Effects of Sex and Obesity on Lipid Metabolism during Low-Intensity Exercise and Postexercise Period

Whether there are sex and obesity differences in fat oxidation and free fatty acid (FFA) mobilization during low-intensity exercise is unclear.

We studied 21 (nine females) participants without and 24 (16 females) with obesity during 5 h of treadmill walking (~2 mph). We measured energy expenditure (EE) and substrate oxidation (indirect calorimetry) and FFA-palmitate kinetics and oxidation with isotopic palmitate infusions ([U- 13 C]palmitate and [1- 14 C]palmitate or [9,10- 3 H]palmitate). Skeletal muscle samples were collected at 1.5 and 5 h of exercise and 24 h after exercise. We calculated fatty acid (FA) oxidation, palmitate rate of disappearance (Rd), and intramyocellular triglyceride-derived palmitate fractions during exercise.

At rest, EE was 1.24 ± 0.23 kcal/min, with FA oxidation contributing 49 ± 13% of total EE. During exercise, FA oxidation accounted for 73 ± 8% of EE, and was not different between males and females ( P = 0.34). Plasma FFA-palmitate concentrations were greater in females than males ( P = 0.003), mean palmitate Rd and plasma palmitate clearance were not different ( P = 0.32 and 0.14, respectively). Plasma FFA-palmitate Rd was correlated with EE ( r = 0.49, P = 0.001), with a proportionally greater Rd response in females ( P < 0.001). Although absolute plasma FFA-palmitate oxidation rates were comparable between sexes, the percent contribution of FFA-palmitate oxidation to EE was greater in females ( P = 0.001). During exercise, intramyocellular triglyceride-palmitate fraction amounts remained stable.

Lipid oxidation during low-intensity exercise is similar in males and females, independent of obesity, although next-day values were greater in participants with normal weight. EE correlates with plasma palmitate disappearance rates, but is proportionally greater in females. The contribution of plasma palmitate oxidation is greater in females than in males during low-intensity exercise.