A variety of health benefits, including protection against
breast cancer, have been attributed to soy food consumption, primarily because of the soybean
isoflavones (
genistein,
daidzein,
glycitein).
Isoflavones are considered to be possible
selective estrogen receptor modulators but possess nonhormonal properties that also may contribute to their effects. Concern has arisen over a possible detrimental effect of soy in
breast cancer patients because of the
estrogen-like effects of
isoflavones.
Genistein exhibits a biphasic effect on the growth of MCF-7 cells in vitro, stimulating proliferation at low concentrations but inhibiting it at high concentrations. In ovariectomized athymic mice implanted with MCF-7 cells, both
genistein and
soy protein stimulate
tumor growth in a dose-dependent manner. In contrast, in intact mice fed
estrogen,
genistein inhibits
tumor growth. Although two studies in premenopausal women suggested that soy exerts estrogenic-like effects on breast tissue, recently conducted year-long studies indicated that
isoflavone supplements do not affect breast tissue density in premenopausal women and may decrease density in postmenopausal women. These latter effects are opposite to those of
hormone replacement therapy (HRT). Importantly, substantial data suggest that the
progestogen, not the
estrogen, component of HRT increases risk of developing
breast cancer. Furthermore, recently conducted studies have failed to find that even HRT reduces survival in
breast cancer patients. Overall, the data are not impressive that the adult consumption of soy affects the risk of developing
breast cancer or that soy consumption affects the survival of
breast cancer patients. Consequently, if
breast cancer patients enjoy soy products, it seems reasonable for them to continue to use them.