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 (V over dot(O2,max)) in an environmental chamber at either 20 degrees C and 20 % relative humidity (RH) (T-20) or 3 degrees C and similar to 50 % RH (T-3). The trials were conducted in random order at least 1 week apart. Mean oxygen comsumption (V over dot(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 T-20 compared with T-3. Heart rate, rectal temperature and plasma catecholamines were higher (P < 0.05) during exercise at T-20 compared with T-3, as was post-exercise muscle temperature (P < 0.01). Muscle and blood lactate and blood glucose concentrations were not significantly different when comparing T-20 with T-3. Net muscle glycogen utilization was greater (P < 0.05) at T-20 compared with T-3. 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.