Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration

TLDR

The study used stable isotope tracers and indirect calorimetry to assess how endogenous fat and glucose metabolism respond to different exercise intensities (25, 65, and 85% VO₂max) in five trained subjects. Higher exercise intensity increased plasma glucose uptake and muscle glycogen oxidation, while peripheral lipolysis peaked at the lowest intensity and fatty acid release fell with intensity; muscle triglyceride lipolysis occurred only at higher intensities, and during prolonged 65% VO₂max exercise plasma substrate oxidation rose over time as muscle glycogen and triglyceride oxidation fell, with recovery from high intensity showing a transient drop in lipolysis but a rise in plasma fatty acid release, indicating that carbohydrate availability is directly intensity‑dependent whereas lipid metabolism regulation is more complex.

Abstract

Stable isotope tracers and indirect calorimetry were used to evaluate the regulation of endogenous fat and glucose metabolism in relation to exercise intensity and duration. Five trained subjects were studied during exercise intensities of 25, 65, and 85% of maximal oxygen consumption (VO2max). Plasma glucose tissue uptake and muscle glycogen oxidation increased in relation to exercise intensity. In contrast, peripheral lipolysis was stimulated maximally at the lowest exercise intensity, and fatty acid release into plasma decreased with increasing exercise intensity. Muscle triglyceride lipolysis was stimulated only at higher intensities. During 2 h of exercise at 65% VO2max plasma-derived substrate oxidation progressively increased over time, whereas muscle glycogen and triglyceride oxidation decreased. In recovery from high-intensity exercise, although the rate of lipolysis immediately decreased, the rate of release of fatty acids into plasma increased, indicating release of fatty acids from previously hydrolyzed triglycerides. We conclude that, whereas carbohydrate availability is regulated directly in relation to exercise intensity, the regulation of lipid metabolism seems to be more complex.