Omega-3

n−3 fatty acids (popularly referred to as ω−3 fatty acids or omega-3 fatty acids) are a family of unsaturated fatty acids which have in common a carbon–carbon double bond in the n−3 position; that is, the third bond from the end of the fatty acid. more...

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Important nutritionally essential n−3 fatty acids are: α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). The human body cannot synthesize n−3 fatty acids de novo, but it can form 20- and 22-carbon unsaturated n−3 fatty acids from the eighteen-carbon n−3 fatty acid, α-linolenic acid. These conversions occur competitively with n−6 fatty acids, which are essential closely related chemical analogues that are derived from linoleic acid. Both the n−3 α-linolenic acid and n−6 linoleic acid are essential nutrients which must be obtained from food. Synthesis of the longer n−3 fatty acids from linolenic acid within the body is competitively slowed by the n−6 analogues. Thus accumulation of long-chain n−3 fatty acids in tissues is more effective when they are obtained directly from food or when competing amounts of n−6 analogs do not greatly exceed the amounts of n−3.

Chemistry

The term n−3 (also called ω−3 or omega-3) signifies that the first double bond exists as the third carbon-carbon bond from the terminal methyl end (n) of the carbon chain.

n−3 fatty acids which are important in human nutrition are: α-linolenic acid (18:3, n−3; ALA), eicosapentaenoic acid (20:5, n−3; EPA), and docosahexaenoic acid (22:6, n−3; DHA). These three polyunsaturates have either 3, 5 or 6 double bonds in a carbon chain of 18, 20 or 22 carbon atoms, respectively. All double bonds are in the cis-configuration, i.e. the two hydrogen atoms are on the same side of the double bond.

Most naturally-produced fatty acids (created or transformed in animalia or plant cells with an even number of carbon in chains) are in cis-configuration where they are more easily transformable. The trans-configuration results in much more stable chains that are very difficult to further break or transform, forming longer chains that aggregate in tissues and lacking the necessary hydrophilic properties. This trans-configuration can be the result of the transformation in alkaline solutions, or of the action of some bacterias that are shortening the carbonic chains. Natural transforms in vegetal or animal cells more rarely affect the last n−3 group itself. However, n−3 compounds are still more fragile than n−6 because the last double bond is geometrically and electrically more exposed, notably in the natural cis configuration.

Read more at Wikipedia.org