Scientists have theorized that since branched-chain amino acids (BCAA) are used extensively by muscle tissue for energy during exercise, the subsequent drop in plasma concentrations of BCAA combined with the displacement of tryptophan from albumin by an increase in concentration of free fatty acids can result in perceived exertion and thus premature fatigue. Tryptophan more readily enters the brain when the ratio of tryptophan to BCAA in the blood is elevated. Serotin production increases when tryptophan enters the brain, which may produce the sensation of fatigue and decrease an individual’s ability to send a signal from the brain to the muscle for contraction. This process has been referred to as the central fatigue theory.