Long-chain Omega-3 Polyunsaturates

Fish and seafoods from cold waters characteristically and uniquely contain significant quantities of long chain omega-3 polyunsaturates (see  page on the Dietary Balance to see the health effects of this). Though there is some evidence that fish can elongate and desaturate the shorter chain omega-3 polyunsaturates, current opinion is that most of the long chain omega-3 polyunsaturates are formed in the microscopic algae, plankton and planktonic crustacea at the bottom of the marine food chain. They are then passed up the food chain into the higher fish, and of course ultimately to humans. There are three significant members of the omega-3 group, all with 20 or more carbon atoms, and all with five or more double bonds.

Eicosapentaenoic acid, 20:5,w-3.

The most widely researched is 5c,8c,11c,14c,17c eicosapentaenoic acid (20:5,w-3), usually referred to as EPA, but also sometimes called timnodonic acid . It is the major omega-3 polyunsaturate in most seafoods. It is capable of being elongated to 7c,10c,13c,16c,19c docosapentaenoic acid (22:5,w-3) which in turn can be converted to 4c,7c,10c,13c,16c,19c docosahexaenoic acid (22:6 w-3), usually called DHA, but sometimes also known as clupadonic acid. 20:5 n-3, or EPA, is also capable of being metabolised to a range of biologically active substances referred to generically as eicosanoids. Prostaglandins and leukotrienes are important members of this group. They are locally produced, powerful regulators of biological activity. A parallel series of eicosanoids can also be produced from 5c,8c,11c,14c eicosatetraenoic acid, (20:4, w-6),usually called arachidonic acid (AA) which tend to have even more potent biological activity. Since the w-6 family tends to dominate human food, by a factor of 8 times or more compared with the w-3 family, most eicosanoids produced by the human body tend to be of the w-6 type. Increasing the dietary intake of the w-3 polyunsaturates alters this balance, and this is thought to be in part responsible for the beneficial health impact of the w-3 polyunsaturates from seafoods (see  page on the Dietary Balance to see the health effects of this).

Docosahexaenoic acid, 22:6 w-3.

The second most abundant long chain n-3 polyunsaturate is 22:6 w-3, or DHA. It is actually the most abundant w-3 polyunsaturate in certain fish, such as tuna, but in most fish, it is present to a lesser extent than EPA. It is not thought capable of being metabolised directly to eicosanoids, but since it can be retroconverted to EPA, it is possible that a high DHA intake could also affect the eicosanoid balance. The most significant aspect of DHA, from the human nutrition point of view, is it’s role as a major structural component of brain, nerve and retinal membranes. In these membranes, it can form up to 60% of the polyunsaturates present, and recent research is leading to the view that functional abnormalities can result from depletion of membrane DHA levels. DHA plays a unique role in the building of these tissues in the foetus, and such is it’s importance, especially during the first few months of life, that breast milk supplies 0.1-0.4% of fatty acids as DHA, while there is almost no EPA present in breast milk. Breast milk DHA can be augmented by dietary intake of fish and fish oils, but the EPA level does not vary much.

Minor Polyunsaturates

7c,10c,13c,16c,19c docosapentaenoic acid (22:5,w-3) sometimes called clupanodonic acid, is a minor component of most fish , present to the extent of 1-3% of the total fatty acids. Little is known of any specific physiological effects of this polyunsaturate, though of course it is in principal capable of being converted either to 20:5 w-3, or to 22:6 w-3, and as such could augment the available supplies of either.

5c,8c,11c,14c eicosatetraenoic acid, (20:4, w-6) is a minor component of some fish lipids. Fish from tropical waters can have significant amounts of 20:4 w-6, but analytical information is not readily available. Small amounts of short chain omega-3 polyunsaturates are also present in fish lipids, chiefly the 9c,12c,15c octadecatrienoic acid (18:3, w-3) alpha-linolenic acid, and 6c,9c,12c,15c octadecatetraenoic acid (18:4,w-3) stearidonic acid, but the amounts rarely exceed 0.1-0.2% of all fatty acids.

Omega-3 Contents

The pattern of individual polyunsaturates in fish can be a characteristic of the species, though in practice, the potential variations which can occur make it difficult to draw conclusions based on this alone. The geographical location of the feeding grounds, water temperature, water salinity, stage of breeding cycle, and the season of the year are all factors which can and do complicate this issue etc.   The  fish oils page provides a table of the various omega-3 polyunsaturate levels typically found in different marine organisms.