Branched-chain amino acid supplementation increases the BCAA:tryptophan ratio during prolonged exercise, which reduces central fatigue by limiting tryptophan transport into the brain and subsequent serotonin synthesis, but this effect does not consistently improve endurance performance.

When someone takes branched-chain amino acid supplements during long exercise, the levels of these amino acids in the blood rise. This causes them to compete with another amino acid called tryptophan for entry into the brain. Less tryptophan gets into the brain, so the brain makes less serotonin, a chemical linked to feeling tired. As a result, the person feels less mentally exhausted during the exercise.