Respiratory exchange ratio
The respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide (CO2) produced in metabolism and oxygen (O2) used. Humans typically inhale more molecules of oxygen than they exhale of carbon dioxide.
The ratio is determined by comparing exhaled gasses 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.