title: Aerobic fitness determines whole-body fat oxidation rate during exercise in the heat.
abstract: 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 °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).
authors.affiliation: Exercise Physiology Lab at Toledo, Universidad de Castilla-La Mancha, Avda. Carlos III, s/n, Toledo, 45071, Spain.
authors.forename: Juan
authors.initials: J
authors.lastname: Del Coso
authors.forename: Nassim
authors.initials: N
authors.lastname: Hamouti
authors.forename: Juan Fernando
authors.initials: JF
authors.lastname: Ortega
authors.forename: Ricardo
authors.initials: R
authors.lastname: Mora-Rodriguez
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D001642:Bicycling; D002153:Calorimetry, Indirect; D004734:Energy Metabolism; D004837:Epinephrine / Q000097:blood; D015444:Exercise / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D050356:Lipid Metabolism; D009638:Norepinephrine / Q000097:blood; D010084:Oxidation-Reduction; D010101:Oxygen Consumption; D010807:Physical Endurance / Q000502:physiology; D010809:Physical Fitness / Q000502:physiology*; D013997:Time Factors; D055815:Young Adult
chemical_list: D009638:Norepinephrine; D004837:Epinephrine