Currently, researchers are unsure of the exact effect of soy on breast cancer. Researchers believe phytoestrogens found in soy may help protect against breast cancer because phytoestrogens compete with estrogen in the body to bind to estrogen receptors on cells. Since estrogen triggers breast cell reproduction, some researchers believe that a higher amount of estrogen in the body may increase a woman's risk for breast cancer. Because phytoestrogens found in soy foods may block estrogen from reaching estrogen receptors, pre-menopausal women who include soy in their diet may decrease their risk of breast cancer. Though research on the effects of soy in pre-menopausal women shows some promise, it is preliminary and needs to be confirmed in large clinical trials.
Researchers are less certain about the effects of soy in post-menopausal women.
Some small studies have shown that soy may provide post-menopausal women with many of the same benefits as hormone replacement therapy . Soy may reduce hot flashes, vaginal dryness, and other menopausal symptoms. Whether soy can help protect against breast cancer in post-menopausal women is unclear at this time. At this time, researchers are not certain what effect soy may have on breast cancer patients or survivors. However, some researchers believe that phytoestrogens found in many soy foods may stimulate cell growth and could actually lead to a higher risk of breast cancer recurrence .