Found this on MSN web page of all places. 
The latest praise and craze has befallen soy. Toted as a heart-healthy, high-protein alternative to beef, chicken and fish, even the U.S. Food and Drug Administration is backing soy-rich products — In October 1999, the FDA allowed food manufacturers to promote soy as a way to help lower the risk of heart disease. But the old adage “everything in moderation” should apply to your intake of this miracle food.
Scientists are discovering that although soy can be great for the health of your heart, it may be a cause of concern when it comes to other problems, such as breast cancer, thyroid disorders and infant development.
Breast cancer
“Soy functions similarly to estrogen in our bodies and breast cancer is an estrogen-dependent cancer,” explains Andrea Miller, a community and family practice dietitian at Sunnybrook and Women’s College Health Sciences Centre. Although more studies are needed, the suspicion is that the estrogen-like components of soy, called isoflavones, may enhance breast cancer in some women, says Miller. In others, however, soy may protect against it.
Thyroid disorders According to Dr. Larrian Gillespie, author of The Menopause Diet, too much isoflavones a day can weaken the function of the thyroid. "More than 30 mg of isoflavones can send your thyroid into shutdown by blocking thyroid peroxidase enzymes and stimulating anti-thyroid antibodies," she wrote in the July 3, 2000, issue of The Menopause Diet Newsletter. And menopausal women are more at risk as they are already prone to hypothyroidism. The only link between soy and thyroid that Miller has come across relates to medication. Large amounts of soy, she explains, may affect the effectiveness of thyroid medication.
Infant development
Concentrated amounts of soy may also alter sexual development. According to a study on soy formula published in a 1997 issue of the medical journal The Lancet, infants had five to 10 times higher levels of isoflavones than women who took soy supplements and demonstrated menstrual changes. To date, the adverse effects have only been tracked in experimental animals.
In a joint statement on nutrition for healthy term infants, Health Canada, the Canadian Paediatric Society Nutrition Committee and the Dietitians of Canada had this to say: “Use soy-based formulas only for those infants who cannot take dairy-based products for health, cultural or religious reasons, such as vegan lifestyle, or galactosemia.”
The fine print
Before you empty the refrigerator and cupboards of soy and soy-based products, you should know that when it comes to health and diet, it’s not all cause and effect. “You can’t single out a part of the diet,” explains Miller. “If you increase soy you reduce red meat — so is it the increase in soy that’s having an effect or the reduction in red meat?”
Furthermore, while soy products may be inundating grocery shelves and health food stores, “For most western people, it’s a real stretch getting soy into a diet,” she says. To consume 30 milligrams of isoflavones in a day would require a concentrated form, such as powder or pill supplement. To limit your soy intake, have one serving a day and combine it with other healthy foods. “Anything can negatively affect health in large amounts,” says Miller. “Keeping a balance in our diets with all foods is important.”
