I also had heard mixed things about soy and breast cancer - one study saying it helps prevent breast cancer and another that says it can increase the possibly of developing a tumor.
So I looked at Breastcancer.org believing they would have the most up to date information. Since October is Breast Cancer awareness month - Here is a summary of just that question posed this month: "
The estrogen-like substances (isoflavones) in soy may stimulate the estrogen receptors of breast cancer cells and make them grow. It's this potential danger that makes many doctors warn their breast cancer patients against eating soy products."
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On the other hand, there are some that believe that soy may protect a woman from breast cancer. Why? Because the strength of the soy isoflavones' estrogen-like powers is weak?-like 1/1000th the "umph" of estradiol, your own body's regular estrogen. If the weaker soy isoflavones occupy the estrogen receptors instead of your body's stronger stuff, then the belief is that the receptors will be less "turned on," and the cells will be less likely to grow. These soy enthusiasts believe that the lower risk of breast cancer seen in Asian women living in Asia may be due in part to their use of soy as their main source of protein
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Here's where you just have to rely on your best judgment and the cautious recommendations of responsible scientists, dietitians, and of course your own doctor. I tell my own patients that it's probably OK to eat soy in the form of soy foods up to a few times per week, as a healthy source of protein (no cholesterol; many fat-free products are also available) instead of chicken and meat. I do not feel comfortable with my patients using significant amounts of the purified soy products that come in the form of powders, tablets, capsules, drinks, etc., because it's entirely unknown if the soy isoflavones are safe in such high concentrations
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For the full Q&A:
http://www.breastcancer.org/faq_risk_soy.html