Soy and Breast Cancer Risk

From Dr. Weil: Japanese scientists have presented new information on the effect of soy foods on breast cancer risk, at least among Japanese women: eating soy-based foods on a regular basis is protective. But excessive amounts of genistein, a compound found in soybeans, might have the opposite effect. The investigators from Japan’s National Cancer Center in Tokyo followed approximately 25,000 women between the ages of 40 and 69 for an average of ten and a half years and compared blood samples from 144 women who developed breast cancer and 288 who did not. The tests showed that women who ate whole soy foods - about 3.5 ounces of tofu daily or 1.75 ounces of natto (Japanese fermented beans) - had protective levels of genistein. This group reduced their breast cancer risk to one-third of what it was among women whose genistein levels were lowest. However, the researchers warned that excessive consumption of genistein and other isoflavone compounds from soy isolates or from supplements rather than whole soy foods may actually increase breast cancer risk.

Note from Marcia: If you take synthetic thyroid hormone supplements for hypothyroidism, check with your doctor about incorporating soy into your diet. Soy can interfere with absorption of the synthetic hormones. Generally, it is considered best to limit consumption to one serving or less of soy per day.