Branched-chain amino acids (BCAAs) are a group of essential amino acids. They are all found in the muscle, accounting for one-third of all amino acids in muscle protein.
l-leucine, l-isoleucine, l-valine.
As with all amino acids, the primary function of BCAAs is as precursors for the synthesis of proteins. In addition, they may be broken down if necessary to serve as an energy source. They may be used directly by skeletal muscle, as opposed to other amino acids which require prior gluconeogenesis in the liver to produce a useful energy source. Muscle tissue appears to demonstrate an increased need for these amino acids during times of intense physical exercise, and there is some evidence that serum BCAA levels fall during exercise.
In addition, because BCAAs are not readily degraded by the liver,theycirculateintheblood and compete with the amino acid tryptophan for uptake into the brain. Tryptophan is a precursor of serotonin (5-hydroxytryptamine), which may produce symptoms of fatigue. It appears that exercise increases the ratio of free tryptophan:BCAA, thus raising serotonin levels in the brain. Some researchers think that supplementation with BCAAs will reduce this ratio and raise serum levels of BCAAs, so improving mental and physical performance.
Researchers have been interested in the potential role of BCAAs in exercise and whether they can improve performance.
A placebo-controlled (not blinded) study involving 193 experienced runners given 16g BCAAs or placebo showed that running performance was improved in the slow runners but not the fast runners with BCAAs. A second part of the study showed that 7.5g BCAAs improved mental performance during exercise compared with placebo, but the study has been criticised for lack of dietary control and poor choice of performance measures.
Another placebo-controlled, but in this case double-blind, study in 16 subjects, participating in a 21-day trek at a mean altitude of 3255m found that supplementation with BCAAs (11.52g) improved indices of muscle loss and concluded that BCAAs could prevent muscle loss during chronic hypoxia of high altitude.
In a double-blind, placebo-controlled study, endurance-trained male athletes were studied during cycle exercise while ingesting in random order drinks containing sucrose, sucrose plus tryptophan, sucrose plus BCAAs (6g) or sucrose plus BCAAs (18g). There were no differences between the treatment groups in time to exhaustion, suggesting that BCAAs did not improve exercise performance. However, BCAAs reduced brain tryptophan uptake and signiﬁcantly increased plasma ammonia levels compared to the control group.
In a double-blind, placebo-controlled, randomised, crossover study, nine well-trained male cyclists performed three laboratory trials consisting of 100km cycling after ingesting either glucose, glucose plus BCAA, or placebo. Neither the glucose nor the BCAAs enhanced performance in these cyclists.
In a further study of seven well-trained male cyclists, perceived exhaustion was 7% lower and ratings of mental fatigue were 15% lower than when they were given placebo, but there was no difference in physical performance. However,the ratio of tryptophan:BCAA,which increased during exercise, remained unchanged or decreased when BCAAs were ingested.
In a double-blind, placebo-controlled trial, six women and seven women participated in a cycle trial in the heat. Cycle time to exhaustion increased with BCAAs, indicating that BCAA supplementation prolongs moderate exercise performance in the heat.
In a further double-blind, placebo-controlled trial, eight subjects performed three exercise trials and were given either carbohydrate drinks, carbohydrate plus BCAAs (7g) or placebo 1hour before and then during exercise.Subjects ran longer with both carbohydrate and carbohydrate plus BCAAs,but there were no differences between the carbohydrate and carbohydrate and BCAA groups, indicating that BCAAs are of no added beneﬁt in exercise.
Other studies have suggested that BCAAs could prevent or decrease the net rate of protein degradation seen in heavy exercise, while others have not. One study indicated that BCAAs might have a sparing effect on muscle glycogen degradation during exercise. There is also some evidence that BCAAs can alter mood and cognitive performance during exercise.
BCAAs can activate glutamate dehydrogenase, an enzyme deﬁcient in amyotrophic lateral sclerosis (ALS). In one double-blind, randomised, placebo-controlled trial of BCAA, supplements helped maintenance of muscle strength and continued ability to walk in such patients.13 However, a larger study was ended early when BCAAs not only failed to cause beneﬁt, but also led to excess mortality. BCAAs have also been investigated for potential value in anorexia and cachexia. A review concluded that they could exert signiﬁcant anti-anorectic and anticachectic effects and that their supplementation may represent a viable intervention for patients suffering from chronic disease such as cancer, chronic renal failure and liver cirrhosis and for patients at risk of muscle wasting due to age, immobility or bed rest.
No problems have been reported, but BCAAs should probably not be used in hepatic and renal impairment without medical supervision. However, BCAAs are used occasionally in patients with these conditions but in a medical setting.
Pregnancy and breast-feeding
No problems have been reported, but there have not been sufﬁcient studies to guarantee the safety of BCAAs in pregnancy and breastfeeding.
None reported, but there are no long-term studies assessing the safety of BCAAs.Large doses of BCAAs (> 20g) may increase plasma ammonia levels and may impair water absorption,causing gastrointestinal discomfort.
None reported. BCAAs compete with aromatic amino acids (e.g. phenylalanine, tyrosine, tryptophan) for transport into the brain.
BCAAs are available in tablet and powder form. The dose is not established.
Dietary supplements provide 7–20g per dose.
Evidence from well-controlled trials shows no beneﬁt of BCAAs in exercise performance. Beneﬁt has been shown mainly in poorly controlled trials.
(extracted from) Dietary Supplements, Third Edition, by Pamela Mason, BSc, MSc, PhD, MRPharmS, published by Pharmaceutical Press, London, 2007.