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Food Lipids: Chemistry, Nutrition, and Biotechnology

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feeding. These findings suggest that the CLA mechanism for body fat reduction in<br />

mice, <strong>and</strong> possibly in other animals, is a result of inhibition of fat transportation <strong>and</strong><br />

storage in adipocytes coupled with both elevated � oxidation in skeletal muscle <strong>and</strong><br />

an increase in skeletal muscle mass (71).<br />

D. Bone<br />

Our laboratory recently investigated the effects of CLA isomers on bone modeling<br />

in growing male rats (73,74). In our studies with rats, 1% dietary CLA combined<br />

with two ratios of n-6/n-3 fatty acids led to differences in CLA enrichment of various<br />

organs <strong>and</strong> tissues. Brain exhibited the lowest concentration of isomers, but bone<br />

tissue (periosteum <strong>and</strong> marrow) contained the highest amounts (73). Both n-3 fatty<br />

acids <strong>and</strong> CLA lowered ex vivo prostagl<strong>and</strong>in E2 (PGE2) production in bone organ<br />

culture. The supplemental CLA isomers also reduced serum insulin-like growth factor<br />

type I (IGF-I) concentration <strong>and</strong> modulated IGF-binding protein (IGFBP) differentially<br />

depending on the ratio of n-6/n-3 fatty acids in the diet. Moreover, CLA increased<br />

IGFBP in rats given a high dietary level of n-6 fatty acids but decreased<br />

IGFBP in rats given a high level of n-3 fatty acids. In tibia, rats given CLA had<br />

markedly reduced mineral apposition rate (MAR) (3.69 vs. 2.79 �m/day) <strong>and</strong> bone<br />

formation rate (BFR) (0.96 vs. 0.65 �m/day) in comparison with those not given<br />

the CLA supplement (74). Dietary lipid treatments did not affect serum intact osteocalcin<br />

or bone mineral content. These results showed that a mixture of CLA isomers<br />

at 1% of the diet modulated local factors that regulate bone metabolism <strong>and</strong> reduced<br />

bone formation rates. This response may be due to the total dietary level of CLA or<br />

varying effects of individual isomers of CLA on bone biochemistry <strong>and</strong> physiology.<br />

In a subsequent study with rats, a 0.5% dietary level of CLA was provided<br />

with or without beef fat (101). The dietary CLA treatments resulted in total CLA<br />

values ranging from 0.27% to 0.43% in the polar lipid fraction <strong>and</strong> from 2.02% to<br />

3.37% in the neutral lipids in liver, bone marrow, <strong>and</strong> bone periosteum. We observed<br />

that CLA accumulated at a higher concentration in neutral lipids compared with polar<br />

lipids consistent with the findings of Ip et al. (56) for rat mammary gl<strong>and</strong>. In rats,<br />

the t10,c12-18:2 isomer was incorporated into the phospholipid fraction of tissue<br />

lipid extracts at the same extent as was the c9,t11 isomer. The ratio of c9,t11/t10,c12<br />

roughly reflected the isomeric distribution of these CLA isomers in the diet or supplement<br />

given to rats. Rat serum osteocalcin, a serum bone formation marker, was<br />

decreased in rats given CLA after 12 weeks of dietary treatment. Serum bone-specific<br />

alkaline phosphatase activity was also significantly decreased in rats given CLA. The<br />

fact that CLA lowered serum bone formation biomarkers, together with our previous<br />

finding that CLA lowered ex vivo PGE2 production in bone organ culture <strong>and</strong> bone<br />

formation rate, suggests that some CLA isomers may exert a down-regulatory effect<br />

on bone metabolism in growing animals. Future research must be conducted to evaluate<br />

the effects of individual CLA isomers on osteoblast function <strong>and</strong> bone formation.<br />

VI. MECHANISMS OF CLA ACTION<br />

Continued research on CLA has focused on elucidating its mode of action. The CLA<br />

isomers have been demonstrated to possess antioxidant properties (54,55,63,64), inhibit<br />

carcinogen-DNA adduct formation (58,102,103), induce apoptosis (60), mod-<br />

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

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