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The question

I've heard that soy is good for you - but I've also read that's it's harmful to eat too much soy. Is it really bad from me? What should I believe?

The answer

You're right: You'll find plenty of information on the Internet touting the health benefits of soy. But among the soy advocates there are also critics who warn that too much soy is harmful to infants, interferes with thyroid function, and may cause cancer.

Soybean nutrition

No one disputes the fact that soybeans deliver plenty of nutrition, especially protein. Soybeans are also very low in saturated fat and cholesterol and they provide fibre, omega-3 fats, vitamins, minerals and antioxidants.

Soy also contains isoflavones, natural compounds that behave like very weak forms of the body's own estrogen. Isoflavones compete for the same place on cells that estrogen does. Some of the risks of excess estrogen, including breast and uterine cancer, are thought to be lowered in this way.

The case against soy revolves around isoflavones. The most controversial and unresolved issue is breast cancer. The concern is that soy isoflavones could increase a woman's total estrogen levels and encourage the growth of estrogen-dependent breast cancer, especially in women who already have the disease.

But it's not that simple. Some studies conducted in animals and test tubes indicate that extracts of isoflavones inhibit the development of breast cancer, while others suggest they may increase breast cancer growth. It depends on the particular isoflavone studied (soybeans are rich in two different isoflavones) and the amount used. There's no compelling evidence that soy foods increase breast cancer. In fact, research studies have found that women who include soy in their diet have a lower risk of breast cancer recurrence.

Toxicologists caution that eating large amounts of soy can result in an underactive thyroid and goiter (an enlarged thyroid gland) by blocking the production of thyroid hormones. However, this appears only occur in people who are iodine deficient, a mineral needed for normal thyroid function. In developed countries salt is fortified with iodine to prevent deficiency. It's possible, however, that people who eat soy foods and who don't enough iodine from their diet could be at risk for goiter.

Critics also warn that soy infant formulas can lead to abnormal sexual development in children. But this has never been recognized clinically. American researchers compared 811 adults who were fed either soy or cow's milk formula in infancy and found no differences in growth, maturation, fertility or other reproductive outcomes.

You should be wary of some soy products: I advise against the use of isoflavone pills, which offer highly concentrated doses. We just don't have data on the long term safety of these supplements.

In my opinion, soybeans, soy nuts, soy beverages (unflavoured), tofu, tempeh and other natural soy products are healthy foods that can be safely consumed.

Send dietitian Leslie Beckyour questions at She will answer select questions, which could appear in The Globe and Mail and/or on The Globe and Mail web site. Your name will not be published if your question is chosen.

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The content provided in The Globe and Mail’s Ask a Health Expert centre is for information purposes only and is neither intended to be relied upon nor to be a substitute for professional medical advice, diagnosis or treatment.

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