Soy Appears Safe for Breast Cancer Survivors

Moderate Intake of Soy Reduced Breast Cancer Death Risk, Recurrence in Study

By Kathleen Doheny
WebMD Health News

Reviewed By Louise Chang, MD

Dec. 8, 2009 -- Moderate intake of soy foods by breast cancer survivors appears to be not only safe but beneficial, according to a new study.

"Women who had a higher soy intake had a lower mortality and lower risk of relapse [than women with a low intake]," says researcher Xiao Ou Shu, MD, PhD, professor of medicine at Vanderbilt University Medical Center in Nashville, Tenn.

Previous research has yielded conflicting findings, with some studies finding that soy foods reduce breast cancer risk but others finding that genistein, an estrogen-like compound known as an isoflavone in soy, helps breast cancer cells grow in the lab and promotes tumor growth in animals.

"Some papers say it's safe for women [with breast cancer] to eat some form of soy, others say [these] women should be cautious," Shu tells WebMD. Her findings, she says, should be reassuring to breast cancer survivors.

But the new study isn't the final word, says an expert who co-authored an editorial accompanying the study, both published in the Journal of the American Medical Association. She cites a relatively short follow-up of four years, along with differences in soy consumption habits of women in the U.S. and women in the study, who were from China.

Soy and Breast Cancer: Study Details

Shu and her colleagues analyzed data from the Shanghai Breast Cancer Survival Study, following 5,042 women who were ages 20 to 75 when diagnosed with breast cancer between March 2002 and April 2006.

Follow-up continued through June 2009, with the researchers analyzing information on the diagnosis, cancer stage, treatment, disease progression, and intake of soy foods. The researchers estimated the nutrients consumed -- including both soy protein and isoflavone intake.

After a median follow-up of nearly four years, 444 women had died (from any cause) and 534 had recurrences or breast cancer-related deaths. The researchers looked at the connections between soy intakes and outcomes.

Soy foods are rich in the phytoestrogens known as isoflavones. Because they are estrogen-like, some experts think they compete with the body's estrogen, thus keeping the overall estrogen in the body low. But others worry these isoflavones may exert an estrogen-like effect, perhaps boosting the risk of cancer recurrence.

Soy and Breast Cancer: Study Results

Women in the group with the highest intake of soy had a 29% lower risk of death during the study and a 32% lower risk of breast cancer recurrence, compared to women in the group with the lowest intake.

At the four-year follow-up,10.3% of the women in the lowest intake group had died, but 7.4% of those in the highest intake group. Although 11.2% of those in the lowest intake groups had recurrences, 8% of those in the highest group did.

Women in the lowest soy protein group ate less than 5.31 grams of soy protein daily and about 20 milligrams of isoflavones, while those in the highest ate more than 15.31 grams of soy protein and more than 62.68 milligrams of isoflavones.

But the benefits of soy leveled off, Shu tells WebMD. "After 11 grams of soy protein a day, "we don't see additional benefits."

"That is about 1/4 cup of firm tofu or 1.5 cups of soy milk," she says. "This is a moderate intake."

Of her findings, "I think it's generalizable to the American population," Shu tells WebMD. She points to a study published in November, in which U.S. researchers found in an analysis of nearly 2,000 U.S. breast cancer survivors that soy isoflavones consumed at levels comparable to those in Asian populations may reduce breast cancer recurrence in women getting tamoxifen therapy.

Shu's study was funded by the U.S. Department of Defense Breast Cancer Research Program and the National Cancer Institute. She reported receiving a research development fund from the United Soybean Board in 2005.

Soy and Breast Cancer: Other Opinions

The new study is strong and scientific, says Rachel Ballard-Barbash, MD, MPH, associate director of the Applied Research Program at the National Cancer Institute in Bethesda, Md., who co-authored the editorial. But "the amount of soy food consumed in China is much higher than in the U.S."

That's a point that Shu also makes, writing that the average isoflavone intake in U.S. women is 1 to 6 mg a day, compared with 47 mg a day in the Chinese women.

"It's not quite clear how this [study] extrapolates to U.S. women," Ballard-Barbash says.

"Other differences in these populations may at least partly explain the results," says Marji McCullough, ScD, an American Cancer Society epidemiologist in an email after reviewing the study. "For example, it is likely that Chinese women have regularly consumed soy throughout their lifetime, whereas in the U.S. consumption is much less common. We don't know whether starting to eat soy regularly after a cancer diagnosis would have the same effect as having a lifelong diet high in soy foods."

What's a breast cancer survivor to do? Moderation may be best. "We think it's unlikely that occasional consumption of soy-based food in the diet would be detrimental," Ballard-Barbash says.

"The study is consistent with our current guidelines for breast cancer survivors, which state that consumption of up to three servings of soy foods per day as part of a healthy diet is safe," McCullough says.

Avoiding high doses such as those found in soy powders and isoflavone supplements should be avoided, she says, because of their possible estrogen-like effects.

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SOURCES: Xiao Ou Shu, MD, PhD, professor of medicine, Vanderbilt University Medical Center, Nashville.

Marji McCullough, ScD, epidemiologist, American Cancer Society, Atlanta.

Rachel Ballard-Barbash, MD, MPH, associated director, Applied Research Program, National Cancer Institute.

Shu, X. Journal of the American Medical Association, Dec. 9, 2009; vol 302: pp 2437-2443.

Ballard-Barbash, R. Journal of the American Medical Association, Dec. 9, 2009; vol 302: pp 2483-2484.

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