The Omega-3 Polyunsaturates
Biochemical Structure
Polyunsaturated fatty acids are characterised by the presence of two or more methylene interrupted double bonds. The metabolic fate of polyunsaturates depends on the position of the first double bond with respect to the terminal methyl group, since mammalian systems in general lack the enzymes necessary to alter the configuration of this part of the polyunsaturate molecule. Thus no matter what changes may be brought about in the biochemical structure of the molecule the section between the terminal methyl end, and the first double bond remains unaltered. Polyunsaturates can be elongated, desaturated, shortened, or converted to other bio-active molecules such as prostaglandins or leukotrienes, but the configuration of the methyl end of the molecule remains unchanged.
From the point of view of human nutrition, there are two groups of omega-3 polyunsaturates, those with a short chain length (18 carbon atoms or less) and those with a long chain length (20 carbon atoms or more). The difference is quite critical, since the body requires the long chain omega-3s, but doesn’t seem to have any particular direct use for the short chain versions. The latter are in theory capable of being converted to the long chain form, but doubts persist as to the extent to which this conversion process is capable, under modern lifestyle and dietary conditions of contributing significantly to the required amounts of long chain omega-3’s.
The Conversion Process
The short chain omega-3 polyunsaturate alpha linolenic acid (18 carbons, three double bonds) can theoretically be converted to the long chain forms as shown below. Under modern dietary and life-style conditions, many experts doubt the effectiveness of the elongation process, which makes the long chain omega-3 polyunsaturates from fish quite critical in the modern diet.
The Importance of Dietary Balance
with respect to the Polyunsaturates
| THE OMEGA-6 POLYUNSATURATES LINOLEIC ACID(18:2,W-6)from vegetable oils such as sunflower, sesame, safflower etc | THE OMEGA-3 POLYUNSATURATES |
 | |
| GAMMA-LINOLENIC ACID(18:3,W-6)from evening primrose oil, borage and blackcurrantseed oils | ALPHA-LINOLENIC ACID (18:3,W-3)( a short chain omega-3)from linseed and rapeseed oils |
| too much of the omega-6 polyunsaturates | |
| encourages | inhibits |
| ARACHIDONIC ACID (20:4,W-6) found in small amounts in meat, eggs | EICOSAPENTAENOIC ACID (20:5, W-3)( a long chain omega-3)only significant diet source is oil-rich fish |
| DOCOSAHEXAENOIC ACID (22:6,W-3)(a long chain omega-3)major source is oil-rich fish; small amounts in meats & eggs. | |
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.
