Another putative mechanism underlying visceral hypersensitivity in a subset of IBS patients is related to bacterial histamine production.64 Urinary histamine is high in a subset of IBS patients and decreases following a low FODMAP diet.103 The source of the increased histamine has not been clearly established as it could originate from dietary sources, be produced by colonic mast cells, or result from metabolic activity of the gut microbiota, as multiple bacteria possess the histidine decarboxylase gene responsible for the conversion of the amino acid L-histidine into histamine. The gene discussed is HDC; the disease is irritable bowel syndrome.