Respiratory exchange ratio (RER/RQ)

The respiratory exchange ratio (RER) is the ratio of carbon dioxide you produce (VCO₂) to oxygen you consume (VO₂) per unit time. At rest and during moderate aerobic effort, it is the same as the respiratory quotient (RQ). It is measured non-invasively by indirect calorimetry. The value tells you what fuel you are burning. Pure fat-burning gives an RER near 0.70. A mix of fuels gives about 0.80 to 0.85. Burning only carbohydrate gives 1.00. Values above 1.00 during hard exercise reflect bicarbonate buffering of lactate, not fuel use. A chronically high fasting RER (above 0.91) signals poor fat-burning. It also predicts future metabolic syndrome and type 2 diabetes (Pujia et al. 2019). In the Energy Balance Study, a higher baseline RQ went with bigger gains in weight and fat over 12 months, in adults aged 21 to 35 (Shook et al. 2016). Aging raises fasting RER and narrows your fuel-switching range, independent of body fat. Sarcopenic older adults (mean age about 81) show a clearly higher resting RQ than non-sarcopenic peers. That points to metabolic inflexibility as both a marker and a possible accelerant of muscle-function decline (Shoemaker et al. 2022). Whether lowering your fasting RER (through aerobic training, fat-adaptation, or calorie restriction) extends healthspan causally still needs randomized-trial evidence.

Last reviewed: June 22, 2026

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