There are two families of essential fatty acids, the linoleic and linolenic.

Linoleic acid (C18:2n-6), found mainly in vegetable seed oils, is desaturated and elongated in the body, forming arachidonic acid (C20:4n-6).

Linolenic acid (C18:3n-3), the main dietary source of which is leaves, is desaturated and elongated, forming two fatty acids that are prevalent in fish oils: timnodonic (C20:5n-3) and clupanodonic (C22:6n-3).

EFA are very easily peroxidized in air, but vitamin E protects against this.

There are three functions of EFA.
1. The most important is as part of phospholipids in all animal cellular membranes: in deficiency of EFA faulty membranes are formed.

2. A second is in the transport and oxidation of cholesterol: EFA tend to lower plasma cholesterol.

3. A third function is as precursors of prostanoids which are only formed from EFA.

Deficiency of EFA in experimental animals causes lesions mainly attributable to faulty cellular membranes: sudden failure of growth, lesions of skin and kidney and connective tissue, erythrocyte fragility, impaired fertility, uncoupling of oxidation and phosphorylation.

In man pure deficiency of EFA has been studied particularly in persons fed intravenously. A relative deficiency (that is, a low ratio in the body of EFA to long-chain saturated fatty acids and isomers of EFA) is common on Western diets and plays an important part in the causation of atherosclerosis, coronary thrombosis, multiple sclerosis, the triopathy of diabetes mellitus, hypertension and certain forms of malignant disease.

Various factors affect the dietary requirement of EFA.