In the context of exercise, what does 'RER' stand for?

In the context of exercise, RER stands for respiratory exchange ratio.

RER compares the volume of carbon dioxide produced by the body with the volume of oxygen consumed. It is written as VCO₂ divided by VO₂ and is commonly measured during metabolic testing, including a VO₂ max test. The ratio gives an estimate of which fuels the body is using during exercise.

An RER near 0.7 is associated mainly with fat oxidation, while a value near 1.0 is associated mainly with carbohydrate oxidation. A mixed diet and mixed fuel use often produce a value around 0.8. These figures are useful rules of thumb rather than exact readings of every molecule being burned.

RER is often confused with respiratory quotient, or RQ. RQ describes gas exchange at the cellular or metabolic level, while RER is measured from expired and inspired air. At rest and during moderate aerobic exercise they are often similar, but intense exercise can push RER above 1.0 because extra carbon dioxide is released while the body buffers rising acidity.