The essential polyunsaturated fatty acids (PUFA) are constituents of the animal and vegetable fats found in food. They are called "essential" because the human body cannot manufacture them, even though they form the framework for the organism's cell membranes, particularly the neurones in the brain. They have to be provided by food or by an appropriate supplement.

Omega-3 and omega-6 are the two families of essential fatty acids. The two are distinguished by the position of the fatty acid's double bond, which determines its spatial structure.

ALA is a fatty acid found mainly in certain foodstuffs and particularly in purslane (a sort of watercress eaten around the Mediterranean), nuts, rapeseed and linseed.

EPA and DHA are found mainly in fish, particularly fish from cold sea areas. They are also found in produce such as eggs, meat and milk, etc, from animals fed partly on linseed.

 

Omega-6 acids are found in the most common vegetable oils such as sunflower oil, palm oil and corn oil, and in animal products when the stock have been fed on grain rich in omega-6 (such as corn or soya).

 

The omega-3 and omega-6 families are two parallel and unconnected chains of highly unsaturated fatty acids called HUFA (Highly Unsaturated Fatty Acids). This is the term used to describe fatty acids with 20 and 22 carbon atoms and 3, 4, 5 or 6 double bonds (Lands 2000).

HUFA are the main precursors of a series of chemical messengers called eicosanoids. These substances work in the human body rather like hormones. Eicosanoids are usually synthesised to respond to intermittent signals from cells. The strength of the response, once initiated, is limited by the ratio of precursors (omega-3 and omega-6) to inhibitors present in the tissue (Liu, Bibus et al. 2001).

Omega-3 and omega-6 eicosanoids have varied and often opposing effects. Those from omega-6 acids may lead to the aggregation of blood platelets ("pro-aggregation") and inflammation ("pro-inflammatory") (Eberhart and Dubois 1995). Those from omega-3 often have the opposite effect ("anti-aggregation" and "anti-inflammatory") (Endres 1993; James, Gibson et al. 2000).

Eicosanoids from omega-3 acids are however less powerful than those from omega-6 acids (Lands, Libelt et al. 1992; Abayasekara, Wathes et al. 1999). Because they lower the level of omega-6 precursors in tissue, omega-3 acids reduce the synthesis of eicosanoids from omega-6 acids.