Plasma amino acid and ammonia responses to altered dietary intakes prior to prolonged exercise in humans.
This study examined the effects of altered dietary intakes on amino acid and ammonia (NH3) responses prior to and during prolonged exercise in humans. Six male recreational cyclists rode to exhaustion at 75% of VO2max following 3 days on a low carbohydrate (LC), mixed (M), or high carbohydrate (HC) diet in a latin square design. There were differences (p less than 0.05) in exercise times among all treatments (58.8 +/- 3.7, 112.1 +/- 7.3, and 152.9 +/- 10.3 min for the LC, M, and HC treatments, respectively). The rate of increase in plasma NH3 during exercise was greater (p less than 0.05) during the LC trial. The LC trial was also characterized by higher (p less than 0.05) resting plasma concentrations of branched chain amino acids (BCAA) and a greater decrease in these amino acids during exercise (p less than 0.05), as compared with the other two treatments. Both plasma BCAA and NH3 were susceptible to dietary manipulations. These findings suggest that limited carbohydrate availability in association with increased BCAA availability results in enhanced BCAA metabolism during exercise. This is reflected in a greater rate of increase in plasma NH3 and is consistent with the hypothesis that a significant fraction of the NH3 released during a prolonged, submaximal exercise bout is from amino acid catabolism.