An essential branched-chain amino acid important for hemoglobin formation. [PubChem]; Branched chain amino acids (BCAA) are essential amino acids whose carbon structure is marked by a branch point. These three amino acids are critical to human life and are particularly involved in stress, energy and muscle metabolism. BCAA supplementation as therapy, both oral and intravenous, in human health and disease holds great promise. 'BCAA' denotes valine, isoleucine and leucine which are branched chain essential amino acids. Despite their structural similarities, the branched amino acids have different metabolic routes, with valine going solely to carbohydrates, leucine solely to fats and isoleucine to both. The different metabolism accounts for different requirements for these essential amino acids in humans: 12 mg/kg, 14 mg/kg and 16 mg/kg of valine, leucine and isoleucine respectively. Furthermore, these amino acids have different deficiency symptoms. Valine deficiency is marked by neurological defects in the brain, while isoleucine deficiency is marked by muscle tremors. Many types of inborn errors of BCAA metabolism exist, and are marked by various abnormalities. The most common form is the maple syrup urine disease, marked by a characteristic urinary odor. Other abnormalities are associated with a wide range of symptoms, such as mental retardation, ataxia, hypoglycemia, spinal muscle atrophy, rash, vomiting and excessive muscle movement. Most forms of BCAA metabolism errors are corrected by dietary restriction of BCAA and at least one form is correctable by supplementation with 10 mg of biotin daily. BCAA are useful because they are metabolized primarily by muscle. Stress state- e.g surgery, trauma, cirrhosis, infections, fever and starvation--require proportionately more BCAA than other amino acids and probably proportionately more leucine than either valine or isoleucine. BCAA and other amino acids are frequently fed intravenously (TPN) to malnourished surgical patients and in some cases of severe trauma. BCAA, particularly leucine, stimulate protein synthesis, increase reutilization of amino acids in many organs and reduce protein breakdown. Furthermore, leucine can be an important source of calories, and is superior as fuel to the ubiquitous intravenous glucose (dextrose). Leucine also stimulates insulin release, which in turn stimulates protein synthesis and inhibits protein breakdown. These effects are particularly useful in athletic training. BCAA should also replace the use of steroids as commonly used by weightlifters. Huntington's chorea and anorexic disorders both are characterized by low serum BCAA. These diseases, as well as forms of Parkinson's, may respond to BCAA therapy. BCAA, and particularly leucine, are among the amino acids most essential for muscle health. (http://www.dcnutrition.com); Leucine (abbreviated as Leu or L) is a branched-chain amino acid with the chemical formula HO2CCH(NH2)CH2CH(CH3)2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons (UUA, UUG, CUU, CUC, CUA, and CUG) and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins. Leucine is an essential amino acid. Leucine is a branched-chain amino acid (BCAA) since it possesses an aliphatic side-chain that is non-linear.