Because of the great breadth of the topic and the voluminous and controversial data generated during the last 50 years, this discussion will be limited to recent developments concerning fatty acids of current interest to the animal industry, namely, trans fatty acids, CLA and n-3 fatty acids. Effects of dietary trans fatty acids on health have been objects of concern for many years, but only rather recently have data shown clearly that these may affect blood lipid profiles negatively. Though the primary dietary source of trans fatty acids is hydrogenated vegetable oils, ruminant fats contain significant amounts of trans fatty acids, mainly as vaccenic acid (trans-11 18:1). In this discussion we will show that not all trans fatty acids are created equal, and in fact, that vaccenic acid can be beneficial to health because it is a direct precursor of CLA in the body when consumed. CLAs (conjugated linoleic acids) compose a group of fatty acid isomers with 18 carbon atoms and containing two unsaturatedbonds in a conjugated arrangement; these bonds exist in many combinations of positional and cis/trans configurations. In recent years certain of the CLA isomers, namely cis-9, frans-11 18:2 (rumenic acid, the predominant form found in ruminant fats) andfrans-10, cis-12 18:2, have been shown to have important biological effects on cell function. These include inhibition of tumors in cell culture and animal models, inhibition of fat synthesis (frans-10, cis-12 isomer only), enhancement of immune function and bone development, inhibition of atherosclerosis, and modification of Type 2 diabetes. Though intense research is continuing to define and describe the mechanisms of CLA in biological systems, only limited epidemiological data are available to demonstrate positive effects of these on human health. Ruminant fats characteristically contain relatively low proportions of n-3 fatty acids; there is interest to increase these. Forage-based diets effectively increase n-3 fatty acids in tissues, although they remain a relatively low proportion of the total fat. There is some interest to increase the n-3 'fish oil' fatty acids, EPA and DHA, in ruminant fats by special feeding regimes; these may have some limited application in human diets. This discussion will review physiological effects of these fatty acids, their occurrence in the diet and estimated intakes and highlight some of the potential benefits of their consumption on human health.
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