In general chemical terms, a branched-chain amino acid (BCAA), is any amino acid that contains a branched carbon chain attached to its alpha-carbon atom. All amino acids contain a hydrogen atom, a carboxyl group, and an amino group, all attached to the same central carbon atom, called the alpha-carbon. The alpha-carbon also bears a fourth group, commonly symbolized as the R-group; in branched-chain amino acids, this R-group is a short, Y-shaped, three- or four-carbon chain. The properties of this R-group determine the properties of the amino acid, and thus its biological importance.
As applied to biology and nutrition, the term branched-chain amino acids (BCAAs) refers to the three naturally occurring amino acids that contain a small, branched, three- or four-carbon chain: L-leucine, L-isoleucine, and L-valine. Since all naturally occurring amino acids are of the L variety, the prefix is often dropped and they're commonly referred to simply as leucine, isoleucine, and valine. All three branched-chain amino acids are essential amino acids, meaning they can't be made by the body but must be obtained through food. They are common in most protein sources, however, such as meat, fish, poultry, and dairy, and so are rarely deficient in the normal Western diet.
Aside from their nutritional value in the diet, where they're vital to protein synthesis, branched-chain amino acids are of some interest as nutraceuticals, or foods with pharmaceutical activity, especially among athletes and body-builders. Leucine is known to be important not only as a component of protein, but as a contributing regulator of muscle synthesis. Ingestion of supplemental BCAAs has been shown to increase muscle protein synthesis following a workout, and decrease the post-exercise catabolic breakdown of muscle tissue. BCAAs thus appear to have the ability to stimulate an anabolic, or protein-building, response in muscles.
BCAAs have also been found to reduce delayed onset muscle soreness, or DOMS, the tenderness that appears several days after strenuous workout. Studies have found that a 5 gram pre-exercise supplement composed of isoleucine:leusine:valine in the ratio of 1:2.3:1.2 significantly decreased DOMS following vigorous exercise among subjects unaccustomed to exercise. The reduced soreness purportedly allows more vigorous and intense workouts.
Additional studies suggest that regular use of BCAAs may be associated with an increase of the amount of growth hormone in the blood, which could thereby help increase muscle mass and overall body size. BCAAs also appear to lower the levels of the amino acid tryptophan in the blood. Excess tryptophan is associated with sleepiness and fatigue, so it's been suggested that this tryptophan-lowering effect of BCAAs might enable longer and more strenuous workouts.
It should be noted, though, that at present there is no direct evidence that BCAAs are of unequivocal benefit either in increasing the size and amount of muscle mass or in enhancing athletic performance. More work is needed to establish these facts. Branched-chain amino acids were also of interest some years ago when a pilot study suggested they might be effective in treating the symptoms of amyotrophic lateral sclerosis, or Lou Gehrig's Disease. Follow-up studies failed to confirm this, however, and one even suggested that excess dietary branched-chain amino acids might even increase mortality in such patients.