DHA

DHA is usually referred to by this abbreviation, because the full name (docosahexaeneoic acid) is so much of a mouthful. DHA is a highly unsaturated fatty acid (six double bonds) with 22 carbon atoms, found in animal tissues - oily fish are particularly high in it - and also in various plant sources. The best plant source of DHA is the alga Schizochytrium, which describes several very closely related species.
DHA is made only in very small amounts in any animal, including humans. It is made from the fatty acid alpha-linolenic acid, found in fairly large quantities in walnuts and in very large amounts in flaxseed (linseed). However, this process is difficult and inefficient and is blocked very easily by lack of any of a fairly large number of nutrients, so it is better to get DHA from a dietary source.
DHA forms a very large proportion of the fat that makes up the cell membranes of the nervous system, including the retina; approximately 40% of the polyunsaturates in the brain, and 60% in the retina, are DHA. DHA is also used to make a variety of hormones and prostaglandins (prostaglandins are short-acting biochemicals similar to hormones).
DHA deficiency can cause ADHD, Alzheimer's disease and depression - or at least contribute to it - and supplementation with DHA can make any and all of these problems less likely.
DHA supplementation can also make the risk of various cancers smaller; however, one point worth mentioning in this context is that being unsaturated, DHA is best taken in combination with vitamin E.
DHA is particularly important for children and pregnant or lactating women. This is probably because DHA is used in very large amounts by a growing and developing brain and nervous system. This is probably also the reason why human breast milk is very high in DHA (as long as the mother is not deficient).
DHA is a nutrient that is more commonly deficient in the diet than formerly, because less oily fish is eaten and because meat animals (particularly cattle) are fed on grain instead of grass, which reduces the level of DHA in meat and milk.
DHA supplements are therefore very useful, particularly for small children and pregnant and breastfeeding women.