The aim of this study was to investigate how supplementation of diet of female Sprague-Dawley rats with different doses of conjugated linoleic acids and for a varied period of time influences breast cancer risk, fatty acids profile and lipids peroxidation in chemically induced mammary tumors. Animals were divided into nine groups with different modifications of diet (vegetable oil, 1.0 or 2.0% of CLA) and period of supplementation, which lasted after (A), before (B) and before and after (BA) carcinogenic agent - 7,12- dimethylbenz[a]anthracene administration at 50th day of life. Mammary adenocarcinomas occurred in all groups, but CLA supplementation decreased the cancer morbidity. Two percent CLA seem to be excessive because of the coexisting cachexia. Two CLA isomers (cis-9, trans-11 and trans-10, cis-12) were detected in tumors but content of rumenic acid was higher. Dietary supplementation significantly influenced some unsaturated fatty acids content (C18:2 n-6 trans, C20:1, C20:5 n-3, C22:2), but the anti- or prooxidant properties of CLA were not confirmed. CLA can inhibit chemically induced mammary tumors development in female rats, but their cytotoxic action seems not to be connected with lipids peroxidation. CLA isomers differ with their incorporation into cancerous tissues and they influence the content of some other fatty acids
