plural: short-chain fatty acids”
short chain fat·ty ac·id, ʃɔːt ˈt͡ʃeɪn ˈfætɪ ˈæsɪd
A type of fatty acid distinguished from the other fatty acids by being comprised of less than six carbon atoms
A fatty acid is a subunit of fats, oils, and waxes. It pertains to any long chain of hydrocarbon, with a single carboxylic group and aliphatic tail. It is produced by the breakdown of fats (usually triglycerides or phospholipids) through a process called hydrolysis. Fatty acids are a subgroup of lipids, which are organic compounds readily soluble in nonpolar solvent (e.g. ether) but not in polar solvent (e.g. water). A fatty acid may be represented by R-COOH, where R stands for the aliphatic moiety and COOH as the carboxylic group (making the molecule an acid). The general formula is CnH2n+1COOH. Almost all natural fatty acids have an even number of carbons. This is because fatty acids are synthesized by adding two carbons each time to malonyl-CoA. Based on the length of the chain, a fatty acid may be short-chain fatty acid, medium-chain fatty acid, long-chain fatty acid, or very long chain fatty acid.
A fatty acid with aliphatic tail of five or fewer carbons is called a short-chain fatty acid (SCFA).
Examples of SCFAs are as follows:
- Formic acid (methanoic acid, chemical formula: CH2O2) is the simplest SCFA (or carboxylic acid). It occurs naturally, such as in ants and in stingless bees (Oxytrigona). These insects use formic acid as a defense mechanism. For instance, wood ants spray formic acid to defend the nest from foes. They may also spray formic acid to catch prey.
- Acetic acid (ethanoic acid, chemical formula: C2H4O2) consists of a methyl group and a carboxyl group. It occurs naturally and is vital to all living things as it binds to coenzyme A, which is involved in the metabolism of fats and carbohydrates.
- Propionic acid (chemical formula: C3H6O2) is a SCFA produced biologically from the breakdown of fatty acids (with odd number of carbons) and from certain amino acids. In humans, it is known as being responsible in part for the odor from sweat and for causing acne. The human skin is host to various bacteria and one of them is the group of Propionibacteria. These bacteria (especially Cutibacterium acnes) are located in the sweat glands are capable of metabolically producing propionic acid from anaerobic metabolism. Propionibacteria are also present in the stomachs of ruminants.
- Butyric acid (butanoic acid, chemical formula: C4H8O2) is a SCFA occurring naturally in animal fat, plant oils, bovine milk, and breast milk. It is a metabolic byproduct of fermentation by obligate anaerobic bacteria (e.g. Clostridium butyricum).
Common biological reactions
Common biological reactions
SCFAs are formed from fermentation. In humans, the indigestible saccharides are metabolized by certain bacteria through fermentation. In particular, the microbiota residing normally in the large intestines of humans ferments cellulose, for instance, and subsequently produces short-chain fatty acids and gases. The short-chain fatty acids are absorbed and metabolized by the body.
In humans, SCFAs in the colon helps in increasing the absorption of dietary minerals as they raise the acidity level in the colon. They are absorbed in the colon and then taken into the portal circulation supplying the liver. They suppress cholesterol synthesis by the liver. The liver, in turn, transports them into the general circulatory system. SCFAs are also able to cross the blood-brain barrier (via monocarboxylate transporters).
In humans, the major SCFAs are acetic acid (acetate), propionic acid (propionate), and butyric acid (butyrate). They provide energy to the cells that have taken them up. For instance, butyrate is the major energy source of colonocytes. They also provide energy to muscle, kidney, and brain.
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