Effect of dietary genistein on estradiol-induced mammary carcinogenesis in the ACI rat

We evaluated the possible effectiveness of genistein (GEN) as an inhibitor of mammary cancer in the ACI rat, a strain genetically susceptible to estrogen-induced mammary carcinogenesis. In this study, three-week old ACI rats were fed a phytoestrogen-free diet or a diet containing 250 ppm GEN. At nine weeks of age, the rats were implanted with 20 mg pellets containing either 2 mg 17 beta-estradiol (E-2)/18 mg cholesterol or cholesterol alone. The appearance of palpable mammary tumors (MTs) was recorded. Our results show that E-2-treated rats fed the GEN diet both prepubertally and as adults had the highest incidence of palpable MTs. E-2-treated rats fed the GEN diet only prepubertally had delayed appearance of palpable MTs but the incidence quickly approached that of the rats fed GEN for the whole treatment period. Dietary GEN fed to adult E-2-treated rats had the lowest incidence of MT. Rats implanted with the cholesterol pellet and fed either diet did not develop MTs.