Respiratory exchange ratio

In one breath, you normally breathe in more molecules of oxygen than you breathe out molecules of carbon dioxide. The ratio between these CO₂ / %O₂ is the respiratory exchange ratio (RER).

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Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (carbohydrate or fat) is being metabolised to supply the body with energy.

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. It is important to note that during exercise, using RER for estimating RQ loses accuracy because of factors including bicarbonate buffering of lactic acid, 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 participant is nearing exhaustion and the limits of their cardio-respiratory system. An RER of approximately 1.2 is reached at this point.