Purpose: Acute oral tyrosine administration has been associated with increased constant-load, submaximal exercise capacity in the heat. This study sought to determine whether self-paced exercise performance in the heat is enhanced with the same tyrosine dosage.
Methods: Following familiarisation, seven male endurance-trained volunteers, unacclimated to exercise in the heat, performed two experimental trials in 30[degrees]C (60% relative humidity) in a crossover fashion separated by at least 7 days. Subjects ingested 150 mg[middle dot]kg body mass-1 tyrosine (TYR) or an isocaloric quantity of whey powder (PLA) in 500 mL of sugar-free flavoured water in a randomised, double-blind fashion. Sixty minutes following drink ingestion subjects cycled for 60 min at 57 +/- 4% peak oxygen uptake (O2peak), then performed a simulated cycling time-trial requiring completion of an individualised target work quantity (393.1 +/- 39.8 kJ).
Results: The ratio of plasma tyrosine plus phenylalanine (tyrosine precursor): amino acids competing for brain uptake (free-tryptophan, leucine, isoleucine, valine, methionine, threonine, lysine) increased 2.5-fold from rest in TYR, and remained elevated throughout exercise (P < 0.001), whereas it declined in PLA from rest to pre-exercise (P = 0.004). Time-trial power output (P = 0.869) and performance (34.8 +/- 6.5 min and 35.2 +/- 8.3 min in TYR and PLA, respectively; P = 0.4167) were similar between trials. Thermal sensation (P > 0.05), RPE (P > 0.05), core temperature (P = 0.860), skin temperature (P = 0.822) and heart rate (P = 0.314) did not differ between trials.
Conclusion: These data indicate that acute tyrosine administration did not influence self-paced endurance exercise performance in the heat. Plasma tyrosine availability is apparently not a key determinant of fatigue processes under these conditions.
(C) 2014 American College of Sports Medicine