Physiological effects of <i>trans</i> fatty acids in humans.

<i>Trans</i> fatty acids include not only mono- and poly-unsaturated fatty acids with methylene-interrupted double bonds but also isomers with conjugated double bonds such as conjugated isomers of linoleic acid (CLA). <i>Trans</i> monounsaturated fatty acids are formed during hydrogenation of oils to produce margarine and shortening but are also present in ruminant meat and milk as a result of biohydrogenation in the rumen. The linoleic and alpha-linolenic acid <i>trans</i> isomers may also be present in non-hydrogenated fats as the result of exposure of these polyunsaturated fatty acids to heat treatment, such as steam deodorization or deep fat frying. In food, conjugated fatty acids or CLA are found in products from ruminants such as meat and dairy products. <i>Trans</i> monounsaturated isomers are the major <i>trans</i> isomers consumed by humans. Data on <i>trans</i> fatty acids were obtained not only from nutritional interventions but also from epidemiological studies. For example, a meta-analysis of controlled trials published between 1970 and 1998 showed that replacing carbohydrates isoenergetically (1% of total energy) by 18:1 <i>trans</i> (eight studies) would increase low-density lipoprotein (LDL) cholesterol, while the effect on high-density lipoprotein (HDL) would be similar. The calculated impact on the ratio of total cholesterol/HDL would be of +0.022 nmol/l (<i>P</i>=0.015). A recent meta-analysis of the four larger epidemiological studies showed that increasing the total energy intake with 2% <i>trans</i> fatty acids would result in a 23% increase in cardiovascular disease risk. Effects of conjugated fatty acids in humans are still not clear as data obtained for intervention studies dealing with body composition, plasma lipid parameters and immune function are still controversial compared with what was found in some animal species. A recent meta-analysis using 18 human intervention trials reported a modest loss in body fat at a daily dose of 3.2 g/day. These data do not seem to be convincing enough to favour supplementation with CLA, especially when considering that adverse effects were also observed in some cases.