Trans fatty acids are present in the diet in esterified form, mainly in triacylglycerols but those from ruminant sources may also be present in phospholi-pids. Before absorption into the body, triacylglycer-ols must be digested by pancreatic lipase in the upper small intestine. There is no evidence of differences in the hydrolysis and absorption of trans fatty acids, in comparison with that of cis fatty acids. Trans fatty acids are transported from the intestine mainly in chylomicrons, but some are also incorporated into cholesteryl esters and phospholipids.
Trans fatty acids are incorporated into the lipids of most tissues of the body and are present in all the major classes of complex lipids. The positional distribution of trans fatty acids tends to show more similarity to that of saturated fatty acids than to that of the corresponding cis fatty acids. Some selectivity between tissues results in an uneven distribution of trans fatty acids throughout the body.
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Trans fatty acids occur mainly in positions 1 and 3 of triacylglycerols, the predominant lipids in adipose tissue. The concentration of trans fatty acids in adipose tissue is approximately proportional to long-term dietary intake, and determination of the concentrations in storage fat is one method used to estimate trans fatty acid intake. However, this is not entirely straightforward as variation has been reported in the composition of adipose tissue obtained from different sites and depths, and factors that influence adipose tissue turnover rates such as dieting and exercise are also complicating factors. Trans-18:1 isomers account for approximately 70% of the trans fatty acids found in adipose tissue, and trans-18:2 isomers (trans,trans, trans,cis, and cis,trans) account for about 20%.
In heart, liver, and brain, trans fatty acids occur mainly in membrane phospholipids. The position of the double bond as well as the conformation of the carbon chain may determine the pattern of trans fatty acid esterification in phospholipids, but there is evidence that trans-18:1 fatty acids are preferentially incorporated into position 1 of the phospho-acylgly-cerols, as are saturated fatty acids; in contrast, oleic acid is randomly distributed.
The turnover of trans fatty acids parallels that of other types of fatty acids in the body, and trans fatty acids are readily removed from the tissues for oxidation. Studies in which human subjects were fed labelled carbon-13 isotope have demonstrated that the whole-body oxidation rate for trans-18:1 is similar to that for cis-18:1. Trans fatty acids are a minor component of tissue lipids, and their concentrations in tissues are much lower than their concentrations in the diet. However, research has focused on C18 trans fatty acids, and more studies are needed to investigate the effects of very long-chain trans fatty acids derived from the hydrogenation of fish oils.
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