Title: EFFECTS OF CARBOHYDRATE AND L-TYROSINE INGESTION ON ENDURANCE EXERCISE PERFORMANCE
Abstract: It has been hypothesized that brain concentrations of serotonin and dopamine may play a role in the onset of fatigue during long-term endurance exercise. Alterations of plasma tyrosine, the precursor amino acid to dopamine, might affect central levels of dopamine and, consequently, endurance exercise performance. PURPOSE: To test the effects of tyrosine ingestion with or without carbohydrate supplementation on endurance performance. METHODS: Nine competitive cyclists cycled at 70% VO2peak for 90 min under four different feeding conditions followed immediately by a time trial. Each subject consumed either 5 mlkg-1 body weight of water sweetened with aspartame [placebo (PLA)], polydextrose (70 gL-1) (CHO), L-tyrosine (25 mgkg-1 bw) (TYR), or polydextrose (70 gL-1) and L-tyrosine (25 mgkg-1 bw) (CHO+TYR) every 30 min prior to and during exercise. The experimental trials were given in random order and carried out using a counterbalanced double blind design. RESULTS: VO2, RPE, and blood lactate were not different at any time during the experimental trials. After 15 min into the time trial, RER was higher in CHO+TYR versus all other trials and higher at test termination compared with TYR and PLA (p < 0.05). Heart rate was higher at test termination in both CHO groups versus PLA (p < 0.05). In TYR and CHO+TYR, plasma tyrosine and the tyrosine/free tryptophan ratio rose continuously from 60 min prior to exercise to test termination (p < 0.05). Plasma free tryptophan was markedly suppressed in CHO and CHO+TYR versus TYR and PLA during exercise (p < 0.05). A significant correlation existed between plasma FFA and plasma free tryptophan for all groups combined (r = 0.70, p < 0.001). CHO and CHO+TYR treatments resulted in a reduced time to complete the endurance time trial compared to PLA (p < 0.05). CONCLUSION: These findings demonstrate that carbohydrate ingestion enhances endurance cycling performance following 90 minutes of steady state exercise and that tyrosine ingestion, either with or without carbohydrates, does not affect cycling performance. Supported by the Gatorade Sports Science Institute and Nature's Sunshine, Inc.