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Wood's study adds new fuel to the ongoing debate surrounding soy's effect on cancer risk. "There's been a good deal of confusing information, particularly with soy's effect on (breast) cancer risk," said Wood.

"Most population-based studies have found that women who consume lots of soy are less likely to develop breast cancer," he said. "A number of studies have been done, and they generally show a positive effect or no effect."

But in lab studies, Wood said, isoflavones from soy -- which have a structure similar to estrogen -- have been found to stimulate breast cancer cells grown in a petri dish and induced estrogen-like effects.

"Our hypothesis was that the amount of estrogen in the body may help determine whether soy was having good or bad effects," he said. "If you have very low estrogen, high doses of soy could have adverse estrogen-like effects on your reproductive tissue. If you had high estrogen, the isoflavones could block the adverse effects of your body's own estrogen. That was our working hypothesis."

Wood and his team used a postmenopausal monkey model. They first selected out a high-estrogen group of monkeys and a low-estrogen group. Next, they fed each group four different diets for 16 weeks each, along with a high or a low dose of estrogen.

The diets included either no isoflavones; 60 milligrams of isoflavones (similar to the typical Asian diet); 120 milligrams (highest amount that can be obtained via diet alone); or 240 milligrams (levels that must be obtained via supplements).

Next, Wood's team measured how the diets affected so-called "markers" for breast cancer risk, such as breast cell proliferation.

"No effect of the isoflavones was seen in the low-estrogen animals," he said.

In contrast, among the high-estrogen groups the researchers observed more breast cell proliferation when isoflavones were NOT added to the diet, and when they were added in smaller doses. High levels of the isoflavones tended to block the effect of estrogen on breast tissue in the high-estrogen animals. The strongest effects were seen at 240 milligrams daily, Wood said.

"In the postmenopausal period, women with high natural estrogen levels have higher breast cancer risk," he said. So the isoflavones may help reduce risk in those who need it most. These women with high estrogen levels may get the most benefit from isoflavones in soy in terms of cancer risk reduction, Wood said.

"Isoflavones may connect with cell receptors normally reserved for estrogen," he speculated, thus reducing the breast's exposure to estrogen, thereby decreasing cancer risk.

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Wood stopped short of giving dietary advice, only noting that the topic warrants further study in humans.

Another expert praised the study and said it gives women reassurance. "This study is basically coming down on the side of, 'Do not worry about the effect of estrogen on the breasts of postmenopausal women,"' said Mindy Kurzer, a professor of nutrition at the University of Minnesota, who has published on the topic of soy intake. "I think it's an excellent study."

The study does have its limitations, she said -- most notably the fact that it was conducted in animals, not humans. However, "the monkey is the absolutely best animal model for this kind of study," Kurzer said, because its physiology is so close to that of humans.

The finding that there was virtually no effect of soy in the low-estrogen group is also good news when it comes to breast cancer risk, she added. "The concern was that the phytoestrogens (isoflavones) might mimic estrogen when estrogen is not around."

Soy is considered good for building bones and good for heart health, Kurzer said, as well as for relieving hot flashes during menopause.

Copyright 2006 The New York Times News Service. All rights reserved.