Branched-chain amino acids (BCAA) are among the nine essential amino acids for humans . Among the proteinogenic amino acids (precursors to proteins), there are three BCAAs: leucine, isoleucine and valine . BCAAs are absolutely required for protein synthesis but some intermediates formed in their catabolism [e.g., branched-chain α-keto acids (BCKA)] can be toxic at high concentrations .
Studies show that ingestion of branched-chain amino acids (BCAA) at rest, particularly leucine, has an anabolic effect on protein metabolism by increasing the rate of protein synthesis and decreasing protein degradation.[4-6]
It has been shown that oral supplement of BCAAs (77 mg/kg) before exercise increased intracellular and arterial BCAA levels during exercise and resulted in suppression of muscle-protein breakdown . Nosaka K. et al.  tested the effectiveness of pre- and post-workout supplementation with 9 essential and 3 non-essential amino acids on muscle soreness. It was concluded that soreness were significantly lower for the amino acid group compared to placebo.
The effects of BCAA on protein metabolism during exercise are less clear. During exercise, amino acid oxidation increases accompanied by a reduction in whole-body protein synthesis and an increase in protein breakdown.  The study by Rennie, Michael J. and colleagues  suggests that BCAA have an anabolic effect on muscle protein metabolism during the recovery period after exercise. The data during exercise was too variable to make any conclusions about its effects. In theory, supplement with BCAA during training may help improve training adaptations .
One other study showed that both mental and physical performance was improved by an intake of BCAA during exercise but only for the “slower” runners. However, there was no significant effect on the performance in the “faster” runners.
Leucine is special among the BCAAs, because it promotes muscle-protein synthesis in vivo when orally administered to animals .
Acceptable dosages seem to be around 4-20 grams depending on body weight and event of use.
BCAA’s research and use in supplementation has been going on for many years. Some authorities believe that the use of BCAA’s for energy, recovery, muscle synthesis and stamina are not conclusive enough.
- Shimomura, Yoshiharu, et al. “Exercise promotes BCAA catabolism: effects of BCAA supplementation on skeletal muscle during exercise.” The Journal of nutrition 134.6 (2004): 1583S-1587S.
- Sowers, Strakie. “A Primer On Branched Chain Amino Acids”. Huntington College of Health Sciences. Retrieved 10 Feb 2013.
- Harper, A. E., R. H. Miller, and K. P. Block. “Branched-chain amino acid metabolism.” Annual review of nutrition 4.1 (1984): 409-454.
- Alvestrand, A., et al. “Influence of leucine infusion on intracellular amino acids in humans.” European journal of clinical investigation 20.3 (2008): 293-298.
- Nair, K. S., R. G. Schwartz, and S. T. E. P. H. E. N. Welle. “Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans.” American Journal of Physiology-Endocrinology And Metabolism 263.5 (1992): E928-E934.
- Blomstrand, Eva, and Bengt Saltin. “BCAA intake affects protein metabolism in muscle after but not during exercise in humans.” American Journal of Physiology-Endocrinology And Metabolism 281.2 (2001): E365-E374.
- MacLean, D. A., T. E. Graham, and B. Saltin. “Branched-chain amino acids augment ammonia metabolism while attenuating protein breakdown during exercise.” American Journal of Physiology-Endocrinology And Metabolism 267.6 (1994): E1010-E1022.
- Nosaka, Kazunori, Paul Sacco, and Kazunori Mawatari. “Effects of amino acid supplementation on muscle soreness and damage.” International journal of sport nutrition and exercise metabolism 16.6 (2006): 620.
- Rennie, Michael J., et al. “Branched-chain amino acids as fuels and anabolic signals in human muscle.” The Journal of nutrition 136.1 (2006): 264S-268S.
- Ratamess, Nicholas A., et al. “The effects of amino acid supplementation on muscular performance during resistance training overreaching.” Journal of Strength and Conditioning Research 17.2 (2003): 250-258.
- Blomstrand, E., et al. “Administration of branched-chain amino acids during sustained exercise—effects on performance and on plasma concentration of some amino acids.” European journal of applied physiology and occupational physiology 63.2 (1991): 83-88.