Blunting the rise in body temperature reduces muscle glycogenolysis during exercise in humans

To examine the effect of blunting the rise in body temperature on exercise metabolism, seven endurance-trained men cycled for 40 min at 65% of maximal oxygen consumption (V̇(O₂)max) in an environmental chamber at either 20°C and 20% relative humidity (RH) (T₂₀) or 3°C and ~ 50% RH (T₃). The trials were conducted in random order at least 1 week apart. Mean oxygen comsumption (V̇(O₂)) during exercise was not different when comparing the two trials. In contrast, the mean respiratory exchange ratio (RER) was lower (P < 0.05) at T₂₀ compared with T₃. Heart rate, rectal temperature and plasma catecholamines were higher (P < 0.05) during exercise at T₂₀ compared with T₃, as was post-exercise muscle temperature (P < 0.01). Muscle and blood lactate and blood glucose concentrations were not significantly different when comparing T₂₀ with T₃. Net muscle glycogen utilization was greater (P < 0.05) at T₂₀ compared with T₃. These results suggest that glycogenolysis in contracting skeletal muscle is reduced during exercise when the rise in body core temperature is attenuated. These changes in carbohydrate metabolism appear to be influenced by alterations in muscle temperature and/or sympatho-adrenal activity.