of hydrogen ions, which affects the CO2 levels being expelled by the respiratory system.
Calculation of RER is commonly done in conjunction with exercise tests such as the VO2 max test and can be used as an indicator that the participants are nearing exhaustion and the limits of their cardio-respiratory system. An RER greater than or equal to 1.15 is often used as a secondary endpoint criterion of a VO2 max test.
An RER of 0.70 indicates that fat is the predominant fuel source, RER of 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above is indicative of carbohydrate being the predominant fuel source.
Oxidation of a carbohydrate molecule
Oxidation of a fatty acid molecule