A few years ago, studies started yielding conflicting results, suggesting that certain dosages of soy protein might actually serve to accelerate cancer-cell growth. "One group studying isoflavones found that high doses suppress various types of tumors," says Suzanne Hendrich, a professor in food science and nutrition at Iowa State University, "and another group showed that high doses stimulate mammary tumors." Even the experts were confused, Hendrich says.
The issue may hinge on when in a woman's life the soy is eaten, says Claude Hughes, M.D., a consulting professor at Duke University who has been researching phytoestrogens for almost 20 years: "The overall effect of taking in isoflavones is to moderate, or decrease, natural estrogen activity," which may benefit premenopausal women. Even soy eaten quite early in life, before puberty, may protect against breast cancer in later life, perhaps by changing the structure of the breast tissue in some way that makes it more resistant to cancer. But in postmenopausal women, Hughes says, "isoflavones may slightly increase estrogen activity, rather than decrease it, because at that age the body has less estrogen occurring naturally." So even the phytoestrogens can have some weak estrogen effects.
"It's an open question whether this is bad or not," Hughes says. Just as in estrogen- replacement therapy, the extra estrogen provided by soy isoflavones may be beneficial to the heart and bones while potentially raising the risk for certain cancers. Women who have had breast cancer or whose family medical histories show a high risk for the disease should be careful about soy intake, because they may be more susceptible to estrogen-activated tumors.
You may have heard of other studies indicating problems that arise from eating isoflavones, but the evidence is sketchy, at best. For instance, although some data show that soy could increase the risk of thyroid disorders, researchers agree that this threat is unlikely for anyone whose diet includes a normal amount of iodine (that is, anyone who eats ordinary table salt), unless she has a preexisting thyroid condition. Other results indicated an adverse effect on immune system function, but those studies involved injecting rats with dosages several times the relative amount of soy that a person would eat, Hendrich says.
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