Histidine (His), also known as L-histidine, 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. Histidine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Histidine is found in all organisms ranging from bacteria to plants to animals.  It is classified as an aliphatic, positively charged or basic amino acid. Histidine is a unique amino acid with an imidazole functional group. The acid-base properties of the imidazole side chain are relevant to the catalytic mechanism of many enzymes such as proteases. In catalytic triads, the basic nitrogen of histidine abstracts a proton from serine, threonine, or cysteine to activate it as a nucleophile. In a histidine proton shuttle, histidine is used to quickly shuttle protons. It can do this by abstracting a proton with its basic nitrogen to make a positively charged intermediate and then use another molecule to extract the proton from its acidic nitrogen. Histidine forms complexes with many metal ions. The imidazole sidechain of the histidine residue commonly serves as a ligand in metalloproteins. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans and other mammals. It was initially thought that it was only essential for infants, but longer-term studies established that it is also essential for adults. Infants four to six months old require 33 mg/kg of histidine. It is not clear how adults make small amounts of histidine, and dietary sources probably account for most of the histidine in the body. Histidine is a precursor for histamine and carnosine biosynthesis. Inborn errors of histidine metabolism, including histidinemia, maple syrup urine disease, propionic acidemia, and tyrosinemia I, exist and are marked by increased histidine levels in the blood. Elevated blood histidine is accompanied by a wide range of symptoms, from mental and physical retardation to poor intellectual functioning, emotional instability, tremor, ataxia and psychosis. Histidine and other imidazole compounds have anti-oxidant, anti-inflammatory and anti-secretory properties (PMID: 9605177 ). The efficacy of L-histidine in protecting inflamed tissue is attributed to the capacity of the imidazole ring to scavenge reactive oxygen species (ROS) generated by cells during acute inflammatory response (PMID: 9605177 ). Histidine, when administered in therapeutic quantities is able to inhibit cytokines and growth factors involved in cell and tissue damage (US patent 6150392). Histidine in medical therapies has its most promising trials in rheumatoid arthritis where up to 4.5 g daily have been used effectively in severely affected patients. Arthritis patients have been found to have low serum histidine levels, apparently because of very rapid removal of histidine from their blood (PMID: 1079527 ). Other patients besides arthritis patients that have been found to be low in serum histidine are those with chronic renal failure. Urinary levels of histidine are reduced in pediatric patients with pneumonia (PMID: 2084459 ). Asthma patients exhibit increased serum levels of histidine over normal controls (PMID: 23517038 ). Serum histidine levels are lower and are negatively associated with inflammation and oxidative stress in obese women (PMID: 23361591 ). Histidine supplementation has been shown to reduce insulin resistance, reduce BMI and fat mass and suppress inflammation and oxidative stress in obese women with metabolic syndrome. Histidine appears to suppress pro-inflammatory cytokine expression, possibly via the NF-Œ∫B pathway, in adipocytes (PMID: 23361591 ). Low plasma concentrations of histidine are associated with protein-energy wasting, inflammation, oxidative stress, and greater mortality in chronic kidney disease patients (PMID: 18541578 ). Histidine may have many other possible functions because it is the precursor of the ubiquitous neurohormone-neurotransmitter histamine. Histidine increases histamine in the blood and probably in the brain. Low blood histamine with low serum histidine occurs in rheumatoid arthritis patients. Low blood histamine also occurs in some manic, schizophrenic, high copper and hyperactive groups of psychiatric patients. Histidine is a useful therapy in all patients with low histamine levels (http://www.dcnutrition.com).