Polyunsaturation

Polyunsaturation is the chemical property of having more than one carbon–carbon double bond in a molecule. In biology and nutrition, it most often refers to polyunsaturated fatty acids (PUFAs), in which the hydrocarbon chain contains two or more carbon-carbon double bonds. These bonds are typically in the cis configuration and separated by one or more methylene groups. This contrasts with monounsaturated fatty acids (one double bond) and saturated fatty acids (no double bonds).

The most studied PUFAs in human nutrition are the omega-6 and omega-3 families. Essential fatty acids, linoleic acid (LA, 18:2n-6) and alpha-linolenic acid (ALA, 18:3n-3), must be obtained from the diet because humans cannot introduce the first double bonds in these series. The body can further desaturate and elongate LA and ALA to longer-chain polyunsaturates such as arachidonic acid (AA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3), though conversion efficiency is limited.

Polyunsaturation affects physical properties: higher unsaturation lowers melting points and increases membrane fluidity, which is important for cellular function. However, multiple double bonds also make lipids more susceptible to oxidation, influencing storage, shelf life, and the need for antioxidants. In physiology, omega-6 and omega-3 PUFAs give rise to distinct signaling molecules called eicosanoids, influencing inflammation and vascular function.

Common dietary sources include vegetable oils, seeds, nuts, and fatty fish. Dietary recommendations often emphasize a balanced intake of omega-6 and omega-3 fatty acids, highlighting the role of PUFAs in health and disease.