What do they do?
Branched-chain amino acids (BCAAs) include leucine, isoleucine and valine. BCAAs are needed for the maintenance of muscle tissue; they are also needed during times of physical stress and intense exercise. Research shows that BCAA supplements help even those with amyotrophic lateral sclerosis (Lou Gehrig disease) maintain muscle strength (1). In addition, individuals with liver disease (hepatic encephalopathy) benefit from intravenous administration of BCAAs (2).

Where can they be found? Dairy and red meat contain the greatest amount of BCAAs. They are present in all protein—containing foods. Whey protein and egg protein supplements are other sources of BCAAs. BCAA supplements provide the single amino acids leucine, isoleucine and valine. Under what conditions might BCAAs be supportive? BCAAs are sufficient for athletic support and post-surgery recovery. Who is likely to be deficient? Periods of physical stress, such as intense weight lifting and long distance running can create a catabolic state in which muscle tissue is broken down. In these situations, supplemental BCAAs—particularly leucine and its derivatives ketoisocaproate (KIC) and hydroxymethylbutyrate (HMB)—can be beneficial in reducing protein breakdown (3, 4). Nonetheless, BCAA supplements may reduce muscle loss and speed muscle gain. BCAAs may also be useful to anyone wanting to prevent muscle breakdown (5, 6, 7). How much should I take? A diet including animal protein provides an adequate amount of BCAA for most people. Athletes involved in intense training often take 5 grams of leucine, 4 grams of valine and 2 grams of isoleucine per day to prevent muscle loss and increase muscle gain. Are there any side effects or interactions? To Date, side effects have not been reported with the use of BCAAs.  A high intake of BCAAs are simply converted into other amino acids or used as energy. It is prudent to take BCAAs along with whole proteins, such as lean meat or poultry and multiple vitamins/minerals, especially the B-complex vitamins.

The pick of  blood  BCAAs concentration presents on 40 after 2 hours of consumption.

References:

(1). Plaitakis A, Smith J, Mandeli J, Yahr MD. Pilot trial of branched-chain amino acids in amyotrophic lateral sclerosis. Lancet May 7, 1988:1015-8.
(2). Wahren J, Denis J, Desurmont P, et al. Is intravenous administration of branched chain amino acids effective in the treatment of hepatic encephalopathy? A multicenter study. Hepatology 1983; 3(4):475-80.
(3). Mero A. Leucine supplementation and intensive training. Sports Med. 1999 Jun; 27(6):347-58.
(4). Holecek M, Muthny T, Kovarik M, Sispera L. Effect of beta-hydroxy-beta-methylbutyrate (HMB) on protein metabolism in whole body and in selected tissues. Food Chem Toxicol. 2009 Jan; 47(1):255-9.
(5). Kelly GS. Sports nutrition: a review of selected nutritional supplements for bodybuilders and strength athletes. Alt Med Rev. 1997; 2:184-201.
(6). MacLean DA, Graham TE, Satlin B. Branched-chain amino acids augment ammonia metabolism while attenuating protein breakdown during exercise. Am J Physiol 1994; 267:E1010-1022.
(7). Blomstrand E, Hassmen P, Ek S, Ekblom B, Newsholme EA. Influence of ingesting a solution of branched-chain amino acids on perceived exertion during exercise. Acta Physiol Scand 1997; 159:41-49.