09.12.2012 Views

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

adipose tissue contained two major CLA isomers, both c9,t11 <strong>and</strong> t9,t11, <strong>and</strong> two<br />

minor isomers t9,c11 <strong>and</strong> c9,c11. The presence of CLA in human tissues is principally<br />

due to dietary intake since the amount of c9,t11-18:2 in human adipose tissue<br />

appears to be directly related to milk fat intake (45). However, the possibility of<br />

endogenously produced CLA in human tissues could not be excluded. Brown <strong>and</strong><br />

Moore (46) have reported the presence of CLA-producing bacterial strains of Butyrivibrio<br />

fibrisolvens isolated from human feces. Moreover, CLA is also produced in<br />

conventional, but not germ-free rats after consumption of LA (47). On the other<br />

h<strong>and</strong>, consuming safflower oil that is high in triacylglycerol-esterified LA, the precursor<br />

for bioisomerization, did not increase the CLA concentrations in total lipids<br />

of human plasma (38). Therefore, the contribution of colon-derived CLA to that<br />

found in human tissues <strong>and</strong> in nonruminant monogastric species is most likely<br />

negligible.<br />

Another possible endogenous origin of tissue CLA is by the desaturation of<br />

trans vaccenic acid, which was demonstrated in rat liver microsomal preparations<br />

(48). Studies performed in human subjects <strong>and</strong> pigs support this hypothesis. The<br />

results of Salminen et al. (42) support such a mechanism of endogenous CLA production<br />

by demonstrating that a high level of dietary trans fatty acids actually increased<br />

serum CLA content over stearic acid. Adlof et al. (49) showed that deuterated<br />

11-trans-octadecenoate (fed as the triglyceride) was converted to 9-cis,11-trans CLA<br />

via the �9 desaturase pathway at a CLA enrichment of about 30% in human subjects.<br />

The conversion of dietary C18:1 trans fatty acid to CLA was also demonstrated in<br />

a pig feeding study using partially hydrogenated fat that was rich in C18:1 trans<br />

fatty acids (50). This indicates that CLA could be produced by endogenous �9<br />

desaturation of dietary trans vaccenic acid in pigs.<br />

V. NUTRITION AND BIOLOGY OF CLA<br />

CLA is the only known anticarcinogen tested in animals associated with foods originating<br />

from animal sources. Ha <strong>and</strong> coworkers (9) provided one of the earliest<br />

observations that CLA from beef was protective against chemically induced cancer.<br />

In that study, CLA isolated from extracts of grilled ground beef were found to reduce<br />

skin tumors in mice treated with 7,12-dimethylbenz[�]anthracene (DMBA), a known<br />

carcinogen (9). Since then, numerous researchers have reported the effects of CLA<br />

isomers. The research on CLA has relied entirely on animal models <strong>and</strong> cell culture<br />

systems employing isomeric mixtures of CLA. Recent studies in human subjects<br />

have been negative (20,51,52). The purported properties of CLA include anticarcinogenic<br />

(9,53–59,60–62) <strong>and</strong> antiatherosclerotic (63,64). Other CLA effects include<br />

antioxidative (54,55,63,64) <strong>and</strong> immunomodulative (47,65–68).<br />

More recently, preliminary data suggest that CLA may have a role in controlling<br />

obesity (69–71), reducing the risk of diabetes (72), <strong>and</strong> modulating bone metabolism<br />

(73,74). In addition, some studies indicate that the biological effects of CLA<br />

are modulated by dietary sources of long chain n-3 fatty acids (68,73,74).<br />

Considerable research has been done to examine the anticarcinogenic properties<br />

of CLA. Isomers of CLA have been shown to reduce chemically induced tumorigenesis<br />

in rat mammary gl<strong>and</strong> <strong>and</strong> colon (53,55–58, 60,62,75). Moreover, CLA<br />

modulated chemically induced carcinogenesis in mouse skin (9,76) <strong>and</strong> forestomach<br />

(54). Sources of CLA also inhibited the growth of human tumor cell lines in culture<br />

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

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!