Ambient temperature influences metabolic substrate oxidation curves during running and cycling in healthy men

Fat oxidation in cold environments and carbohydrate (CHO) use in hot environments are increased during exercise at steady-state submaximal workloads. However, the influence of cold and heat on fat and CHO oxidation curves remain unknown. We therefore examined the influence of a cold and warm ambient temperature on fat and CHO oxidation across a wide range of exercise intensities during treadmill and cycle ergometer exercise. Nine, young, healthy, male subjects completed four trials, during which they performed an incremental peak oxygen consumption (⩒O_2peak) test on a cycle ergometer or treadmill in a 4.6°C or 34.1°C environment. Substrate oxidation, maximal fat oxidation rate (MFO), and exercise intensity where MFO occurs (Fat_max) were assessed via indirect calorimetry. MFO was significantly greater in the cold vs. warm during the treadmill exercise (0.66 ± 0.31 vs. 0.43 ± 0.23 g min^-1; p = 0.02) but not during cycling (0.45 ± 0.24 vs. 0.29 ± 0.11 g min^-1; p = 0.076). MFO was also greater during treadmill vs. cycling exercise, irrespective of ambient temperature (0.57 g min^-1 vs. 0.37 g min^-1; p = 0.04). Fat_max was greater in the cold vs. warm for both treadmill (57 ± 20 vs. 37 ± 17%⩒O_2peak; p = 0.025) and cycling (62 ± 28 vs. 36 ± 13%⩒O_2peak; p = 0.003). Multiple, linear, mixed-effects regressions revealed a strong influence of ambient temperature on substrate oxidation. We demonstrated that exercising in a cold environment increases MFO and Fat_max, predominantly during treadmill exercise. These results validate the implication of ambient temperature on energy metabolism over a wide range of exercise intensities.