Effects of muscle CHO-loading manipulations on hormonal responses during prolonged exercise

This study examined the effects of a pre-experimental period of muscle carbohydrate (CHO)-loading manipulations followed by a 24-h CHO-poor diet, intended to increase muscle glycogen content and reduce hepatic glycogen levels, on substrate and endocrine responses during a period of prolonged exercise. Seven subjects pedaled a cycle ergometer for 70 min at 64% leg VO2max (1) after normal CHO intake (CHON) and (2) after leg muscle CHO loading (CHOL), both of these procedures being followed by a period of arm exercise (70 min; 70% arm VO2max) and 24 h CHO-poor intake. CHON, as compared to CHOL condition, resulted in greater blood concentrations of free fatty acids (1.8 vs 1.3 mmol X L-1), glycerol (0.41 vs 0.28 mmol X L-1), norepinephrine (2.2 vs 1.5 ng X ml-1), epinephrine (0.90 vs 0.27 ng X ml-1), and cortisol (47 vs 23 g X dl-1) at min 70 of exercise. Insulin concentrations during exercise showed a strong tendency to be lower in CHON than in CHOL condition, although the differences were not significant. There were no significant differences between the two conditions in blood glucose, lactate, and glucagon concentrations. These data indicate that muscle CHO-loading manipulations intended to specifically increase the muscle glycogen content are associated with the difference in metabolic adaptation and hormonal changes during exercise.