You may have read that the amino acid supplement is an effective endurance booster. However, a study conducted at Brigham Young University (BYU) and published in the November 2002 issue of the Journal of Applied Physiology showed it to be practically inconsequential for improving endurance.
Tyrosine travels through the same portal to the brain that tryptophan (the amino acid in turkey known to make people drowsy after Thanksgiving dinner) travels. Consequently, some scientists have thought that it could block the action of tryptophan so one might not feel fatigue. Allen Parcell, assistant professor in the Human Performance Research Center at BYU, and colleagues tested this idea by standardizing the diets and exercise regimens of nine cyclists; giving each cyclist a sports drink containing a placebo, tyrosine, the carbohydrate polydextrose or a combination of tyrosine and polydextrose; and then conducting time trials. The cyclists who consumed only tyrosine experienced no change in their endurance. “There wasn’t any indication from our tests that tyrosine had an effect in the blood or in the brain,” Parcell said. “Tyrosine didn’t improve endurance performance in our subjects.”
On the other hand, those who consumed the carbohydrate did experience improved endurance. Of course, the time trials lasted only 2 1/2 hours; a previous study on humans indicated that tyrosine might affect endurance after 5 or more hours of physical activity. Nonetheless, unless you’re a triathlete, opt for more traditional aids, such as sports drinks and carbohydrate-fortified nutrition bars, to boost your endurance to enhance your workout. “In most cases, a well-balanced diet with an emphasis on carbohydrates has definitely been shown to significantly improve endurance,” Parcell said.