Aerobic fitness determines whole-body fat oxidation rate during exercise in the heat

The purpose of this study was to determine whole-body fat oxidation in endurance-trained (TR) and untrained (UNTR) subjects exercising at different intensities in the heat. On 3 occasions, 10 TR cyclists and 10 UNTR healthy subjects (peak oxygen uptake = 60 +/- 6 vs. 44 +/- 3 mL.kg(-1).min(-1); p < 0.05) exercised at 40%, 60%, and 80% peak oxygen uptake in a hot, dry environment (36 degrees C; 25% relative humidity). To complete the same amount of work in all 3 trials, exercise duration varied (107 +/- 4, 63 +/- 1, and 45 +/- 0 min for 40%, 60%, and 80% peak oxygen uptake, respectively). Substrate oxidation was calculated using indirect calorimetry. Blood samples were collected at the end of exercise to determine plasma epinephrine ([EPI](plasma)) and norepinephrine ([NEPI](plasma)) concentrations. The maximal rate of fat oxidation was achieved at 60% peak oxygen uptake for the TR group (0.41 +/- 0.01 g.min(-1)) and at 40% peak oxygen uptake for the UNTR group (0.28 +/- 0.01 g.min(-1)). TR subjects oxidized absolutely (g.min(-1)) and relatively (% of total energy expenditure) more fat than UNTR subjects at 60% and 80% peak oxygen uptake (p < 0.05). At these exercise intensities, TR subjects also had higher [NEPI](plasma) concentrations than UNTR subjects (p < 0.05). In the heat, whole-body peak fat oxidation occurs at higher relative exercise intensities in TR than in UNTR subjects (60% vs. 40% peak oxygen uptake). Moreover, TR subjects oxidize more fat than UNTR subjects when exercising at moderate to high intensities (>60% peak oxygen uptake).