RER (sometimes called the respiratory quotient, RQ, in resting studies) reveals which substrate the body is burning. At low intensity, trained endurance athletes have RER values near 0.80 — about 60% fat, 40% carbohydrate. As intensity rises, RER climbs toward 1.0 as carbohydrate contribution increases. RER above 1.0 indicates buffering of lactic acid. Increasing fat oxidation — shifting RER down at any given intensity — is a key adaptation from long zone 2 training. Lab metabolic carts measure RER directly; most amateurs infer shifts from training-induced RHR and fuelling tolerance changes.