Do phytoestrogens lower the risk of breast cancer?

Nutritional factors modulate the risk of breast cancer in women. Epidemiologic studies suggest that diets high in soy, unrefined grain and vegetables may lower the risk of breast cancer. Asian women with a traditional diet high in plant estrogens have a low incidence of and mortality from breast cancer. Migration studies show a low risk of breast cancer in first-generation female migrants with an Asian home country; however, in the second generation with an increasingly Western diet this protection is lost. Of eight case-control studies performed between 1991 and 1999, five suggest a protective effect for premenopausal women, two a protective effect for premenopausal and postmenopausal women, and three no effect. Short-term perinatal or prepubertal phytoestrogen exposure in rat models reduced the incidence of chemically-induced breast cancer and the number of tumors in adulthood by 50%. This pronounced protective effect is associated with ductal differentiation of the mammary gland tree resulting in a lowered susceptibility of the breast epithelium to mutagenic alterations, comparable to the protective effect of an early first pregnancy. The low risk of breast cancer in Asian females might thus be due to prepubertal exposure to phytoestrogens in a soy-rich diet. The data suggest that phytoestrogens may lower the risk of breast cancer, but there is no direct evidence in humans because current information is based on the consumption of phytoestrogens in food. The existing data do not justify a recommendation for increased phytoestrogen intake to prevent breast cancer. Focused clinical intervention studies are necessary to assess benefits and risks of phytoestrogen intake in Western women to prove a putative cancer protective effect.