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% (V)over dotO(2) 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 ((V)over dotO(2) 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 [C-13] palmitate and [H-2(5)] 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 similar to65% greater in women than in men (glycerol R-a: 317 +/- 40 and 195 +/- 33 mumol/kg, respectively; FFA R-a: 652 +/- 46 and 453 +/- 70 mumol/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.