GLUTAMINE SUPPLEMENTATION: EFFECTS ON GROWTH HORMONE AND TIME-TRIAL PERFORMANCE AFTER PROLONGED EXERCISE

Abstract

Dietary glutamine supplementation has been proposed as a potential aid to athletic performance. The purpose of the investigation was to determine if glutamine supplementation might improve cycling performance, perhaps by stimulating growth hormone release, in athletes consuming a high-carbohydrate diet. Six trained young adult men participated in two four-day supplementation trials. In both trials, the subjects underwent an identical carbohydrate-loading regimen of controlled diet (75% carbohydrate) and exercise. The only difference in the trials was whether the subjects ingested dietary supplements of glutamine or glucose placebo. During the first three days of each trial, the diets were supplemented with either glutamine or glucose placebo at a dosage of 150 mg/kg body weight (~10.4 g/d). On the fourth day, the supplements were dissolved in a sports drink (4 g/L). The sports drink initially contained a 6% solution of carbohydrate. On the fourth day the subjects ingested 7 ml/kg of the supplemented sports drink 2 h before exercise, 5 ml/kg 1 h before exercise, and 3 ml/kg after 15, 30, 45, 60, and 90 min of exercise. The total amount of glutamine supplement or placebo consumed on the fourth day was 108 mg/kg or about 7.4 g, of which 3.3 g were consumed before and 4.1 g during exercise. On the fourth day, subjects completed 90 min of cycle ergometry, divided into six 15-min periods, each of which included in sequence 12 min @75% VO2max, 1 min @ 120% VO2max, and 2 min @ 50% VO2max. The 90 min exercise session was immediately followed by 15 min of rest and then a final 20-km time trial at ~85% VO2max. Plasma concentrations of glycerol and free fatty acids and serum growth hormone concentrations were measured before and during exercise, and time-trial performance was recorded. Times (± SEM) to complete the 20-km performance rides were 31.03 ± 0.58 min for the placebo trial and 30.93 ± 0.60 min for the glutamine trial and were not significantly different (P>0.05). There were also no significant effects on serum growth hormone or on plasma glycerol or free fatty acids attributable to glutamine supplementation at any sampling time. In summary, we found no evidence that glutamine supplementation in moderate dosages has any effect on circulating growth hormone, glycerol, or free fatty acids or on time-trial performance in athletes who consume a high-carbohydrate diet