Effect of gender on lipid kinetics during endurance exercise of moderate intensity in untrained subjects

We evaluated lipid metabolism during 90 min of moderate-intensity (50% V̇O₂ peak) cycle ergometer exercise in five men and five women who were matched on adiposity (24 ± 2 and 25 ± 1% body fat, respectively) and aerobic fitness (V̇O₂ peak: 49 ± 2 and 47 ± 1 ml·kg fat-free mass^-1·min^-1, respectively). Substrate oxidation and lipid kinetics were measured by using indirect calorimetry and [¹³C]palmitate and [²H₅]glycerol tracer infusion. The total increase in glycerol and free fatty acid (FFA) rate of appearance (R_a) in plasma during exercise (area under the curve above baseline) was ∼65% greater in women than in men (glycerol R_a: 317 ± 40 and 195 ± 33 μmol/kg, respectively; FFA R_a: 652 ± 46 and 453 ± 70 μmol/kg, respectively; both P < 0.05). Total fatty acid oxidation was similar in men and women, but the relative contribution of plasma FFA to total fatty acid oxidation was higher in women (76 ± 5%) than in men (46 ± 5%; P < 0.05). We conclude that lipolysis of adipose tissue triglycerides during moderate-intensity exercise is greater in women than in men, who are matched on adiposity and fitness. The increase in plasma fatty acid availability leads to a greater rate of plasma FFA tissue uptake and oxidation in women than in men. However, total fat oxidation is the same in both groups because of a reciprocal decrease in the oxidation rate of fatty acids derived from nonplasma sources, presumably intramuscular and possibly plasma triglycerides, in women.