L-Tyrosine Not Effective During Short-Term Exercise
PROVO, Utah--In a study appearing in the November issue of the Journal of Applied Physiology (93:1590-7, 2002) (http://jap.physiology.org), researchers found that L-tyrosine supplementation may not enhance endurance during regular bouts of exercise.
Researchers from the Human Performance Research Center at Brigham Young University (BYU) tested nine competitive cyclists' reactions to ingesting 5 ml/kg body weight of water containing: 1) aspartame, 2) 70 g/l of polydextrose, 3) 25 mg/kg of L-tyrosine or 4) 70 g/l of polydextrose and 25 mg/kg of L-tyrosine. The cyclists cycled at 70-percent peak oxygen uptake for 90 minutes under the four different dietary regimens, consuming the allotted mixture at 30-minute intervals. Even though tyrosine plasma levels rose significantly from 60 minutes prior to the test to the end of the test, L-tyrosine ingestion did not enhance performance during cycling.
According to Allen Parcell, a study author, tyrosine's effect on endurance may kick in after physical activity of five or more hours; the BYU tests lasted no longer than 2.5 hours. "In most cases, a well-balanced diet with an emphasis on carbohydrates has definitely been shown to significantly improve endurance," Parcell noted. "Maybe there's a case to pursue [L-tyrosine supplementation] with some ultra-endurance, really long-term exercise."
The study was funded in part by the Gatorade Sports Science Institute.