Trans Fatty acids dietary sources

Conjugated linoleic acid (CLA) is naturally present in milk, dairy products, and the meat of ruminants (1). Ruminants are the major dietary source of this fatty acid because of the unique abihty of rumen bacteria to convert linoleic acid into cis-9,trans- CLA (c9,tll-CLA) (1). This reaction is part of a process that takes place in the rumen it is called biohydrogenation and it converts linoleic acid [or, less efficiently, other 18-carbon polyunsaturated fatty acids (PUFA) with double bonds located at 9 and 12 positions] to stearic acid (1). During this process, vaccenic acid (tl 1-18 1) is formed. This acid can be converted to CLA in all organisms that possess A9-desaturase (2). 

Except for eicosanoic acid, the proportions of all fatty acids in egg yolk lipids were significantly (P < 0.01) influenced by the dietary CLA (59). The proportions of myristic, palmitic, and stearic acids, and CLA cis-9,trans- CLA and transit),cis-f2 CLA) in egg yolk lipids were increased by dietary CLA, but those of palmitoleic, oleic, linoleic, and linolenic, arachidonic acids, and DHA were decreased. These changes in fatty acid composition of yolk lipids are similar to those reported by Chamruspollert and Sell (60), although the total CLA concentration observed in the current research when a 5% CLA diet was fed (8.5-8.6%) was less than the 11.2% reported by those authors. The decrease in the concentrations of linoleic and linolenic acids in yolk lipids of hens fed CLA likely reflects the relatively low concentration of these fatty acids in the CLA source compared with soybean oil. Decreases in arachidonic acid and DHA in yolk lipids from hens fed CLA also could be related to the low concentration of dietary linoleic and linolenic acids, which serve as precursors to the formation of arachidonic acid and DHA. Another possibility is that CLA may compete with Unoleic and/or linolenic add for A6-desaturase, the rate-limiting step for the conversion of these fatty acids into arachidonic acid and/or DHA in liver microsomes (48). Feeding CLA increased the concentration of stearic acid in yolk lipids. 

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