Responses of plasma glutamine, free tryptophan and branched-chain amino acids to prolonged exercise after a regime designed to reduce muscle glycogen.

Prolonged exercise can elicit a reduction in the plasma glutamine concentration and an increase in the plasma concentration ratio of free tryptophan (FTrp) to branched-chain amino acids (BCAA). The purpose of this investigation was to study the effects of a 60-min bout of vigorous treadmill running with dietary manipulation on plasma concentrations of glutamine, FTrp and BCAA after an exercise and diet regime designed to reduce muscle glycogen. Seven male distance runners [mean (SD) age: 29.3 (2.1) years; VO2max: 62.7 (3.3) ml.kg-1.min-1] acted as subjects. Each undertook a regime designed to reduce muscle glycogen, then performed a 60-min treadmill run (75% VO2max) under two dietary conditions: after a 14-h fast (fasted) and after ingestion of a high carbohydrate meal (30 kJ.kg-1: 80% carbohydrate, 10% protein, 10% fat) 3 h before running (fed). Plasma concentrations of glutamine, FTrp, BCAA, free fatty acids (FFA), glycerol and glucose were measured 5 min before and 5 min after the run, under each dietary condition. Plasma glutamine did not change in response to exercise when fasted (P > 0.05), but increased when fed (P = 0.007). Plasma FTrp increased under both dietary conditions (P < 0.001), but the magnitude of this increase was greater when fasted than when fed (P < 0.001). Plasma BCAA did not change under either dietary condition (P < 0.05). Increases in the plasma FTrp/BCAA ratio reflected increases in plasma FFA and glycerol concentrations (P < 0.001; both dietary conditions) and these changes were all greater under fasted conditions when a fall in blood glucose concentration was observed (P = 0.007). These data emphasise the importance of dietary carbohydrate intake between repeated bouts of prolonged exercise on responses of plasma glutamine, FTrp and BCAA during subsequent exercise.