Effect of ambient temperature on protein breakdown during prolonged exercise.

Male subjects (n = 8) cycled for 90 min in 5, 20, and 30 degrees C environments. Rectal (Tre), chest, and thigh temperatures, O2 consumption (VO2), respiratory exchange ratio (R), and venous concentrations of glucose, free fatty acids (FFA), urea N, lactic acid (LA), norepinephrine (NE), epinephrine (E), and cortisol (C) were measured before, during, and after exercise. Urea N excretion was measured in 72 h of nonexercise, in 72 h of exercise (exercise day + 2 post-exercise days) urine samples, and in exercise sweat. Calculated 72-h protein utilization (means +/- SE) was significantly greater (P less than 0.05) for the 5 (86.9 +/- 27.1 g) and 20 (82.9 +/- 22.7 g) compared with 30 degrees C (34.01 +/- 19.1 g) trial. Regardless of ambient temperature exercise increased the venous concentration of C, E, and NE. These catabolic hormones were greatest in 5, lowest in 20, and intermediate in 30 degrees C. Exercise Tre and VO2 were greatest in the 30 degrees C environment. Venous FFA concentration was significantly higher and R significantly lower in 5 vs. 20 or 30 degrees C, and venous LA concentration was significantly greater in 30 vs. 20 or 5 degrees C. Although these results indicate that exercise protein breakdown is affected by ambient temperatures, the mechanism of action is not due solely to circulating NE, E, and C. Differences in venous FFA and LA across environmental temperatures suggest that alterations in carbohydrate and fat metabolism may have contributed to the observed variable protein utilization.