Fat metabolism during high-intensity exercise in endurance-trained and untrained men

To determine whether trained individuals rely more on fat than untrained persons during high-intensity exercise, six endurance-trained men and six untrained men were studied during 30 minutes of exercise at 75% to 80% maximal oxygen consumption (V̇O₂max). The rates of appearance (R(a)) and disappearance (R(d)) of glycerol and free fatty acids (FFAs) were determined using [1,1,2,3,3-²H]glycerol and [1-¹³C]palmitate, respectively, whereas the overall rate of fatty acid oxidation was determined using indirect calorimetry. During exercise, the whole-body rate of lipolysis (ie, glycerol R(a)) was higher in the trained group (7.1 ± 1.2 v 4.5 ± 0.7 μmol · min^{- 1} · kg^-1, P<.05), as was the R(a) (≃R(d)) of FFA (9.0 ± 0.9 v 5.0 ± 1.0 μmol · min^-1 · kg^-1, P < .001). FFA utilization was higher in trained subjects even when expressed as a percentage of total energy expenditure (10% ± 1% v 7% ± 1%, P < .05). However, this difference in plasma FFA flux could not account for all of the difference in fatty acid oxidation between trained and untrained subjects (20.8 ± 3.3 v 7.9 ± 1.6 μmol · min^-1 · kg^-1, or 23% ± 3% v 13% ± 2% of total energy expenditure, both P < .05). Thus, the oxidation of fatty acids derived from some other source also must have been greater in the trained men. We conclude that trained athletes use more fat than untrained individuals even during intense exercise performed at the same percentage of V̇O₂max. The additional fatty acids appear to be derived from both adipose tissue and, presumably, intramuscular triglyceride stores.