Aim: This study examined the effects of a hot environment on metabolic responses, thermoregulation, and performance during simulated cycling and running in triathletes. Methods: Seven male triathletes completed two trials in moderate (22±0.2 °C, 76±2% relative humidity, M) and hot (31.2±0.2 °C, 76.4±1.6% relative humidity, H) environmental conditions separated by at least 7 days. The subjects were required to complete a self-paced 40 km simulated cycling, followed by a 10 km run on a treadmill for as fast as possible in both trials. Results: The overall performance time was faster in M than H (M vs H, 119±6 min vs 127±6 min, P<0.01). Moreover, there were no differences in the cycling time between the two trials, but the run time was faster in M (M vs H, 51±4 min vs 59±5 min, P<0.05) than in H. Ad libitum water consumption was higher in H than in M (H vs M, 970±231 mL×min-1 vs 547±131 mL×min-1 P<0.05), and the mean skin temperature was also higher in H than in M throughout the exercise (H vs M, 35.3±0.1 vs 33.3±0.1 °C, P<0.05). However, there were no differences in rectal temperature, blood lactate, blood glucose, body mass change, plasma volume change, osmolality, carbohydrate oxidation, and fat oxidation between the trials. Conclusion: The results suggested that triathletes reduced their running performance after a 40 km simulated cycling when the ambient temperature was high.
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