Soy has also received widespread media coverage. For the most part, it's considered a healthy addition to your diet—it's been shown to lower blood pressure, decrease LDL (bad) cholesterol levels, and increase bone mass, possibly warding off osteoporosis. The FDA even permits use of the claim that incorporating soy products into a diet low in saturated fat and cholesterol may decrease the risk of heart attack. But several research studies have also suggested that soy might accelerate the growth of existing breast cancers and possibly increase the risk of Alzheimer's disease in men. Here's a look at this conflicting information to help you decide if you should add soy to your diet.

Breast Cancer
The role of soy isoflavones in breast cancer is controversial, to say the least. Many studies suggest that soy isoflavones may protect the body from breast cancer by mimicking natural estrogen, which cancer feeds on. Breast cells have estrogen receptors that accept soy isoflavones, mistaking them for human estrogen, thereby blocking the action of the estrogen receptors in breast tissue. This reduces the potential for the body's natural estrogen to promote cancer growth.

But in 2000, many women read a New York Times article that said soy may actually increase their risk of breast cancer. Specifically, the article pointed out that consuming soy isoflavones in supplement form may increase cancer risk because soy supplements generally have higher levels of isoflavones than natural soy foods. Experts recommend getting the benefits of soy isoflavones from whole foods rather than supplements, which in some cases may provide more soy isoflavones than is typical in even an Asian diet; the effects of such high consumption have not yet been studied.

Coral Lamartiniere, PhD, a professor of pharmacology and toxicology at the University of Alabama, Birmingham, studies the effects of the main soy isoflavone, genistein, on cancer. He believes that much of the negative attention soy has received comes from those who assume that because genistein has estrogenlike effects, the isoflavone also has estrogen's negative effects. "Genistein has a lot of other properties that are very distinct from its estrogenlike qualities. Genistein is able to modulate a lot of proteins that lead to the way cells differentiate. Genistein also acts like an antioxidant. It is misleading to say that soy has harmful effects for cancer just because it acts like estrogen in some ways."

The majority of studies support the idea that soy helps prevent breast cancer. In 1993, for example, a study found that genistein blocked blood vessel growth, and therefore overall growth, in tumors (Cancer Research, 1993, vol. 53, no. 23).

Alzheimer's disease

Soy has also been implicated in an increased risk for Alzheimer's disease. In an April 2000 study, researchers examined the tofu intake of Japanese-American men in Hawaii and found that those who ate the most tofu had a higher-than-expected risk of developing this debilitating mental illness. However, no other studies have corroborated a negative relationship between soy and dementia; soy protein has actually been shown to protect the brain during traumatic events, such as stroke or seizure. And in Japan, where tofu is a staple, Alzheimer's cases are relatively few.

Thyroid disease

Numerous studies have suggested that a diet high in soy can cause goiter, an enlargement of the thyroid gland (Biochemical Pharmacology, 1997, vol. 54, no. 10). However, the risk of this occurring in healthy people who have sufficient iodine in their diets is low. In fact, no controlled studies in humans have shown soy to have statistically significant effects on thyroid function. And, in one study, consumption of soy-based foods was shown to reduce the risk of thyroid cancer (Cancer Epidemiology Biomarkers & Prevention, 2002, vol. 11, no. 1).

—G.T.