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Decreased energy expenditure during prolonged sub-maximal exercise in a warm environment

Abstract The purpose of this study was to investigate the influence of a warm environment on thermoregulation and energy expenditure during sub-maximal prolonged exercise in humans. Six healthy male subjects cycled for 120 min at an intensity of 60% maximal oxygen uptake (Vo2max) at three environmental temperatures (10°C, CT; 20°C, MT; and 30°C, WT). Although oxygen uptake at WT showed a significantly lower value compared to those at MT and CT, no significant differences of respiratory exchange ratio were observed among the three environmental trials. A remarkable decrease in total energy expenditure during the 120-min exercise at WT was observed in comparison with those at MT and CT (p<0.05). Changes in rectal temperature, mean skin temperature, and mean body temperature at WT were significantly higher than those at both MT and CT. Although increases in mean body temperature from rest every five minutes during exercise were not different among three environmental temperatures, mean energy expenditures every five minutes at WT were lower compared with those at MT and CT (p<0.05). These results suggest that the increase in energy expenditure for physical exertion is substantially reduced during prolonged sub-maximal exercise in a warm environment. This acute alteration in the energy metabolism may contribute to inhibition of excess heat production and enable prolonged exercise in a warm environment.