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Conjugated Linoleic Acid (9,11- and 10,12-Octadecadienoic Acid) Is Produced in Conventional but Not Germ-Free Rats Fed Linoleic Acid^1,2,

Food Research Institute, Department of Food Microbiology and Toxicology, University of Wisconsin-Madison, Madison, WI 53706

Conjugated linoleic acid (CLA) is an anticarcinogen in several model animal systems. Conjugated linoleic acid occurs naturally in food and is present at higher concentrations in products from ruminant animals. Given that certain rumen microorganisms produce CLA from free linoleic acid, we studied the effect of feeding free or esterified linoleic acid on tissue CLA concentrations using conventional and germfree rats. Conventional rats were fed a 5% (wt/wt) corn oil control diet alone or supplemented with 5% free linoleic acid or 8.63% corn oil (equivalent to 5% linoleic acid in triglyceride). Germ-free rats were fed autoclavable nonpurified diet alone or supplemented with 5% free linoleic acid. Analyses of CLA concentrations were performed on lipids extracted from liver, lung, kidney, skeletal muscle and abdominal adipose tissue, and on liver phospholipid and neutral lipid fractions. Tissue CLA concentrations were higher in conventional rats fed free linoleic acid (the major isomers were cis-9, trans-11 and trans-9, cis-11) than in control animals. Conjugated linoleic acid concentrations in free linoleic acid-fed rats were maximal at 4 wk, and levels were 5–10 times higher than those of controls. Elevated CLA concentrations were also observed in liver phospholipid and neutral lipid fractions. In contrast, CLA concentrations in the tissues of germ-free rats were not affected by diet. Feeding the corn oil-fortified diet to conventional rats did not increase CLA concentration in the tissues. We conclude that the intestinal bacterial flora of rats is capable of converting free linoleic acid (but not linoleic acid esterified in triglycerides) to cis-9, trans-11 and trans-9, cis-11 CLA isomers.

KEY WORDS: • conjugated linoleic acid • rats • linoleic acid

¹ Supported in part by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, a grant from the Wisconsin Milk Marketing Board, and gift funds administered through the Food Research Institute, University of Wisconsin-Madison.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 9 August 1993. Revision accepted 16 December 1993.

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