Isoleucine (Ile) or L-isoleucine 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-isolecuine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Isoleucine 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. Isoleucine is an essential amino acid in humans, meaning the body cannot synthesize it and that it must be obtained from the diet. In plants and microorganisms, isoleucine is synthesized starting from pyruvate and alpha-ketobutyrate. Isoleucine is classified as a branched chain amino acid (BCAA). BCAAs include three amino acids: isoleucine, leucine and valine. They are alpha amino acids whose carbon structure is marked by a beta branch point. Despite their structural similarities, 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. Isoleucine is catabolized via with alpha-ketoglutarate where upon it is oxidized and split into propionyl-CoA and acetyl-CoA. Propionyl-CoA is converted into succinyl-CoA, a TCA cycle intermediate which can be converted into oxaloacetate for gluconeogenesis (hence glucogenic). The acetyl-CoA can be fed into the TCA cycle by condensing with oxaloacetate to form citrate or used in the synthesis of ketone bodies or fatty acids. The different metabolism of BCAAs 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 are required 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. BCAAs are decreased in patients with liver disease, such as hepatitis, hepatic coma, cirrhosis, extrahepatic biliary atresia. An inability to break down isoleucine, along with other amino acids, is associated with maple syrup urine disease (MSUD) (PMID: 34125801). Isoleucine, like other BCAAs, is associated with insulin resistance. In particular, higher levels of isoleucine are observed in the blood of diabetic mice, rats, and humans (PMID 25287287). Mice fed an isoleucine deprivation diet for one day have improved insulin sensitivity, and feeding of an isoleucine deprivation diet for one week significantly decreases blood glucose levels (PMID: 24684822).