3-Methylhistidine, also known as 3-MHis or 3MH, belongs to the class of organic compounds known as histidine and derivatives. 3MH is also classified as a methylamino acid. Methylamino acids are primarily proteogenic amino acids (found in proteins) which have been methylated (in situ) on their side chains by various methyltransferase enzymes. 3-Methylhistidine is also classified as a member of the class of compounds known as L-alpha-amino acids. L-alpha-Amino acids are alpha amino acids which have the L-configuration of the alpha-carbon atom. 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. 3-Methylhistidine is generated from histidine residues found in proteins. Histidine can be methylated at either the N1 or N3 position of its imidazole ring, yielding the isomers 1-methylhistidine (1MH; also referred to as pi-methylhistidine) or 3-methylhistidine (3MH; tau-methylhistidine), respectively. There is considerable confusion with regard to the nomenclature of the methylated nitrogen atoms on the imidazole ring of histidine and other histidine-containing peptides such as anserine. In particular, older literature (mostly prior to the year 2000) designated anserine (Npi methylated) as beta-alanyl-N1-methyl-histidine, whereas according to standard IUPAC nomenclature, anserine is correctly named as beta-alanyl-N3-methyl-histidine. As a result, many papers published prior to the year 2000 incorrectly identified 1MH as a specific marker for dietary consumption or various pathophysiological effects when they really were referring to 3MH (PMID: 24137022).  Histidine methylation on the 3- or tau site is mediated by the enzyme known as METTL18. METTL18 is a nuclear methyltransferase protein that contains a functional nuclear localization signal and accumulates in nucleoli. Urinary concentrations of 3-methylhistidine can be used as a biomarker for skeletal muscle protein breakdown in humans who have been subject to muscle injury (PMID: 16079625). 3-methylhistidine is formed by the posttranslational methylation of histidine residues of the main myofibrillar proteins actin and myosin. During protein catabolism, 3-methylhistidine is released but cannot be reutilized. Therefore, the plasma concentration and urine excretion of 3-methylhistidine are sensitive markers of myofibrillar protein degradation (PMID: 32235743). Approximately 75% of 3-methylhistidine is estimated to originate from skeletal muscle (PMID: 32235743). In addition to the degradation of muscle proteins, the 3-methylhistidine level is affected by the degradation of intestinal proteins and meat intake. 3-Methylhistidine exists in all eukaryotes, ranging from yeast to humans. In humans, 3-methylhistidine is involved in methylhistidine metabolism. 3-Methylhistidine has been found to be associated with several diseases such as diabetes mellitus type 2, eosinophilic esophagitis, and kidney disease. The normal concentration of 3-methylhistidine in the urine of healthy adult humans has been detected and quantified in a range of 3.63‚Äì69.27 micromoles per millimole (umol/mmol) of creatinine, with most studies reporting the average urinary concentration between 15‚Äì20 umol/mmol of creatinine.  The average concentration of 3-methylhistidine in human blood plasma has been detected and quantified at 2.85 micromolar (uM) with a range of 0.0‚Äì5.9 uM. As a general rule, urinary 1MH is associated with white meat intake (p< 0.001), whereas urinary 3MH is associated with red meat intake (p< 0.001) (PMID: 34091671).