The hype about soy and breast cancer all started when researchers started seeing a positive correlation between women taking estrogen supplements and the increased prevalence of breast cancer. What does soy have to do with estrogen? Well, soy contains beneficial isoflavones, which actually act as plant estrogens.
Currently the information people are receiving is very mixed, but from a professional point of view I have read plenty of journal articles and taken enough classes (especially a supplement class I took last quarter) to confidently recommend soy as a part of a healthy diet. It seems that taking supplemental doses of soy, which is usually means you are consuming well above the 26 grams that are recommended each day, is where the problems begin.
My grandmother had breast cancer, therefore I have a higher risk for developing breast cancer. Do I still consume soy? Indeed. Do I take soy supplements? Not a chance. More and more research is now coming out that actually suggests that childhood soy intake has a significant association with reduced breast cancer. Another study actually found that a high intake (there was no definition of "high", but I am assuming they mean 26 grams or slightly more, not supplemental doses) in adolescent and adult years was associated with 20-25% reduction in breast cancer (Source: Today's Dietitian Magazine).
Other health benefits of soy:
Bone health for post menopausal women
Cholesterol reduction
Triglyceride reduction
May increase HDL (good cholesterol)
Sources: Soybeans, tofu, soymilk, tempeh, soy nuts, soy flour, miso, SOYJOY!

Bottom Line: If you still aren't convinced about the benefits of soy, you're not alone and it's ok to not include it in your diet. However, research has convinced me that it is ok, and actually beneficial, to include about 26 grams of soy into your diet regardless of any family history of breast cancer. The most readily absorbed form of soy is fermented soy (miso and tempeh) and soy is absorbed better when consumed with some type of probiotic (Source: Maureen Garaghty of OSU).
