Effect of carbohydrate ingestion on ammonia metabolism during exercise in humans

The present study was undertaken to examine the effect of carbohydrate ingestion on plasma and muscle ammonia (NH3 denotes ammonia and ammonium) accumulation during prolonged exercise. Eleven trained men exercised for 2 h at 65% peak pulmonary oxygen consumption while ingesting either 250 mi of an 8% carbohydrate-electrolyte solution every 15 min (CHO) or an equal volume of a sweet placebo. Blood glucose and plasma insulin levels during exercise were higher in CHO, but plasma hypoxanthine was lower after 120 min (1.7 +/- 0.3 vs. 2.6 +/- 0.1 mu mol/l; P < 0.05). Plasma NH3 levels were similar at rest and after 30 min of exercise in both trials but were lower after 60, 90, and 120 min of exercise in CHO (62 +/- 9 vs. 76 +/- 9 mu mol/l; P < 0.05). Muscle NH3 levels were similar at rest and after 30 min of exercise but were lower after 120 min of exercise in CHO (1.51 +/- 0.21 vs. 2.07 +/- 0.23 mmol/kg dry muscle; P < 0.05; n = 5). These data are best explained by carbohydrate ingestion reducing muscle NH3 production from amino acid degradation, although a small reduction in net AMP catabolism within the contracting muscle may also make a minor contribution to the lower tissue NH3 levels.