Effects of caffeine on muscle glycogen utilization and the neuroendocrine axis during exercise

To examine the effect of caffeine ingestion on muscle glycogen utilization and the neuroendocrine axis during exercise, we studied 20 muscle glycogen- loaded subjects who were given placebo or caffeine (6 mg/kg) in a double bl inded fashion 90 min before cycling for 2 h at 65% of their maximal oxygen consumption. Exercise-induced glycogen depletion in the thigh muscle was no ninvasively measured by means of C-13 nuclear magnetic resonance spectrosco py (NMR) spectroscopy, and plasma concentrations of substrates and neuroend ocrine hormones, including beta-endorphins, were also assessed. Muscle glyc ogen content was increased 140% above normal values on the caffeine trial d ay (P < 0.001). After cycling for 2 h, caffeine ingestion was associated wi th a greater increase in plasma lactate (caffeine: +1.0 +/- 0.2 mmol/L; pla cebo, +0.1 +/- 0.2 mmol/L; P < 0.005), epinephrine(caffeine, +223 +/- 82 pg /mL; placebo, +56 +/- 26 pg/mL; P < 0.05), and cortisol (caffeine, +12 +/- 3 mg/mL; placebo, +2 +/- 2 mg/mL; P < 0.001) levels. However, plasma free f atty acid concentrations increased (caffeine, +814 +/- 133 mmol/L; placebo, +785 +/- 85 mmol/L; P = NS), and muscle glycogen content decreased (caffei ne, -57 +/- 6 mmol/L, muscle; placebo, - 53 +/- 5 mmol/L muscle; P = NS) to the same extent in both groups. At the same time, plasma beta-endorphin le vels almost doubled (from 30 +/- 5 to 53 +/- 13 pg/mL; P < 0.05) in the caf feine-treated group, whereas no change occurred in the placebo group. We co nclude that caffeine ingestion 90 min before prolonged exercise does not ex ert a muscle glycogen-sparing effect in athletes with high muscle glycogen content. However, these data suggest that caffeine lowers the threshold for exercise-induced beta-endorphin and cortisol release, which may contribute to the reported benefits of caffeine on exercise endurance.