Leucine (abbreviated as Leu or L) is an -amino acid that is used in the biosynthesis of proteins. It contains an -amino group (which is in the protonated H3+ form under biological conditions), an -carboxylic acid group (which is in the deprotonated-COO-form under biological conditions), and a side chain isobutyl group, making it a non-polar aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it, and it must be obtained from the diet. Human dietary sources are foods that contain protein, such as meats, dairy products, soy products, beans and legumes. In the genetic code, it is encoded by the six codons UUA, UUG, CUU, CUC, CUA, and CUG. L-Leucine is a branched-chain amino acid (BCAA). BCAAs 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. 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. BCAA and other amino acids are frequently fed intravenously (TPN) to malnourished surgical patients and, in some cases, those with 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). L-Leucine can be found in most biofluids, including blood, cerebrospinal fluid (CSF), feces, and sweat, as well as throughout most human tissues. L-Leucine exists in all living species, ranging from bacteria to humans.