Trans unsaturated fatty acids in natural products and processed foods.

Various representatives of trans unsaturated fatty acids have been detected in plants. The main trans monoenoic acids in ruminants, elaidic acid and vaccenic acid are not found in plants. Most oil seeds used for production of edible fats do not contain any trans fatty acids. Trans fatty acids in non-ruminant animals are derived from food. In adult ruminants, however, trans fatty acids are produced by microbial hydrogenation of linoleic acid and linolenic acid in the rumen; in consequence, a variety of positional and stereoisomers of both cis and trans fatty acids appear in both meat and milk. The total trans content of 5-10% in beef fat is largely trans monoene, mainly 9-trans (elaidic acid), 10-trans and 11-trans (vaccenic acid). The trans fatty acid content of human fat is dependent on the uptake of trans fatty acid from food. The usual content is about 2%, though up to 15% has been recorded. The isomer distribution is similar to that of the trans isomers of butter and margarine. The trans fatty acid content in ruminant products such as milk, butter, cheese, curds and tallow is 5-10%. These products contain nearly all possible stereo- and positional isomers of oleic and linoleic acid. The trans fatty acid content of margarines fluctuates widely according to raw material and process conditions. According to one investigation, an average of 16% of all unsaturated fatty acids in margarines consists of geometrical and positional isomers of the "natural" acids, a similar percentage to that in butter. Diet and health margarines have much lower contents of trans fatty acids. The total trans content of margarines is largely due to trans-18:1 monoenes. The positional isomers of the trans-monoenes are more evenly distributed in margarine than in butter. Hardened oils do not contain trans fatty acid isomers other than those produced by the microflora of ruminants. Therefore, claims that trans fatty acid isomers are "synthetic", "nonphysiological" or "unnatural" are unjustified if these words are used to imply "not produced by the living organism".