Leucine (Leu) or L-leucine is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon).  Amino acids are organic compounds that contain amino (‚ÄìNH2) and carboxyl (‚ÄìCOOH) functional groups, along with a side chain (R group) specific to each amino acid. L-leucine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Leucine is found in all organisms ranging from bacteria to plants to animals. It is classified as a non-polar, uncharged (at physiological pH) aliphatic amino acid. Leucine 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. L-Leucine is a branched chain amino acid (BCAA).  The BCAAs consist of leucine, valine and isoleucine (and occasionally threonine). BCAAs are essential amino acids whose carbon structure is marked by a branch point at the beta-carbon position. BCAAs 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. BCAAs have different metabolic routes, with valine going solely to carbohydrates (glucogenic), leucine solely to fats (ketogenic) and isoleucine being both a glucogenic and a ketogenic amino acid. 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. The primary metabolic end products of leucine metabolism are acetyl-CoA and acetoacetate; consequently, it is one of the two exclusively ketogenic amino acids, with lysine being the other. Leucine is the most important ketogenic amino acid in humans. The vast majority of l-leucine metabolism is initially catalyzed by the branched-chain amino acid aminotransferase enzyme, producing alpha-ketoisocaproate (alpha-KIC). alpha-KIC is metabolized by the mitochondrial enzyme branched-chain alpha-ketoacid dehydrogenase, which converts it to isovaleryl-CoA. Isovaleryl-CoA is subsequently metabolized by the enzyme isovaleryl-CoA dehydrogenase and converted to beta-methylcrotonyl-CoA (MC-CoA), which is used in the synthesis of acetyl-CoA and other compounds. During biotin deficiency, HMB can be synthesized from MC-CoA via enoyl-CoA hydratase and an unknown thioesterase enzyme, which convert MC-CoA into HMB-CoA and HMB-CoA into HMB respectively.  Leucine has the capacity to directly stimulate myofibrillar muscle protein synthesis (PMID 15051860). This effect of leucine arises results from its role as an activator of the mechanistic target of rapamycin (mTOR) (PMID 23551944) a serine-threonine protein kinase that regulates protein biosynthesis and cell growth. The activation of mTOR by leucine is mediated through Rag GTPases.  Leucine, like other BCAAs, is associated with insulin resistance. In particular, higher levels of leucine are observed in the blood of diabetic mice, rats, and humans (PMID 25287287). BCAAs such as leucine have different deficiency symptoms. Valine deficiency is marked by neurological defects in the brain, while isoleucine deficiency is marked by muscle tremors. Persistently low leucine levels can result in decreased appetite, poor feeding, lethargy, poor growth, weight loss, skin rashes, hair loss, and desquamation. Many types of inborn errors of BCAA metabolism exist and these are marked by various abnormalities. The most common form is 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 BCAAs and at least one form is correctable by supplementation with 10 mg of biotin daily. - BCAAs are useful because they are metabolized primarily by muscle. Stress states - e.g surgery, trauma, cirrhosis, infections, fever and starvation--require proportionately more BCAAs than other amino acids and probably proportionately more leucine than either valine or isoleucine. BCAAs and other amino acids are frequently fed intravenously (TPN) to malnourished surgical patients and in some cases of severe trauma. BCAAs, 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. Huntington's chorea and anorexic disorders both are characterized by low serum BCAAs. These diseases, as well as forms of Parkinson's, may respond to BCAA therapy.