Branched Chain Amino Acids (BCAA)

Branched-chain amino acid (BCAA) supplements are popular with athletes these days. One popular idea is that the three BCAAs - valine, leucine, and isoleucine - can move through the blood to the brain and decrease the production of serotonin in the brain's interior, thereby lowering mental fatigue (serotonin can create a sense of fatigue). A fairly limited amount of scientific research supports this hypothesis.
However, new studies cast doubt on an ergogenic role for BCAAs. 10 subjects cycled until exhaustion at 70-75 per cent of maximal power output while ingesting sugary sports drinks. On one occasion, nothing had been added to the beverage, but during other trials the drink was laced with either a small amount of BCAAs, a large quantity of BCAAs, or a modest amount of 'L-tryptophan,' an amino acid which is used to synthesise serotonin.
Tryptophan boosted blood levels of the chemical by 500 per cent, and consumption of ample amounts of BCAAs quintupled BCAA -blood concentrations, but neither supplement had any effect on performance; time to exhaustion averaged about 120 minutes on all four trials.
A second study confirmed the idea that BCAAs have little impact on endurance. At the University of Cape Town, eight endurance-trained cyclists pedalled away for four hours in the laboratory at 70 per cent of maximal heart rate and then tried to cover 40 kilometres as quickly as possible.
Use of the BCAAs didn't help the athletes at all; time for the 40-K trial averaged about 68 minutes, regardless of whether BCAAs were ingested, and perceived exertion (how the effort actually felt) was the same in all three trials.
However, a final study detected a possibly positive role for BCAAs. Subjects completed two trials in which they exercised their quadriceps muscles continuously for 60 minutes. Prior to only one of the trials, the athletes ingested 77 mg of BCAAs per kilogram of body weight (about a five-gram dose for a 154-pound individual).
As a result of the pre-workout BCAA supplementation, degradation of proteins inside the quadriceps muscles was considerably reduced during exercise. This is a potentially positive effect, since proteins broken down during exercise must be replaced before subsequent workouts so that an athlete can continue training at a high level.
In previous studies carried out over the last year or so, BCAA supplementation has been linked with better immune-system functioning and superior preservation of muscle mass in athletes undergoing heavy training. Therefore, although an acute dose of BCAAs probably won't directly boost endurance during a competitive event, it's possible that taking in three to five grams of BCAAs before and after strenuous workouts might produce some long-term benefits in serious endurance athletes.
To sum up; BCAAs taken at or around the time of competition have little immediate benefit; but BCAA taken on a regular basis, before and probably after strenuous exercise (the type usually done by bodybuilders and strength athletes) will help recovery and thus speed up muscle growth in that way.