Influence of a 36 h fast followed by refeeding with glucose, glycerol or placebo on metabolism and performance during prolonged exercise in man.

Five men were studied during exercise to exhaustion on an electrically braked cycle ergometer at 70% of VO2max. The four experimental treatments were as follows: fasted for 36 h (A); fasted (36 h) and refed with glucose (B) or glycerol (C); postabsorptive (overnight fast, D). In B and C the subjects were given a drink containing glucose or glycerol (1g per kg body weight) 45 min before starting exercise. A placebo drink was given 45 min before exercise on treatments A and D. Despite an increased availability of circulating free fatty acids, beta-hydroxybutyrate and glycerol exercise time to exhaustion was significantly lower after fasting (treatment A 77.7 +/- 6.8 min) compared with treatment D (119.5 +/- 5.8 min). Refeeding with glucose or glycerol did not significantly improve performance (92.4 +/- 11.8 min and 80.8 +/- 3.6 min respectively) compared with treatment A and lowered circulating levels of FFA and beta-HB during exercise compared with A. Despite the probability of low liver glycogen levels after fasting, none of the subjects became hypoglycaemic (blood glucose less than 4 mmol.l-1) during exercise and their blood lactate concentrations were not high at exhaustion. Plasma levels of branched chain amino acids (BCAA) decreased progressively during exercise on treatments A, B and C and were considerably lower at exhaustion compared with treatment D. Falling plasma concentrations of BCAA during prolonged exercise may be implicated in the generation of central fatigue.