Dietary purified cis-9,trans-11 conjugated linoleic acid isomer has anticarcinogenic properties in chemically induced mammary tumors in rats.

To determine whether the purified 9c,11t conjugated linoleic acid (CLA) isomer, the main dietary isomer, is biologically active on mammary tumor growth, we carried out a dietary intervention study designed to compare its effects with those of a mixture of CLA isomers on the incidence and growth of autochthonous mammary tumors induced by methylnitrosourea in rats. After the initiation step, rats were fed a sunflower oil-based diet (5%) and separated into three experimental groups supplemented with either a 1% homemade synthesized 9c,11t isomer, a 1% CLA isomer mixture, or free fatty acids prepared from sunflower oil for the control group. We found that, in the two CLA groups compared with the control group, CLA levels were about 30 times higher in mammary fat pads and about 10 times higher in tumor tissues. Compared with the control group, there was a 44% and 45% decrease in tumor mass per rat in the CLA mixture and the 9c,11t groups, respectively, at 20 wk of diet (P < 0.05). There was a nonsignificant trend for a decrease multiplicity in CLA groups compared with the control group, with a 30% and 35% decrease in the CLA mixture and the 9c,11t groups, respectively. Incidence and latency were not significantly different between the dietary groups. Although the effect was specifically restricted in reduction in tumor mass, we concluded that the main CLA isomer found in human diet has anticarcinogenic properties in experimental mammary carcinogenesis.