Respiratory exchange ratio (RER/RQ)

The respiratory exchange ratio (RER), synonymous with the respiratory quotient (RQ) under resting and moderate aerobic conditions, is the ratio of carbon dioxide produced (VCO₂) to oxygen consumed (VO₂) per unit time, measured non-invasively by indirect calorimetry. Pure fat oxidation yields an RER near 0.70, mixed macronutrient combustion yields ~0.80–0.85, and exclusive carbohydrate oxidation yields 1.00; values above 1.00 during intense exercise reflect bicarbonate buffering of lactate rather than substrate use. A chronically elevated fasting RER (>0.91) signals reduced fat oxidation and prospectively predicts incident metabolic syndrome and type 2 diabetes (Pujia et al. 2019). In the Energy Balance Study, a higher baseline RQ was associated with greater gains in body weight and fat mass over 12 months in adults aged 21–35 years (Shook et al. 2016). Aging elevates fasting RER and blunts fuel-switching range independently of adiposity; sarcopenic older adults (mean age ~81 years) show significantly higher resting RQ than age-matched non-sarcopenic peers, suggesting metabolic inflexibility as both a correlate and a potential accelerant of muscle-function decline (Shoemaker et al. 2022). Whether restoring a lower fasting RER through aerobic training, dietary fat adaptation, or caloric restriction extends healthspan causally requires further randomised trial evidence.