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This study examined the effects of branched-chain amino acid (BCAA) supplementation on amino acid and ammonia (NH3) responses during prolonged exercise in humans. Seven men cycled for 60 min at 75% of maximal O2 uptake after 45 min of either placebo (dextrose, 77 mg/kg) or BCAA (leucine + isoleucine + valine, 77 mg/kg) supplementation. Plasma samples (antecubital vein) were collected at rest and during exercise and analyzed for plasma NH3 and amino acids, whole blood glucose and lactate, and serum free fatty acids and glycerol. After BCAA administration, plasma BCAA levels increased from 375 +/- 22 to 760 +/- 80 microM (P < 0.05) by the onset of exercise and remained elevated throughout the experiment. Plasma NH3 concentrations increased continually during exercise for both trials and were higher (P < 0.05) after BCAA supplementation than after placebo administration. The mean plasma NH3 increase from rest to 60 min was 79 +/- 10 and 53 +/- 4 microM for BCAA and placebo trials, respectively. Plasma alanine and glutamine concentrations were elevated (P < 0.05) during exercise for both treatments. However, only glutamine concentrations were greater (P < 0.05) for BCAA trial than for placebo trial during exercise. There were no significant differences between treatments for glucose, lactate, free fatty acids, and glycerol or any other plasma amino acid. These data suggest that increased BCAA availability before exercise, when initial muscle glycogen is normal, results in significantly greater plasma NH3 responses during exercise than does placebo administration.
MacLean et al. (Tue,) studied this question.