Butyric acid is a short-chain fatty acid (SCFA) formed in the mammalian colon by bacterial fermentation of carbohydrates (including dietary fibre). It is a straight-chain alkyl carboxylic acid that appears as an oily, colorless liquid with an unpleasant (rancid butter) odor. The name butyric acid comes from the Greek word for "butter", the substance in which it was first found. Triglycerides of butyric acid constitute 3‚Äì4% of butter. When butter goes rancid, butyric acid is liberated from the short-chain triglycerides via hydrolysis. Butyric acid is a widely distributed SCFA and is found in all organisms ranging from bacteria to plants to animals. It is present in animal fat and plant oils, bovine milk, breast milk, butter, parmesan cheese, body odor and vomit. While butyric acid has an unpleasant odor, it does have a pleasant buttery taste. As a result, butyric acid is used as a flavoring agent in food manufacturing. Low-molecular-weight esters of butyric acid, such as methyl butyrate, also have very pleasant aromas or tastes. As a result, several butyrate esters are used as food and perfume additives. Butyrate is naturally produced by fermentation processes performed by obligate anaerobic bacteria found in the mammalian gut. It is a metabolite of several bacterial genera including Anaerostipes, Coprococcus, Eubacterium, Faecalibacterium and Roseburia (PMID: 12324374; PMID: 27446020). Highly-fermentable fiber residues, such as those from resistant starch, oat bran, pectin, and guar can be transformed by colonic bacteria into butyrate. One study found that resistant starch consistently produces more butyrate than other types of dietary fibre (PMID: 14747692). The production of butyrate from fibres in ruminant animals such as cattle is responsible for the butyrate content of milk and butter. Butyrate has a number of important biological functions and binds to several specific receptors. In humans, butyric acid is one of two primary endogenous agonists of human hydroxycarboxylic acid receptor 2 (HCA2), a G protein-coupled receptor. Like other SCFAs, butyrate is also an agonist at the free fatty acid receptors FFAR2 and FFAR3, which function as nutrient sensors that facilitate the homeostatic control of energy balance. Butyrate is essential to host immune homeostasis (PMID: 25875123). Butyrate's effects on the immune system are mediated through the inhibition of class I histone deacetylases (specifically, HDAC1, HDAC2, HDAC3, and HDAC8) and activation of its G-protein coupled receptor targets including HCA2, FFAR2 and FFAR3. Among the short-chain fatty acids, butyrate is the most potent promoter of intestinal regulatory T cells in vitro and the only SCFA that is an HCA2 ligand (PMID: 25741338). Butyrate has been shown to be a critical mediator of the colonic inflammatory response. It possesses both preventive and therapeutic potential to counteract inflammation-mediated ulcerative colitis and colorectal cancer. As a short-chain fatty acid, butyrate is metabolized by mitochondria as an energy source through fatty acid metabolism. In particular, it is an important energy source for cells lining the mammalian colon (colonocytes). Without butyrate, colon cells undergo autophagy (i.e., self-digestion) and die.