Trans fatty acid isomers:: origin and occurrence in food

Experimental and epidemiological studies have recently pointed out the influence of trans fatty acids on plasmatic cholesterol levels. These studies have tried to demonstrate the incidence of the occurrence of such isomers in food on coronary heart diseases. Conjugated Linoleic Acid (CLA) would exhibit a protective effect against chemio-inducted cancers. The main food sources of trans fatty acids are milk fat products, partially hydrogenated oils and heated oils. In this review, the geometrical and positional isomer contents and distribution in milk fat products are compared to the trans fatty acid contents in margarines and partially hydrogenated vegetable fat, and in cooking oils. Trans fatty acids in milk fat products have their origin in the hydrogenation of polyunsaturated fatty acids by rumen micro-organisms. The effect of the diet of mammals on the trans fatty acid production in milk is preponderant. The bio-hydrogenation results mainly in 18:1 trans isomers, especially the Ii-trans vaccenic acid, and produces also noticeable amounts of 9-cis, 11-trans 18:2 rumenic acid, a conjugated linoleic acid (CLA) having anti-carcinogenic properties. Trans isomers in margarine are mainly 18:1 acids as well, but the positional isomer distribution is Gaussian, centred around 9-trans 18:1 elaidic acid. Fatty acid isomerisation during heating treatments affects linoleic (18:2) and alpha -linolenic (18:3) acids, two essential fatty acids. All these topics are discussed in this review.