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Effects of dietary equol administration on the mammary gland in ovariectomized Sprague-Dawley rats

Objective: To evaluate the effects of dietary equol, a metabolite of soy-derived daidzein or formononetin present in red clover, on the mammary gland of ovariectomized Sprague-Dawley rats. Design: Sixty ovariectomized rats were divided into five groups (n = 12) and fed soy-free chow with the addition of equol (50 mg/kg chow and 400 mg/kg chow) or estradiol-3 benzoate (E2B) (4.3 mg/kg chow and 17.3 mg/kg chow). The control group received soy-free chow only. After 3 months animals were killed, blood was collected, and the mammary glands were removed for histological and immunohistochemical evaluation. Results: Equol and E2B treatment significantly increased serum equol and 17&bgr;-estradiol concentrations, respectively. Serum prolactin in animals treated with high-dose equol was also significantly higher than in the controls. Animals treated with high-dose equol had a significantly higher number of terminal ducts and type II lobules compared with controls. This was also apparent in animals treated with low- and high-dose E2B, but a higher number of type I lobules also was seen. Compared with controls, animals treated with high-dose equol had a significantly higher percentage of proliferating cell nuclear antigen-positive cells in terminal ducts and type II lobules. The percentage of progesterone receptor-positive cells in animals treated with high-dose equol was significantly higher only in type II lobules. In animals treated with low- and high-dose E2B, the percentage of proliferating cell nuclear antigen- and progesterone receptor-positive cells was significantly higher in all the mammary structures. Low-dose equol did not have any effects on the parameters listed above. Conclusions: High-dose dietary equol administration to ovariectomized rats exerts clear mammotropic effects.