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Trans fatty acids belong to a group of substances forming part of the fat content of the human diet (in the UK). The characteristic feature of these fatty acids is that one or more of the unsaturated sites in the molecule is in the unusual geometry known as 'trans', in contrast to the more usual 'cis' form. Trans fatty acids occur naturally in small amounts in milk and dairy products and in the meat from sheep and cattle as a result of the hydrogenation process in the animal's rumen. Larger amounts are formed as a result of the hydrogenation of vegetable and fish oils for the manufacture of margarine, cooking fats and oils.

Average consumption of trans fatty acids in the UK at the end of the 1980s was estimated to be 4-6g per person per day, representing approximately 2% of dietary energy intake. Some individuals may, however, consume 12g or more of trans fatty acids per day and others as little as 1-2g. Spreading fats, particularly margarine, provide an average of 30% of the trans fatty acids in the adult diet. Cereal products, e.g. cakes and biscuits, provide a further 27%, meat and meat products provide 18% and milk products (e.g. cheese) provide 10%.

In 1987, a major review in the UK concluded that there was insufficient evidence (allegedly: DA) to make firm conclusions about the effects of trans fatty acids on health. Since then, an American epidemiological study has reported an association between consumption of trans fatty acids from hydrogenated vegetable oils and risk of coronary heart disease and breast cancer while several experimental studies have concluded that consumption of high levels of trans fatty acids results in adverse patterns of blood lipoproteins. It has also been suggested that trans fatty acids are readily transferred to the foetus and may influence birth weight.

Thus, over the period since 1987, there has been a shift of scientific opinion towards the view that consumption of high levels of trans fatty acids may increase the risk of CHD insofar as they influence blood lipoprotein concentrations, although there is no conclusive evidence concerning their influence on thrombosis. However, average intakes of trans fatty acids in the UK are low and are falling slowly. Available evidence does not suggest that current average levels of consumption are hazardous in any way provided that an adequate consumption of essential polyunsaturated fatty acids is maintained, which is the case for most people. However, a continuation of the downward trend seems advisable and high intakes by individuals should be avoided.

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