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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Yamasaki et al. studied CLA <strong>and</strong> antibody production in vivo <strong>and</strong> in vitro (133).<br />

In CLA-fed (0.05–0.5%) Sprague–Dawley rats, the production of IgG, IgM, <strong>and</strong><br />

IgA in spleen lymphocytes was dose-dependently increased, whereas the serum concentration<br />

of these immunoglobulins were not affected. In an in vitro assay with<br />

spleen lymphocytes, CLA at 100 �M suppressed Ig production (133).<br />

Dietary CLA supplement of 0.25 g per 100 g of diet for a week significantly<br />

reduced histamine <strong>and</strong> PGE 2 release from female Hartley guinea pig trachea tissue<br />

superfusate when sensitized with antigens, indicating a reduced release of some inflammatory<br />

mediators during type 1 hypersensitivity reactions (134). Feeding healthy<br />

young women subjects 3.9 g/day CLA did not change the indices of immune status,<br />

such as number of circulating white blood cells, granulocytes, monocytes, lymphocytes,<br />

<strong>and</strong> their subsets; lymphocyte proliferation in response to phytohemagglutinin;<br />

<strong>and</strong> influenza vaccine, serum influenza antibody titers, <strong>and</strong> DTH response (20,51).<br />

VII. POTENTIAL ADVERSE EFFECTS OF CLA<br />

Since CLA has become a widely advertised nutritional supplement for human use<br />

(135), it is important to study both its positive <strong>and</strong> potential negative effects on<br />

health using cell culture or animal models. Our recent dietary studies using an isomeric<br />

mixture of CLA revealed a negative effect on rat bone metabolism (74,101).<br />

Rats given 1% CLA in the diet demonstrated decreased mineral apposition rate <strong>and</strong><br />

bone formation rate in the tibia compared with rats not given CLA. In a follow-up<br />

experiment with rats, a lower level of CLA (0.5% diet) reduced serum osteocalcin<br />

<strong>and</strong> bone-specific alkaline phosphatase activity, both biomarkers of bone formation,<br />

after 12 weeks of dietary treatment (136). The negative effect of CLA was likely<br />

due to a high dietary level of isomeric mixtures that do not reflect the usual food<br />

sources of CLA.<br />

In a recent study by Belury et al. (137), CLA displayed the typical peroxisome<br />

proliferation response in rodent liver. Peroxisome proliferators may enhance tumorigenesis<br />

in liver, testes, <strong>and</strong> pancreas by acting as promoters, resulting in enhanced<br />

cell proliferation, altered cell differentiation, <strong>and</strong> inhibition of apoptosis in initiated<br />

cells (137). This response might suggest that the chemoprotective effect of CLA in<br />

extrahepatic tissues (55,56,58,76) may be at the expense of enhanced hepatocarcinogenesis.<br />

Jones et al. (138) also reported that CLA lowered blood LDL cholesterol<br />

but increased VLDL cholesterol <strong>and</strong> resulted in liver hypertrophy in mice.<br />

The results of Belury et al. (137) indicate that the peroxisome proliferation<br />

response to CLA may be greater in mice than in rats. These data suggest a species<br />

difference in the response to CLA, <strong>and</strong> such information might be relevant to a<br />

proper risk assessment for human consumption of isomeric mixtures of CLA. Thus<br />

far, the information on CLA action in humans is still very limited; additional research<br />

is necessary to clarify safety issues related not only to isomeric supplements of CLA<br />

but also to individual isomers that could behave differently from each other in various<br />

biological systems <strong>and</strong> physiological conditions.<br />

Many CLA isomers present in commercial CLA supplements for human use<br />

do not exist in natural food products. Furthermore, even for the naturally occurring<br />

CLA isomers, human consumption without dietary supplementation is normally at a<br />

very low level. The possible negative effects of CLA at trace levels could not be<br />

easily detected. Therefore, before promoting the human use of dietary CLA supple-<br />

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

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