Respiratory Exchange Ratio

The ratio between the amount of CO₂ produced and O₂ consumed in one breath (determined from comparing exhaled gasses to room air) 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).

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.