This study was conducted to quantify fat metabolism among endurance-trained men during high-intensity interval running. Purpose: We: 1) quantified the total energy expenditure and substrate oxidation among well-trained men during a high-intensity interval session and a continuous session matched for average workload, 2) compared relative fat oxidation during a high-intensity interval session and a continuous session to the maximal fat oxidation rate identified by fatmax protocol and 3) compared the energy expenditure after a high-intensity interval session and a continuous session, based on EPOC. Methods: Nine well-trained male runners (VO2max: 68.1 ± 3.6 ml/kg.min) completed preliminary testing followed by an interval session and a continuous session in randomized order. Blood lactate (La-) and blood gasses were quantified throughout. The sessions lasted 48 minutes and were performed at a 1.7% incline. Individual running velocities were calculated corresponding to 40% (rest periods), 65% (continuous session) and 90% (work periods) of VO2max. Interval bouts consisted of 6 x 4 minutes work periods separated by 4 minutes rest periods. Substrate oxidation was calculated from gas exchange with corrections for RER >1.0. Results: Despite identical work, a significant difference was found in energy expenditure between the interval session and the continuous session, 3207 ± 325 kJ vs. 2962 ± 309 kJ, (p < 0.001). Fat oxidation tended to be higher during the continuous session (755 ± 440 kJ) compared with interval exercise (533 ± 219 kJ), but the difference was not significant (p = 0.170). Fatmax occurred at 65 ± 8% of VO2max (0.55 ± 0.17 g/min), while fat oxidation accounted for 25 ± 14% (0.39 ± 0.22 g/min) and 17 ± 8% (0.27 ± 0.11 g/min) during continuous and interval sessions respectively. Blood pH and [HCO3-] stabilized from work bout one to six, (from 7.35 ± 0.03 to 7.35 ± 0.05) and (from 20.8 ± 1.9 to 19.4 ± 3.5 mmol/L) respectively. We found no difference in energy expenditure based on 15 min EPOC between the continuous session (25 ± 28 kJ) and interval session (27 ± 28 kJ). Conclusion: Well-trained runners oxidized significant amounts of fat during high-intensity exercise, equal to ~50% of fat oxidation achieved at fatmax.
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