Acute oral administration of a tyrosine and phenylalanine-free amino acid mixture reduces exercise capacity in the heat

Acute tyrosine administration is associated with increased exercise capacity in the heat. To explore whether reduced plasma tyrosine and phenylalanine (tyrosine precursor) is associated with impaired exercise capacity in the heat, eight healthy, moderately trained male volunteers, unacclimated to exercise in the heat, performed two tests in a crossover design separated by at least 7 days. In a randomised, double-blind fashion, subjects ingested 500 mL flavoured, sugar-free water containing amino acids [(TYR-free; isoleucine 15 g, leucine 22.5 g, valine 17.5 g, lysine 17.5 g, methionine 5 g, threonine 10 g, tryptophan 2.5 g)] to lower the ratio of plasma tyrosine plus phenylalanine:amino acids competing for blood–brain barrier uptake (CAA), a key determinant of brain uptake, or a balanced mixture (BAL:TYR-free plus 12.5 g tyrosine and 12.5 g phenylalanine). One hour later, subjects cycled to exhaustion at 63 ± 5 %
V O2peak in 30 C and 60 % relative humidity. Pre-exercise ratio of plasma tyrosine plus phenylalanine:ΣCAA declined 75 ± 5 % from rest in TYR-free (P<0.001), but was unchanged in BAL(P = 0.061). Exercise time was shorter in
TYR-free (59.8 ± 19.0 min vs. 66.2 ± 16.9 min in TYRfree and BAL respectively; P = 0.036). Heart rate (P =0.298), core (P = 0.134) and skin (P = 0.384) temperature, RPE (P>0.05) and thermal sensation (P>0.05) were
similar at exhaustion in both trials. These data indicate that acutely depleting plasma catecholamine precursors:ΣCAA is associated with reduced submaximal exercise capacity in the heat.