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HNMT controls histamine levels in the brain and airways. The stimulatory activity of histamine in the brain may benefit memory and concentration, however, it can also negatively affect sleep and cause allergy-type symptoms. Genetic variants cause up to a five-fold decrease in HNMT activity resulting in symptoms such as headaches, watery, itchy eyes and nose, coughing, wheezing, shortness of breath, epilepsy, insomnia, and anxiety.

Histamine is metabolized by two major pathways: N(tau)-methylation via histamine N-methyltransferase and oxidative deamination via diamine oxidase. This gene encodes the first enzyme which is found in the cytosol and uses S-adenosyl-L-methionine as the methyl donor. In the mammalian brain, the neurotransmitter activity of histamine is controlled by N(tau)-methylation as diamine oxidase is not found in the central nervous system. A common genetic polymorphism affects the activity levels of this gene product in red blood cells. 

When looking at your HNMT genotype, it is important to consider your methylation profile too by looking at your MTHFR SNP.

Zinc and Salacia oblonga both have an agnostic impact on this enzyme.