Omega-3 and omega-6 fatty acids
The particular properties of omega-3 PUFAs, compared to omega-6 and omega-9 fatty acids, are due to their structural characteristics. In omega-3 PUFAs, the first double bond is placed three carbon atoms away from the methyl terminal, whereas in omega-6 PUFAs the first double bond is found in the sixth carbon (starting from the methyl carbon).
The essential alpha-linolenic acid (ALA) is the precursor molecule of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is an essential nutrient since humans do not have the biochemical pathways required to synthesize it. On the other hand, linoleic acid (LA) is the precursor of arachidonic acid (AA), an omega-6 PUFA. At the same time, these omega-3 and omega-6 fatty acids are the precursors of a family of eicosanoids that mediate the beneficial effects conferred by these PUFAs.
LA and ALA compete for the same elongase and desaturase enzymes in a slow desaturation-elongation enzymatic process, which leads to the formation of EPA, DHA and also AA. Because of the dietary excess of LA, there is a greater formation of AA compared to omega-3 fatty acids, and thus human conversion from ALA to EPA and DHA is relatively inefficient. This is why these omega-3 fatty acids are considered essential and must be obtained from dietary sources or supplements. In fact, the ingestion of fish or fish oil provides directly omega-3 PUFAs, and this way the competition for enzymes in the conversion pathway is avoided.
Sources:
- Colussi,L.; Catena,C.; Baroselli,S.; Nadalini,E.; Lapenna,R.; Chiuch,A.; Sechi,L.A. Omega-3 fatty acids: from biochemistry to their clinical use in the prevention of cardiovascular disease. Recent Patents on Cardiovascular Drug Discovery 2007, (2): 1.13-21. 1574-8901.
- http://lpi.oregonstate.edu/infocenter/othernuts/omega3fa/
