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

To determine whether trained individuals rely more on fat than untrained pe rsons during high-intensity exercise, six endurance-trained men and six unt rained men were studied during 30 minutes of exercise at 75% to 80% maximal oxygen consumption (V(over dot)o(2)max). The rates of appearance (R-a) and disappearance (R-d) Of glycerol and free fatty acids (FFAs) were determine d using [1,1,2,3,3(-2)H]glycerol and [1-C-13]palmitate, respectively, where as the overall rate of fatty acid oxidation was determined using indirect c alorimetry. 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 mu mol.min (-1) kg(-1), P < .05), as was the R-a (approximate to R-d) of FFA (9.0 +/- 0.9 v 5.0 +/- 1.0 mu mol.min(-1).kg(-1), P < .001). FFA utilization was hig her 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 mu mol min(-1) kg(-1), or 23% +/- 3% v 13% +/- 2% of total energy expendi ture, both P < .05). Thus, the oxidation of fatty acids derived from some o ther source also must have been greater in the trained men. We conclude tha t trained athletes use more fat than untrained individuals even during inte nse exercise performed at the same percentage of V(over dot)O(2)max. The ad ditional fatty acids appear to be derived from both adipose tissue and, pre sumably, intramuscular triglyceride stores. Copyright (C) 2000 by W.B. Saun ders Company.