We examined the effects of exercise intensity and a 10-wk cycle ergometer training program [5 days/wk, 1 h, 75% peak oxygen consumption (O_{2 peak})] on plasma free fatty acid (FFA) flux, total fat oxidation, and whole body lipolysis in healthy male subjects (n = 10; age = 25.6 ± 1.0 yr). Two pretraining trials (45 and 65% of O_{2 peak}) and two posttraining trials (same absolute workload, 65% of old O_{2 peak}; and same relative workload, 65% of new O_{2 peak}) were performed by using an infusion of [1-¹³C]palmitate and [1,1,2,3,3-²H]glycerol. An additional nine subjects (age 25.4 ± 0.8 yr) were treated similarly but were infused with [1,1,2,3,3-²H]glycerol and not [1-¹³C]palmitate. Subjects were studied postabsorptive for 90 min of rest and 1 h of cycling exercise. After training, subjects increased O_{2 peak} by 9.4 ± 1.4%. Pretraining, plasma FFA kinetics were inversely related to exercise intensity with rates of appearance (R_a) and disappearance (R_d) being significantly higher at 45 than at 65% O_{2 peak} (R_a: 8.14 ± 1.28 vs. 6.64 ± 0.46, R_d: 8.03 ± 1.28 vs. 6.42 ± 0.41 mol · kg¹ · min¹) (P  0.05). After training, when measured at the same absolute and relative intensities, FFA R_a increased to 8.84 ± 1.1, 8.44 ± 1.1 and R_d to 8.82 ± 1.1, 8.35 ± 1.1 mol · kg¹ · min¹, respectively (P  0.05). Total fat oxidation determined from respiratory exchange ratio was elevated during exercise compared with rest, but did not differ among the four conditions. Glycerol R_a was elevated during exercise compared with rest but did not demonstrate significant intensity or training effects during exercise. Thus, in young men, plasma FFA flux is increased during exercise after endurance training, but total fat oxidation and whole-body lipolysis are unaffected when measured at the same absolute or relative exercise intensities.

stable isotopes; substrate utilization; free fatty acids; fat metabolism; glycerol; lipolysis; crossover concept

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