Respiratory exchange ratio (RER) is the ratio of carbon dioxide produced to oxygen consumed during respiration. It is used to determine the type of fuel (carbohydrate or fat) that is predominantly being used for energy production in the body during physical activity.
The RER can be calculated by measuring the volume of oxygen consumed and carbon dioxide produced by the body during exercise. A RER of 0.7 indicates that fat is the primary fuel source, while a RER of 1.0 indicates that carbohydrates are the primary fuel source.