The branched-chain amino acids are a group of three essential amino acids: leucine, valine, and isoleucine. The recommended dietary intake for BCAAs is about 3 g/day, an amount that should be easily obtained from protein foods. Supplemental levels of around 5-20 g/day have been used to increase endurance, delay fatigue, and improve mental performance during prolonged exercise.

For endurance athletes, particularly those competing in longer races (more than 2 hours) such as marathons, triathlons, road cycling, backpacking, and orienteering, BCAA supplements can help delay central fatigue and maintain mental performance. Participation in shorter-duration events is unlikely to result in substantial changes in blood levels of BCAAs, tryptophan, or fatty acids, so BCAA supplementation is not needed.

The idea behind BCAA supplements relates to a phenomenon known as central fatigue, which holds that mental fatigue in the brain can adversely affect physical performance in endurance events. The central fatigue hypothesis suggests that low blood levels of BCAAs may accelerate the production of serotonin, a key neurotransmitter in the brain, and prematurely lead to fatigue (Blomstrand, 1988). Tryptophan, an amino acid that circulates in the blood, is a precursor of serotonin and can be more easily transported into the brain to increase serotonin levels when BCAA levels in the blood are low, because high blood levels of BCAAs can block tryptophan transport into the brain (Castell, 1999). During exercise, as muscle and liver glycogen are depleted for energy, blood levels of BCAAs also decrease while fatty acid levels increase to serve as an additional energy source (Davis, 1995). The problem with extra fatty acids in the blood is that they need to attach to albumin as a carrier protein for proper transport. In doing so, the fatty acids displace tryptophan from its place on albumin and facilitate the transport of tryptophan into the brain for conversion into serotonin (Hassmen, 1994). Therefore, the combination of reduced BCAAs and elevated fatty acids in the blood causes more tryptophan to enter the brain and more serotonin to be produced, leading to central fatigue (Tanaka, 1997). Supplementing the diet with additional levels of BCAA is thought to block the tryptophan transport and therefore delay fatigue (Blomstrand, 1997).

In addition to their effects on prolonging endurance and delaying central fatigue, BCAA supplements have been associated with a reduced rate of protein and glycogen breakdown during exercise and an inhibition of muscle breakdown following exhaustive exercise (Blomstrand, 1991; Gastmann, 1998).

Although the general theory of central fatigue and BCAA supplementation is sound, not all research findings have been positive. In general, acute BCAA supplementation (immediately before or during exercise) has been shown to increase mental performance, improve cycling endurance, and reduce the time to complete a marathon (Davis, 1999; Hassmen, 1994). Chronic BCAA supplementation (2 weeks) has also been shown to be effective in improving time-trial performance in trained cyclists (Mittleman, 1998). A number of studies in trained and untrained subjects, however, have shown no effect of BCAA supplements on exercise performance or mental performance (van Hall, 1995). In some cases, BCAAs have been compared with carbohydrate supplementation during exercise, with results showing that both delay fatigue to similar degrees (Davis, 1992).

Supplemental intakes of BCAAs have been studied in the range of 3-25 g/day in tablet and liquid form with no adverse side effects. Higher intakes should be avoided because of the possibility of competitive inhibition of the absorption of other amino acids from the diet and the risk of gastrointestinal distress. Effective doses are in the range of 3-20 g/day taken before or during exercise for delaying fatigue or immediately following exercise as an aid to recovery.

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