title: Failure of oral tyrosine supplementation to improve exercise performance in the heat.
abstract: OBJECTIVE
  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
  After familiarization, seven male endurance-trained volunteers, unacclimated to exercise in the heat, performed two experimental trials in 30°C (60% relative humidity) in a crossover fashion separated by at least 7 d. Subjects ingested 150 mg·kg(-1) body mass tyrosine (TYR) or an isocaloric quantity of whey powder (PLA) in 500 mL of sugar-free flavored water in a randomized, double-blind fashion. Sixty minutes after drink ingestion, the subjects cycled for 60 min at 57% ± 4% peak oxygen uptake (VO2peak) and then performed a simulated cycling time trial requiring completion of an individualized target work quantity (393.1 ± 39.8 kJ).
  
  
  RESULTS
  The ratio of plasma tyrosine plus phenylalanine (tyrosine precursor) to amino acids competing for brain uptake (free-tryptophan, leucine, isoleucine, valine, methionine, threonine, and 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 preexercise (P = 0.004). Time-trial power output (P = 0.869) and performance (34.8 ± 6.5 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: 1Department of Sport and Exercise Science, Aberystwyth University, Wales, UNITED KINGDOM; 2Endurance Research Group, Centre for Sports Studies, University of Kent at Medway, Chatham, UNITED KINGDOM; and 3Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Wales, UNITED KINGDOM.
authors.forename: Les
authors.initials: L
authors.lastname: Tumilty
authors.forename: Glen
authors.initials: G
authors.lastname: Davison
authors.forename: Manfred
authors.initials: M
authors.lastname: Beckmann
authors.forename: Rhys
authors.initials: R
authors.lastname: Thatcher
mesh_terms: D000284:Administration, Oral; D000328:Adult; D000596:Amino Acids / Q000097:blood; D001628:Beverages; D001642:Bicycling / Q000502:physiology; D001831:Body Temperature; D018592:Cross-Over Studies; D019587:Dietary Supplements; D004311:Double-Blind Method; D017079:Exercise Tolerance / Q000502:physiology*; D006339:Heart Rate; D006358:Hot Temperature; D006801:Humans; D008297:Male; D010649:Phenylalanine / Q000097:blood; D012881:Skin Temperature; D013697:Thermosensing / Q000502:physiology; D014443:Tyrosine / Q000008:administration & dosage* / Q000097:blood; D055815:Young Adult
chemical_list: D000596:Amino Acids; D014443:Tyrosine; D010649:Phenylalanine