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 blinded fashion 90 min before cycling for 2 h at 65% of their maximal oxygen consumption. Exercise-induced glycogen depletion in the thigh muscle was noninvasively measured by means of 13C nuclear magnetic resonance spectroscopy (NMR) spectroscopy, and plasma concentrations of substrates and neuroendocrine hormones, including beta-endorphins, were also assessed. Muscle glycogen content was increased 140% above normal values on the caffeine trial day (P < 0.001). After cycling for 2 h, caffeine ingestion was associated with a greater increase in plasma lactate (caffeine: +1.0 +/- 0.2 mmol/L; placebo, +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 fatty acid concentrations increased (caffeine, +814 +/- 133 mmol/L; placebo, +785 +/- 85 mmol/L; P = NS), and muscle glycogen content decreased (caffeine, -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 levels almost doubled (from 30 +/- 5 to 53 +/- 13 pg/mL; P < 0.05) in the caffeine-treated group, whereas no change occurred in the placebo group. We conclude that caffeine ingestion 90 min before prolonged exercise does not exert 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.