Effects of Muscle CHO-Loading Manipulations on Hormonal Responses During Prolonged Exercise

Abstract

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 V̇O_2max (1) a,fter normal CHO intake (CHO_N) and (2) after leg muscle CHO loading (CHO_N), both of these procedures being followed by a period of arm exercise (70 min; 70% arm V̇O_2max) and 24h CHO-poor intake. CHO_N, as compared to CHO_L condition, resulted in greater blood concentrations of free fatty acids (1.8 vs 1.3 mmol·L”^-1), glycerol (0.41 vs 0.28 mmol·L^-1), norepinephrine (2.2 vs 1.5 ng·ml^-1), epinephrine (0.90 vs 0.27 ng·ml^-1), and Cortisol (47 vs 23 g·dl_-M at min 70 of exercise. Insulin concentrations during exercise showed a strong tendency to be lower in CHO_N than in CHO_L 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.

Key words

muscle glycogen - norepinephrine - insulin - free fatty acids - glycemia - prolonged exercise in humans