Conjugated Linoleic Acid Modulation of Risk Factors Associated With Atherosclerosis

Yukiko K. Nakamura; Nichole Flintoff-Dye; Stanley T. Omaye

Disclosures

Nutr Metab 

In This Article

Abstract and Conjugated Linoleic Acid

Abstract

Conjugated linoleic acid (CLA) has been the subject of extensive investigation regarding its possible benefits on a variety of human diseases. In some animal studies, CLA has been shown to have a beneficial effect on sclerotic lesions associated with atherosclerosis, be a possible anti-carcinogen, increase feed efficiency, and act as a lean body mass supplement. However, the results have been inconsistent, and the effects of CLA on atherogenesis appear to be dose-, isomer-, tissue-, and species-specific. Similarly, CLA trials in humans have resulted in conflicting findings. Both the human and animal study results may be attributed to contrasting doses of CLA, isomers, the coexistence of other dietary fatty acids, length of study, and inter-and/or intra-species diversities. Recent research advances have suggested the importance of CLA isomers in modulating gene expression involved in oxidative damage, fatty acid metabolism, immune/inflammatory responses, and ultimately atherosclerosis. Although the possible mechanisms of action of CLA have been suggested, they have yet to be determined.

Conjugated Linoleic Acid

A group of trans-fatty acids, conjugated linoleic acid (CLA) has been purported to have diverse physiological functions and potential health benefits.[1,2,3,4,5,6] These unique geometric and positional isomers of octadecadienoic acid derived from linoleic acid (18:2n-6) have been found in only a limited number of foods or food products mostly derived from the fat of range animals. The highest levels of CLA are found in ruminant animals (beef, lamb and dairy cows) with beef, milk-fat, and cheese, the most common animal products containing CLA. During the biohydrogenation of linoleic acid to stearic acid, CLA is synthesized in the rumen as an intermediate by gram-negative bacteria, Butyrivibrio fibrisolvens.[7] CLA is also found in fish, monogastric animal products, and plant products, however, in much lower concentrations.[3] CLA isomers have been identified during the hydrogenation of fat, e.g., margarine production, and are found primarily in foods considered high in fat. Also, CLA is found in low concentrations in the lipids of human blood, tissue, and milk,[8] presumably from dietary intakes. Although there are 28 different CLA isomers, the cis-9, trans-11 CLA isomer is predominantly found in the ruminant foods discussed earlier and accounts for >90% of CLA intake in the human diet.[9] The structures, shown in Figure 1, consist of 18 carbon atoms with two conjugated double bonds separated by a single bond, unlike linoleic acid, which is a non-conjugated diene.[1] The conjugated double bonds of CLA isomers contribute to their higher susceptibility to autioxidation than the non-conjugated bonds of linoleic acid.[10] Differences in chain length, degree of unsaturation, and position and stereoisomeric configuration of the double bonds affect fatty acid oxidation or lipid peroxidation. Usually, long-chain fatty acids are oxidized more slowly and unsaturated fatty acids are oxidized more rapidly than are saturated fatty acids. Lauric acid is highly oxidized, but PUFAs and monounsaturated fatty acids are fairly well oxidized.[11] Oxidation of the long-chain, saturated fatty acids decreases with increasing carbon number.

Figure 1.

Chemical structures of linoleic acid and isomers of conjugated acid (CLA).

Many research groups have looked at the possibility of CLA isomers as anti-carcinogens. Most anti-carcinogens are plant products (phytochemical), therefore, CLA isomers are unusual find because it occurs in the highest concentration in animal products (zoochemical) with only trace amounts found in plant lipids. The possibility of CLA isomers working as a feed efficiency supplement and a lean body mass supplement has also been examined, along with its role in cancer prevention and stimulation of the immune system.

With regard to potential health benefits, considerable attention has been given to anti-carcinogenic effects of CLA isomers; however, less attentions has been devoted toward its usefulness in preventing and reversing atherosclerosis and related diseases. The majority of research studies have been done using experimental animals and in vitro, with only recent investigations showing the effects of CLA isomers on humans. The purpose of this review is to assess and summarize current literature and knowledge on the possible health benefits of CLA isomers, particularly with respect to atherosclerosis as a chronic inflammatory disease.

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