Trans fatty acids are unsaturated fatty acids that have hydrogen molecules oriented on the same side of the carbon chain at the double bond. This is in contrast to cis fatty acids that have hydrogen molecules orientated on the opposite side of the carbon chain at the double bonds. While some trans fats are found naturally, for example in ruminant fats, the historical intake of trans fats has been very low in humans because they are not widely found in nature. Most natural fatty acids are present only in their cis form. Within the last 50 years the trans fat content of the human diet has increased significantly because the use of refined polyunsaturated fatty acids has increased in the manufacture of food. Trans fats are produced as part of the hydrogenation and processing of polyunsaturated fatty acids and these modified fats are then added to a wide range of typical Western foods that are characteristically processed. Evidence suggests that trans fats interfere with essential fatty acid metabolism and this may have deleterious effects on health.
The metabolism of trans fats has been studied by researchers in order to assess the fate of these modified fats in mammalian tissues. Much of this work has relied on animal experiments to provide details of the metabolic routes they follow. For example in one study1, researchers investigated the metabolism of the trans fats, trans-octadecanoic acid and trans-octadecadienoic acid, in rats fed a 15 % trans fat diet for 3 months. During this time some of the animals had their tissues examined for trans fatty acid concentrations. Following this 3 month period some the remaining rats were fed a trans fat free diet and then their tissues analysed for trans fatty acids concentrations. The results showed that trans-octadecenoate was incorporated preferentially into phospholipids and triglycerides of the plasma, liver, kidneys, heart, adipose tissue and red blood cells. However trans-octadecadienoate accumulated preferentially in the triglycerides of plasma, liver, heart, kidney and adipose tissue, with little accumulation in the phospholipids.
Therefore trans fatty acids may be incorporated into different tissues within mammals, suggesting the composition of the trans fats in the diet may influence the pathology seen from trans fat ingestion. Following removal of all trans fats from the diet of the rats, tissue levels of trans fatty acids declined such that by 8 weeks following cessation of trans fats, no trans fatty acids remained in most of the tissues of the animals. The exception to this was the adipose tissue which continued to contain measurable concentrations of both trans fats. Elimination of trans fats from the tissues following adherence to trans fat free diets may therefore take many months to accomplish, which suggests that disease risk may take similar lengths of time to adjust following dietary changes. The reason for the incorporation of the different trans fatty acids into different tissues likely relates to structural differences in the acetyltransferase enzymes between tissues, that are specific for particular fatty acid substrates
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