Dr. Neal Barnard has a high-quality, evidence-based summary of soy research on Huffington Post. It’s readable, referenced and answers all the questions you might have.
Here’s the breast cancer section:
Women who include soy products in their routines are less likely to develop breast cancer, compared with other women. In January 2008, researchers at the University of Southern California found that women averaging one cup of soymilk or about one-half cup of tofu daily have about a 30 percent less risk of developing breast cancer, compared with women who have little or no soy products in their diets (1). However, to be effective, the soy consumption may have to occur early in life, as breast tissue is forming during adolescence (2-3).
What about women who have already been diagnosed with breast cancer? A study published in the Journal of the American Medical Association in 2009 shows that soy products may reduce the risk of recurrence (4). In a group of 5,042 women previously diagnosed with breast cancer who were participating in the Shanghai Breast Cancer Survival Study over a four-year period, those who regularly consumed soy products, such as soymilk, tofu, or edamame, had a 32 percent lower risk of recurrence and a 29 percent decreased risk of death, compared with women who consumed little or no soy. An accompanying editorial suggested that inconsistencies in prior research may be attributable to the comparatively low soy consumption in the United States, making beneficial effects harder to identify (5).
