By ROBERT J. DAVIS
Special to THE WALL STREET JOURNAL
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May 10, 2005; Page D4

Few claims concerning food and health are more confusing than those about soy and breast cancer. Some studies suggest that soy may reduce the risk, while others have reached the opposite conclusion: that it may promote the growth of breast tumors. Still, one food company, Solae, is asking the Food and Drug Administration for permission to claim that soy products help prevent breast and other cancers, and a ruling is expected soon. Many researchers say the truth is probably complicated, with soy's effects depending on the type and amount consumed, and at what age.

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Research shows that soy may help lower cholesterol, and some women swear by it for reducing symptoms of menopause. Though it's also touted to prevent several types of cancer, including prostate and colon, much of the focus in recent years has been on breast cancer -- largely because of concerns soy may do harm.

The theory that soy may prevent breast cancer stems from research showing that women in Asian countries, where soy is a traditional staple of the diet, have lower rates of the disease than women in the U.S. After moving here and adopting Western diets, their rates increase and become similar to those of non-Asians within a few generations. It's thought that reduced soy consumption may be a key reason.

Population studies putting this hypothesis to the test have been mixed, with some showing an association between high soy intake and a reduced risk of breast cancer, and others finding no link. Methodological shortcomings and inconsistencies make it hard to draw any firm conclusions overall. For example, most of the studies were backward looking, asking women with breast cancer to recall what they ate -- a method that can yield erroneous findings. What's more, different studies focused on different forms of soy -- whether fermented soybeans, tofu or soy milk. Notably, though, no human research has so far tied soy to an increased risk of breast cancer.

The same can't be said of laboratory studies. Some research in rodents shows that compounds in soy can stimulate the growth of breast tumors. But other studies find they prevent cancers. Research involving breast-cancer cells has also yielded conflicting results.

There's a scientific rationale for both effects. Soy contains plant estrogens known as isoflavones, which can bind to estrogen receptors in the body. By doing so, it's believed they may help block the effects of human estrogen, high levels of which are associated with breast cancer. On the other hand, it's also possible they may act like estrogen, increasing the risk of cancer.

Some researchers say the effects may depend on the type and amount of soy consumed. They believe that, unlike unprocessed soy such as tofu typically found in Asian diets, highly concentrated isoflavones (often labeled as soy protein isolates) in some foods and supplements sold in the U.S. may be harmful. The researchers say that people consuming soy-enriched foods like bars, shakes and cereals throughout the day can end up getting several times the levels of isoflavones normally found in Asian diets.

Other experts believe that soy is most beneficial if consumed regularly beginning at an early age. Many say a moderate intake of up to 30 mg of isoflavones a day -- the amount in a typical Asian diet -- is probably safe for most women, including those who have had breast cancer or are at high risk for it. But until there's more definitive research, any recommendation is at best an educated guess.

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