Dietary phytoestrogens, plant substances that are structurally or functionally similar to estrogen, have been proposed to act as estrogen antagonists in breast, prostate, and endometrial cells. However, epidemiological studies of dietary phytoestrogens and breast cancer have been inconclusive. Some studies indicate that soy intake may reduce a woman's risk of premenopausal breast cancer;77-79 however, at least one study found no association between dietary soy and breast cancer.80 One study has also reported a decrease in risk for postmenopausal breast cancer.79 Ingram et al.81 re-examined the association between phytoestrogens and breast cancer by measuring urinary excretion rates of two classes of phytochemicals and found that an inverse relationship exists with the risk of both pre and postmenopausal breast cancer and urinary excretion. It remains untested whether the measured phytochemicals actually serve as markers of other correlated dietary components, such as fiber, which also has been postulated to reduce breast cancer risk.82
The results of animal experiments and dietary intervention studies using soy products are also inconsistent. Experimental studies show that phytoestrogens can exhibit both an estrogenic and an antiestrogenic effect, depending on the study conditions.83-94 Two dietary studies did find that premenopausal women on soy-rich diets had lower levels of leutinizing hormone, follicle-stimulating hormone, and progesterone and longer menstrual cycles.95,96 Of potential concern are two reports that found women on soy-rich diets had elevated numbers of hyper-plastic epithelial cells in their breast fluid97 and significantly increased rates of breast lobular epithelial proliferation.98
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