Effects of a serotonin re-uptake inhibitor and oral amino acid supplementations on physical and mental performance as well as neuroendocrine variables were investigated. 10 male subjects cycled in four trials until exhaustion. Participants ingested a placebo in trial (T) I, 20 mg paroxetine in T II, 21 g branched-chain amino acids (BCAA) in I Ill and 20 g tyrosine (TYR) in T IV. Heart rate, capillary lactate, plasma insulin, free fatty acids, glucose, serotonin and fI-endorphine did not differ in trials. Plasma ammonia increments during exercise were higher in T Ill. Plasma BCAA in T Ill and plasma TYR in T IV were increased after 30 mm of exercise according to the supplemented substances. In contrast to all other trials, the ratio of plasma free TRP/BCAA did not increase in T Ill. Plasma TYR/BCAA was augmented in I IV and decreased in T III after 30 mm of exercise, whereas it did not change in I I and II. Plasma prolactin (PRL), growth hormone, cortisol, adrenocorticotropic hormone, norepinephrine and epinephrine increased during all trials. Plasma PRL increments were higher in I IV. Exhaustion was reached earlier in I II. No significant differences were found between other trials. Drive during psychometric testing subsequent to exercise was improved in I III and IV. The results indicate that fatigue during endurance exercise was increased by pharmacological augmentation of the brain serotonergic activity. However, a reduction of 5-HI synthesis via BCAA supplementation did not affect physical fatigue. TYR administration did not alter physical performance either although plasma PRI increments suggest that changes in the monoaminergic system were induced. Precaution is necessary before assuming an ergogenic value of amino acids.

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