Thankfully, those fears can be put to rest: a recent scientific study published in the prestigious Journal of the American Medical Association suggests that soy can help decrease risk of death and decrease the chances of a breast cancer relapse. The study looked at a 4-year outcome period amongst 5,000 breast cancer survivors, and measured their intakes of soy foods in their diets. Women with the highest soy intakes had a 29 percent lower risk of dying and a 32 percent lower rate of their breast cancer returning than those who had the lowest intake of soy.
What I think is a very important point here is that these are not women who took soy supplements, or packed their diets with isolated soy protein. Tofu, soy milk and fresh soybeans (cooked soybeans or young soybeans, aka edamame) were the soyfoods most commonly consumed. So the study is NOT suggesting that breast cancer survivors eat soy bars, soy shakes, soy meat alternatives, and soy ice cream all day: the most protective soy foods are those that are the least processed/fractioned: tofu, tempeh, natto, miso, soy milk, and just plain soybeans.
Much of the worry and confusion surrounding soy and breast cancer stem from the fact that soyfoods are super sources of isoflavones, naturally-occurring, estrogen-like compounds. To many, that sounds like we're "eating estrogen," but actually the opposite is true: Breast cancer depends on estrogens to grow, and soy's "weak" estrogens compete for space on cell receptors and bind there, but do not exert estrogen's dangerous effects. So in essence, isoflavones are thought to BLOCK estrogen's deleterious effects.
In addition, the study authors reported that the results eased previous concern that isoflavones might interfere with tamoxifen, a cancer drug designed to block estrogen. The study found higher soy food consumption was beneficial regardless of whether a patient was taking tamoxifen.
The USDA recommends a daily intake of 25 grams of soy protein; this study suggested that 11 grams is enough for a significant benefit. This works out to be 1 to 2 servings of soy a day.
Click here for the study abstract, and option to purchase the full text: