The total C18 trans isomers include elaidic acid, petroselaidic, and transvaccenic acids. The presence of these eighteen-carbon long trans fatty acids in human tissue can disrupt or impair cell membrane function. A person with high levels of total C18 trans isomers should avoid hydrogenated oils.

Trans fatty acids are prevalent in most diets because of the widespread use of hydrogenated oils used by manufacturers of margarines, bakery products, and peanut butters.

Fat is necessary for cell membranes, nerve coverings, hormone production, vitamin absorption, and more. Most of us get a lot of fat in our diet, but it usually is not the quality fat we should be getting. We need to eat more “good” fats from flax seed, olive, vegetable, and nut oils (omega- 3, 6, and 9 fats respectively) and less saturated oils and trans fats (or hydrogenated oils) contained in processed foods.

Elevations in Total C:18 trans indicate excessive intake of foods containing trans fats.

These fatty acids contain one double bond and thus are included in the unsaturated category. Because of the geometry of the trans bond, however, they behave like saturated fats on the one hand, leading to elevated cholesterol levels. On the other hand they mimic unsaturated fats that bind to desaturase enzymes and antagonize the normal production of necessary products. The net effect is to raise plasma LDL cholesterol and lower HDL. It is now the consensus among experts in lipid nutrition that foods containing hydrogenated oils are to be avoided. These fatty acids are also produced by the bacteria in the gut of ruminant animals which is the reason that beef and milk contain small amounts of elaidic acid.

References:

https://grey.colorado.edu/mediawiki/sites/shortgut/images/6/62/Plasmafattyacids.pdf