Respiratory Exchange Ratio

 Respiratory Exchange Ratio 

The respiratory exchange ratio (RER) is the ratio between the amounts of CO₂ produced and O₂ used in metabolism.

The ratio is between exhaled gases with that of room air. Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (e.g. carbohydrate or fat) is being metabolized to supply the body with energy.

Using RER to estimate RQ is only accurate during rest and mild to moderate aerobic exercise without the accumulation of lactate. The loss of accuracy during more intense anaerobic exercise is among others due to factors including the bicarbonate buffer system. The body tries to compensate for the accumulation of lactate and minimize the acidosis by expelling more CO₂ through the respiratory system.

An RER near 0.7 indicates that fat is the predominant fuel source, a value of 1.0 is indicative of carbohydrate being the predominant fuel source, and a value between 0.7 and 1.0 suggests a mix of both fat and carbohydrate. In general a mixed diet corresponds with an RER of approximately 0.8. The RER can also exceed 1.0 during intense exercise. A value above 1.0 cannot be attributed to the substrate metabolism, but rather to the aforementioned factors regarding bicarbonate buffering