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The ratio between the amount of CO2 exhaled and O2 inhaled in one breath is the respiratory exchange ratio (RER). In one breath, humans normally breathe in more molecules of oxygen (O2) than they breathe out molecules of carbon dioxide (CO2).
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 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 1.1 is often used as a secondary endpoint criteria 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.