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Food Lipids: Chemistry, Nutrition, and Biotechnology

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ulate tissue fatty acid composition <strong>and</strong> eicosanoid metabolism (61,62,67,68,73,74,<br />

104–106), <strong>and</strong> affect the expression <strong>and</strong> action of cytokines <strong>and</strong> growth factors (68).<br />

Since most of the studies mentioned above were conducted with mixtures of several<br />

CLA isomers (mainly c9,t11-18:2 <strong>and</strong> t10,c12-18:2), these proposed mechanisms of<br />

action might be specific for individual CLA isomers.<br />

A. Antioxidative Action<br />

Antioxidants originating from both natural <strong>and</strong> synthetic sources with diverse structures<br />

have been known to demonstrate some anticarcinogenic activity (107). Since<br />

CLA is proposed to possess antioxidative action, this could explain an important<br />

mechanism related to its anticarcinogenic activity. Several studies have been conducted<br />

using both in vivo <strong>and</strong> in vitro systems to clarify the role of CLA as an<br />

antioxidant; however, the results obtained from various test systems were conflicting.<br />

Ha et al. (54), for the first time, showed that CLA was a potent in vitro antioxidant.<br />

In an in vivo study, Ip et al. (55) reported that CLA was as effective as vitamin E<br />

in inhibiting the formation of TBARs in the mammary gl<strong>and</strong>. Similar to the results<br />

of Ha et al. (54), Ip et al. observed no dose–response relationship in the dietary<br />

range of 0.25–1.5% CLA. All doses tested produced a 30–40% inhibition of peroxide<br />

formation.<br />

A study with female rats given diets containing different lipids (20% corn oil<br />

or lard) or amounts of a fat blend (from 10–20% by weight) with or without 1%<br />

CLA (56) showed that CLA reduced malondialdehyde (a peroxidation product) production<br />

in rat mammary gl<strong>and</strong> homogenate. In addition, CLA produced a greater<br />

reduction in rats fed a diet high in PUFA (corn oil, 35%) than in those given a high<br />

saturated fat lard diet (25%). CLA failed to show any inhibitory effect on 8-hydroxydeoxyguanosine<br />

(8-OHdG, a marker of oxidative damage to DNA) level in rats.<br />

Leung <strong>and</strong> Liu (108) found that the t10,c12 CLA isomer exhibited a strong antioxidative<br />

property compared to the c9,t11 isomer <strong>and</strong> even �-tocopherol at a lower<br />

concentration of 2 <strong>and</strong> 20 �M in a total oxyradical scavenging capacity assay. The<br />

c9,t11 isomer yielded a weak antioxidant activity at lower concentrations of 2 <strong>and</strong><br />

20 �M, but at a higher concentration (200 �M) it performed as a strong pro-oxidant.<br />

Although CLA has been shown to be antioxidative in both in vitro <strong>and</strong> in vivo<br />

studies, the anticancer effect of CLA in these studies cannot be satisfactorily explained<br />

based on the current findings. First, the maximal effective concentration of<br />

CLA in inhibiting peroxide formation did not agree with the most effective concentration<br />

in tumor inhibition (55). Second, neither of the studies demonstrated a dose–<br />

response relationship between CLA concentration <strong>and</strong> its antioxidative efficacy,<br />

which is usually true for antioxygenic nutrients studied thus far.<br />

van den Berg et al. (21) reinvestigated the antioxidative property of CLA using<br />

a lipid membrane system consisting of 1-palmitoyl-2-linoleoyl phosphatidylcholine.<br />

The results of this study indicated that CLA did not show any protective effect under<br />

the test conditions <strong>and</strong> was more susceptible to oxidative damage than LA <strong>and</strong> comparable<br />

to arachidonic acid. In agreement with the findings of van den Berg et al.<br />

(21), Banni et al. (109) in a recent study also showed that CLA was more prone to<br />

oxidation than LA, <strong>and</strong> no significant antioxidant effect of CLA was detected in the<br />

models tested. It would appear that CLA <strong>and</strong> its metabolites seem to behave like<br />

other PUFAs under conditions of oxidative stress.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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