polyunsaturated fat

(materials) A fat or oil based on fatty acids such as linoleic or linolenic acids which have two or more double bonds in each molecule; corn oil and safflower oil are examples.

Fats that have carbon chains with more than one double bond. A polyunsaturated fat has four or more fewer hydrogen atoms than its equivalent saturated fat. Polyunsaturates are classified as cis and trans fatty acids according to the arrangement of the hydrogen atoms closest to the double bond. In the cis form, the two hydrogen atoms are on same side of the double bond; in the trans form, they are on opposite sides of the double bond. In nature, polyunsaturated fatty acids are usually in the cis form, but hydrogenated polyunsaturates (e.g. some margarines) have a greater proportion in the trans form. This has important dietary implications because trans fatty acids have characteristics very similar to saturated fats and are regarded as being more harmful than cis fatty acids. Consequently, although it is recommended that up to 6 per cent of food energy may be obtained from polyunsaturates, no more than 2 per cent of food energy should be obtained from trans fatty acids.

Polyunsaturated fatty acids include the essential fatty acids, linoleic acid and alpha-linolenic acid. A number of other polyunsaturated fatty acids occur in high concentrations in fish and vegetable oils and are thought to be particularly beneficial to health (see eicosapentaenoic acid and gamma-linolenic-acid). Nutritionists advise us to make sure that we have sufficient quantities of the essential fatty acids in our diets (in the UK, the Committee on Medical Aspects of Food Policy, recommend that linoleic and alpha-linolenic acid should provide at least 1.0 and 0.2 per cent, respectively, of total dietary energy). At the same time, however, we are also told to make sure that our total fat intake does not exceed 35 per cent of dietary intake. See also unsaturated fat.