Fatty acids are unsaturated if they possess a double bond in their carbon chain, and can be classified as monounsaturated (one double bond) or poly unsaturated (many double bonds). Unsaturated fatty acids were historically grouped together in one category, but recent advances in nutrition has caused the splitting of the unsaturated fats into sub categories. One of these divisions of interest to health is the distinction between the cis and trans variants of various of the unsaturated fatty acids. A cis configured unsaturated fatty acid has the hydrogen atoms of the double bond on the same side of the molecule which causes the fatty acid to become kinked due to the repelling forces of the hydrogen atoms. In contrast, a trans configuration has the hydrogen atoms on opposite sides of the double bond, with the result that the fatty acid is straight (figure 1).
Figure 1. A trans and cis double bond in a fatty acid.
The trans configuration of a fatty acid is named a trans fatty acid (TFA). Structural differences between the cis and trans version of unsaturated fatty acids produces quite different biological effects, which can have important health implications. Some TFA are present in the diet naturally and are found in food products from ruminants (e.g. goats, sheep, cattle, deer) such as milk, cheese or meat products. The TFA found in ruminant fats comprise about 2 to 3 % of the total fatty acid content. The 18 carbon TFA found in ruminant fat tend to have the trans double bond in the Δ-11 position, and this may not be detrimental to health. This TFA is called trans-vaccenic acid which is the precursor to conjugated linoleic acid (CLA). Ruminants form CLA in their rumen, but humans can form CLA from trans-vaccenic acid using the same desaturase enzyme that creates oleic acid from stearic acid
Alternatively TFA can be formed synthetically during the processing of natural cis containing vegetable oils (such as sunflower and corn oils) into products such as hydrogenated vegetable oils or deodorised supermarket oils. Around half of the fatty acids in hydrogenated vegetable oils are trans fats, and they tend to have their trans double bonds in the Δ-8, Δ-9, Δ-10, Δ-11 and Δ-12 positions. Researchers such as Mary Enig have highlighted the health problems associated with the Δ-9, Δ-10 and Δ-12 trans fats in our diets. In particular, trans Δ-9 (elaidic acid) has been identified as having some important negative health effects. While trans Δ-9, Δ-10 and Δ-12 make up about one fifth of the 2 to 3 % of TFA in ruminant fats, they make up over half of the 50 to 60 % of TFA found in hydrogenated vegetable oil.
The main problem with trans fats is that they accumulate in cell membranes and cause cells and tissues to function in abnormal ways. This may be due to their interference with normal essential fatty acid metabolism and eicosanoid production. Trans fats have made an increasing contribution to the total fat content of our diets as use of hydrogenated vegetable oil has increased. In some individuals trans fats detrimentally alter levels of certain plasma lipoproteins which may be risk factor for cardiovascular disease. Other effects of feeding TFA include reductions in visual acuity in babies, reproductive changes, possible increases in cancer rates, reductions in insulin sensitivity and decreases in testosterone levels. Because of the quantity of TFA they contain, it is recommended that all synthetic trans fats are removed from the diet by total avoidance of hydrogenated vegetable oils and deodorised supermarket oils.
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