Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, sperm, testicles and retina. It can be synthesized from alpha-linolenic acid or obtained directly from maternal milk or fish oil. DHA's structure is a carboxylic acid(~oic acid) with a 22-carbon chain (docosa- is Greek for 22) and six (Greek "hexa") cis double bonds (-en~); the first double bond is located at the third carbon from the omega end. Its trivial name is cervonic acid, its systematic name is all-cis-docosa-4,7,10,13,16,19-hexa-enoic acid, and its shorthand name is 22:6(n-3) in the nomenclature of fatty acids.
Cold-water oceanic fish oils are rich in DHA. Most of the DHA in fish and multi-cellular organisms with access to cold-water oceanic foods originates from photosynthetic and heterotrophic microalgae, and becomes increasingly concentrated in organisms the further they are up the food chain. DHA is also commercially manufactured from microalgae; Crypthecodinium cohnii and another of the genus Schizochytrium. DHA manufactured using microalgae is vegetarian.
Some animals with access to seafood make very little DHA through metabolism, but obtain it in the diet. However, in strict herbivores, and carnivores that do not eat seafood, DHA is manufactured internally from α-linolenic acid, a shorter omega-3 fatty acid manufactured by plants (and also occurring in animal products as obtained from plants). Eicosapentaenoic and docosapentaenoic acids are the principal products of α-linolenic acid metabolism in young men and illustrates the importance of DHA production for the developing fetus and healthy breast milk. Giltay, Gooren, Toorians, and Katan (2004) found rates of conversion 15% higher for women, and that those taking oral contraceptives demonstrated 10% higher DHA levels. Administration of testosterone or the aromatase inhibitor anastrozole, which blocks conversion of testosterone to estradiol, reduces DHA conversion. DHA is a major fatty acid in sperm and brain phospholipids and in the retina. Dietary DHA may reduce the risk of heart disease by reducing the level of blood triglycerides in humans. Below-normal levels of DHA have been associated with Alzheimer's disease. A low level of DHA is also spotted in patients with retinitis pigmentosa.