For consumers, the connection between soy and breast cancer may be the most confusing. After over 15 years of research on this topic, we don’t have any clear-cut answers. In theory, the plant estrogens in soy foods act as anti-estrogens. This means they may block natural estrogen from reaching the cells’ estrogen receptors. Therefore, soy is probably beneficial when the breasts are developing during childhood, making them less vulnerable to cancer. Later in life, when pre-menopausal women experience high levels of natural estrogen, the estrogens in soy may compete with natural estrogen resulting in positive benefits.
Post-menopause women, however, have low levels of natural estrogen. Adding plant estrogens at this time may increase the risk of breast cancer. Soy isoflavones may enhance tumor growth in women who have (or have had) estrogen-dependent cancers (like some breast cancers). The bottom line: Studies have not reliably demonstrated an increased or decreased risk of breast cancer among women eating soy. The American Institute for Cancer Research (AICR) states that soy consumption early in life may help protect against breast cancer later in life. The American Cancer Society suggests that those at risk for breast cancer should not consume soy isoflavones.
Japanese men have lower rates of prostate cancer than men in the United States, and some experts contribute that to the Japanese diet, which is high in soy foods. The results of a few animal studies showed promise in this area. However, two small studies on humans (men) yielded controversial results. Soy isoflavones appeared to lower and improve the PSA (prostate-specific antigen) score for some men. For others, PSA level increased, but at a slower rate. And for certain men, the isoflavones worsened the prostate cancer condition. The bottom line: There are not enough human studies to say whether the isoflavones in soy affect prostate cancer for better or for worse.