Branched-chain amino acids (BCAAs) include the amino acids leucine, isoleucine, and valine. All three are considered essential amino acids; they cannot be made in the body and must be consumed in the diet from protein-rich foods. BCAAs account for 35% of the essential amino acids found in muscle proteins. BCAAs are unique in that they can be oxidized in skeletal muscle, whereas other essential amino acid are mainly catabolized, or broken down, in the liver.

BCAAs and especially leucine are key stimulators of protein synthesis or muscle building and also play a role in the prevention of muscle breakdown. BCAAs are unique from other amino acids in that they can be oxidized in the muscle for fuel. As a result, it is thought that BCAAs can be used as fuel during exercise, producing performance benefits especially in longer-duration endurance sports.

Because of the role BCAAs have in the regulation of protein synthesis and protein breakdown, it is believed that they can be used to prevent the catabolic effects of exercise when supplemented before or during training and used to enhance muscle building or protein synthesis when supplemented post-exercise. In addition, many believe that BCAAs can have performance-enhancing effects due to their ability to be oxidized and used for fuel by the muscles. This belief has prompted many endurance athletes to supplement with BCAA before and during competition or training.

There is a significant amount of evidence that BCAAs can prevent protein breakdown and muscle damage during exercise. Many studies have revealed that BCAAs supplemented before training can decrease markers of muscle damage such as creatine kinase and lactate dehydrogenase for both endurance- and resistance-trained athletes. Interestingly, it appears that this effect is less consistent for older athletes versus younger athletes. Supplementation with BCAAs after exercise also produces beneficial effects by stimulating muscle building and recovery.

Research related to the performance-enhancing effects of BCAAs for endurance athletes is less promising, and most studies show no additional benefit from consuming. While immediate performance improvements are not seen, the benefits of decreased muscle damage and improved rebuilding or recovery after exercise could produce performance improvements over time. In addition, BCAAs appear to limit immune suppression resulting from exercise. BCAAs limit reductions of serum glutamine, an important nutrient for immune health often lowered as a result of intense and prolonged training. Questions still remain in terms of the effectiveness of BCAAs over the use of whole proteins. Since whole proteins, most notably whey protein, contain sufficient quantities of BCAAs, and in particular leucine, it does not appear that supplementation with BCAAs before or after training provides additional benefits if adequate amounts of high-quality whole proteins are consumed. A relatively new application of BCAAs in athletes is during recovery from injury. Early research appears positive that BCAAs can limit muscle wasting or atrophy associated with immobilization and decreases in physical training.

A variety of supplementation protocols have been used with BCAAs. Most protocols use a higher dosage of leucine and smaller dosages of valine and isoleucine. Protocols have ranged from 6 to 14 g of BCAAs per day in a ratio of leucine:valine:isoleucine of 2 or 3:1:1. No adverse effects of BCAA toxicity have been reported in relation to exercise and sport.