BCAA is a group of three ketogenic essential amino acids (leucine, valine, isoleucine).
BCAA is called a branched chain because its structure has branches in the body of the molecule. BCAA is a free (non-protein) form and has a minimum amount in skeletal muscle, but the combination of these three essential amino acids constitutes approximately two-thirds of human skeletal muscle in protein form. The
BCAA is traded as crystalline powder or crystalline powder and is odorless but has a bitter taste. All BCAAs are readily soluble in formic acid but are sparingly soluble in water and almost insoluble in alcohol.

Since the enzyme BCAA aminotransferase does not exist in the liver where many other amino acids are converted, BCAA is metabolized only in muscle.
The rate-determining enzyme of BCAA metabolism is branched-chain keto dehydrogenase, which is in the muscle and is activated efficiently by exercise and fasting.
BCAA is used in various combinations and contents between leucine, valine and isoleucine.
Leucine oxidizes very easily and is most effective in stimulating insulin secretion from the pancreas.
Leucine lowers high blood sugar and stimulates growth hormone secretion.

In the pharmaceutical field, BCAA is used to maintain the nitrogen balance in patients after surgery for gastric cancer and to treat various liver injuries.
There are three targets for BCAA supplementation in liver disease: (1) hepatic encephalopathy, (2) liver regeneration, and (3) cirrhosis (Urata, 2007, Kawamura, 2009).
BCAA can improve hepatic encephalopathy by promoting detoxification of ammonia, correcting plasma amino acid imbalance, and suppressing the inflow of aromatic amino acids into the brain.
BCAA supplementation for hepatic encephalopathy is particularly effective for chronic liver injury with hyperammonemia and low blood BCAA levels.
BCAA’s effects on liver regeneration and nutritional status in the body are related to protein synthesis, hepatocyte growth factor secretion, glutamine production, and protein degradation (Holecek, 2010).

In recent clinical studies, it is possible to suppress muscle damage and muscle pain caused by exercise by oral loading with BCAA, suggesting improvement of muscle function over a long period of time. There is a controversy about whether there is an anabolic effect later.
However, research on the topic of anabolic effects is still incomplete because the methods for studying differences in muscle function and BCAA intake (the proportion of leucine, valine, and isoleucine) are not uniform.

There is no data showing the side effects of increasing the BCAA intake by healthy individuals with regard to the upper limit intake in the main foods of humans (4th ICAAS Workshop on Evaluation of Appropriate Amounts of Amino Acid Diet).
The only “model” exposed to extremely high amounts of BCAAs is maple syrup urine disease, a very rare genetic disease, or serious brain dysfunction, but useful models that overdose BCAAs in the general population (See Fernstrom, 2005 for an overview).
Patients with cirrhosis received high doses (approximately 12 grams of BCAA per day) for several months, but no side effects were reported (see Marchesini, 2005 for an overview).
According to a recent project supported by ICAAS (Elango, 2010), the average metabolic upper limit for oxidation of leucine in healthy subjects is 0.56 g / kg body weight, maintaining homeostasis of intake of leucine, one of the amino acids of BCAA. Control methods are established.

References
1.Elango, R., Chapman, K., Rafii, M., Ball, R. O., Pencharz, P. B. (2010). Abstract for the Experimental Biology.
2.Fernstrom, J. D. (2005). J. Nutrition, 135S, 1539 – 1546.
3.Holecek, M. (2010). Nutrition, 26, 482-490.
4.Jackman, S. R., Witard, O. C., Jeukendrup, A. E., Tipton, K. D. (2010). Med Sci Sports Exercice, 42, 962-970.
5.Kawamura, E., Habu, D., Morikawa, H., Enomoto, M., Kawabe, J., Tamori, A., Sakaguchi, H., Saeki, S., Kawada, N., Shiomi. S. (2009). Liver Transpl., 15, 790-797.
6.Marchesini, G., Marzocchi, R., Noia, M., Bianchi, G. (2005). J. Nutrition, 135S, 1596 – 1601.
7.Shimomura, Y., Inaguma, A., Watanabe, S., Yamamoto, Y., Muramatsu, Y., Bajotto, G., Sato, J., Shimomura, N., Kobayashi, H., Mawatari, K. (2010). Int J Sport Nutr Exerc Metabolism, 20, 236-244.
8.The 4th Workshop on the Assessment of Adequate Intake of Dietary Amino Acids (2005). J. Nutrition, 135S.
9.Urata, Y., Okita, K., Korenaga, K., Uchida, K., Yamasaki, T., Sakaida, I. (2007). Hepatol Research, 37, 510-516.

・Branched-chain amino acids in liver protection
・Branched-chain amino acids (Bcaa) and delayed muscle soreness
・Adverse effects of leucine overdose depend on dietary protein levels: identification of effective biomarkers by bio-transcriptomic analysis
・Correlation between branched-chain amino acid levels and insulin resistance improvement during weight loss
・Inhibition of hepatoma cell insulin-induced proliferation of branched-chain amino acids by inducing apoptosis through mTORC1- and mTORC2-dependent mechanisms
・Dose-dependent effect of leucine supplementation on muscle mass maintenance in cancer cachexia mice