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Experts said the findings, reported in the Sept. 4 Journal of the National Cancer Institute, shouldn't scare women off from eating tofu.
And for women who don't have breast cancer? "This study doesn't tell us anything about whether soy raises the risk of developing breast cancer," said researcher Dr. Jacqueline Bromberg, a breast cancer specialist at Memorial Sloan Kettering Cancer Center in New York City.
The relationship between soy and breast cancer is complicated. On one hand, in countries where soy is a dietary staple -- like Japan -- women who eat more of it tend to have a lower breast cancer risk.
On the other hand, soy contains phytoestrogens -- plant compounds that have weak estrogen-like properties. And lab research has found that those soy compounds may promote the growth of breast tumors.
In the new study, Bromberg and her colleagues randomly assigned 140 women with newly diagnosed, early-stage breast cancer to one of two groups. In one, women took a soy protein supplement every day for anywhere from one to four weeks; those in the other group were given milk powder as a comparison. The women were premenopausal or just past menopause.
The soy supplement -- a powder added to water or juice -- was the equivalent of about four cups of soy milk a day, Bromberg said. Women in the study typically used it for two weeks.
Even in that short time, the study found, about 20 percent of the women using soy developed high blood concentrations of genistein, a soy phytoestrogen. Among those women, some showed heightened activity in certain genes that promote breast tumor growth and spread.
But it isn't clear just what that could mean, Bromberg stressed.
"Does that necessarily mean the tumor is growing more rapidly?" she said. "No."
Bromberg said there was no evidence of "tumor proliferation" in women with revved-up gene activity, but the study may have been too short to detect such an effect.
"All we can say is that two weeks of soy supplementation was enough to increase expression of genes related to tumor proliferation," Bromberg said.
But to be safe, she said, women with breast cancer should probably not take soy supplements, and should eat soy foods, such as tofu and tempeh, only in moderation.
A breast cancer researcher who wasn't involved in the study agreed that women should play it safe and avoid soy supplements, particularly within a few years of going through menopause.
But a remaining question is whether using soy later in life could be beneficial when it comes to breast cancer, according to Craig Jordan, scientific director of the Lombardi Comprehensive Cancer Center at Georgetown University in Washington, D.C.
With estrogen replacement, there is evidence from lab research and human studies that "timing is everything," said Jordan, who wrote an editorial published with the study.
As an example, he pointed to a large U.S. study called the Women's Health Initiative, which looked at the effects of hormone replacement therapy. In that study, women given estrogen alone (not with progesterone) had a lower risk of developing breast cancer than women given a placebo.
And the women in that study were typically in their 60s.
Based on other research, Jordan said, it seems that estrogen can fuel either the growth or death of breast cancer cells, depending on when it's given. During and soon after menopause, estrogen -- and possibly soy phytoestrogens -- may put fuel on the fire, Jordan said.
"But soy might be good when used farther out from menopause," he said. "If we designed a clinical trial of women in their 60s, we might find more [breast cancer] cell death than survival."
That trial, however, has yet to be done.
Copyright © 2014 HealthDay. All rights reserved.
SOURCES: Jacqueline Bromberg, M.D., Ph.D., physician/scientist, Memorial Sloan Kettering Cancer Center, New York City; V. Craig Jordan, Ph.D., D.Sc., scientific director, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D.C.; Sept. 4, 2014 Journal of the National Cancer Institute