Current United States health statistics show that one out of eight women will develop breast cancer. In Japan, where women consume a diet rich in soy foods such as tofu and miso, the breast cancer rate is four hundred percent lower! Studies have shown that soy may promote breast health through several mechanisms, however, most research has focused on soy's anti-estrogen properties. As was discussed above, estrogen can stimulate the growth of breast cancer cells. Soy, however, works against the effects of estrogen on these cells. The estrogen blocking mechanism of soy can reduce the size of the cancer or even prevent it from forming.

Medications such as tamoxifen that, like soy, interfere with the activity of estrogen have been used successfully for over twenty years to treat patients with breast cancer. When researchers in the Department of Cancer Research at Hiroshima University, Japan, combined miso with tamoxifen to treat animals with mammary cancer they found the combination had a powerful synergistic effect. "These results," researchers reported in the Japanese Journal of Cancer Research, "indicate that miso is useful in protecting against mammary cancer and it can be expected to have a potent anti-tumor effect, especially when used in combination with tamoxifen."