Is soy food helpful or harmful for women with breast cancer? Studies have yielded mixed results. A new study published today suggests that breast cancer survivors may benefit from eating moderate amounts of soy products.

In a large group of breast cancer survivors in China, researchers found that a higher intake of soy food -- up to 11 grams daily -- was associated with a lower risk of death or recurrence of breast cancer during follow up. (For comparison, a slice of bread generally weighs between 30 and 40 grams.)

The key take home message from our study is that moderate amount of soy food intake is safe and may reduce risk of mortality and recurrence among women with breast cancer, Dr. Xiao Ou Shu, of Vanderbilt University Medical Center in Nashville, Tennessee noted in an email to Reuters Health.

Soy foods are rich in compounds called isoflavones -- a major group of plant-derived phytoestrogens possessing both estrogen-like and anti-estrogen actions.

Eating soy has been linked to a reduced of risk of breast cancer in some studies, while other studies have suggested that soy may help breast cancer cells grow and multiply, the study team explains in Wednesday's issue of the Journal of the American Medical Association.

To investigate further, Shu and colleagues analyzed the dietary habits of more than 5,000 women aged 20 to 75 years who were diagnosed with breast cancer between March 2002 and April 2006 and were followed up through June 2009 as part of the Shanghai Breast Cancer Survival Study.

Among 5,033 women who had surgery to remove the breast cancer, 444 women died and 534 had recurrences or breast cancer-related deaths during a median of 3.9 years.

Women who ate the most soy protein had a 29 percent lower risk of dying during the study period, and a 32 percent lower risk of having their cancer return compared to women who ate the least amount of soy protein.

At 4 years, death rates were 10.3 percent and 7.4 percent for women with the lowest and highest intakes of soy protein, and recurrence rates at 4 years were 11.2 percent and 8.0 percent, respectively.

The benefits of soy food intake on death and breast cancer recurrence peaked at 11 grams per day, the researchers note. No additional benefits on mortality and recurrence were observed with higher intakes of soy food, they wrote.

Eating soy was beneficial regardless of whether the women's breast tumors were driven by estrogen (that is, estrogen-receptor positive breast cancer) or were estrogen receptor-negative.

The benefits of soy were also seen in both users and nonusers of tamoxifen, a drug commonly used to treat and prevent breast cancer. Prior studies have suggested that soy isoflavones may interact with tamoxifen, and both beneficial and possibly harmful interactions have been reported.

The authors of a commentary on the study caution that while it provides important information, there are several concerns, including differences in the quality, type and quantity of soy food intake between Chinese and American women.

For one thing, the average isoflavone intake in Chinese women is 47 milligrams per day compared with 1 to 6 milligrams per day for American women, Dr. Rachel Ballard-Barbash, of the National Cancer Institute, Bethesda, Maryland and Dr. Marian L. Neuhouser, of the Fred Hutchinson Cancer Research Center, Seattle, point out.

Larger studies, they say, are needed to understand the effects of these foods among diverse subsets of women with breast cancer.

In the meantime, they add, women with breast cancer should know that soy foods are safe to eat and that these foods may offer some protective benefit for long-term health.

Patients with breast cancer can be assured that enjoying a soy latte or indulging in pad thai with tofu causes no harm and, when consumed in plentiful amounts, may reduce risk of disease recurrence, Ballard-Barbash and Neuhouser advise.

They point out, however, that any potential benefits are from soy foods. Inferences should not be made about the risks or benefits of soy-containing dietary supplements.

SOURCE: Journal of the American Medical Association, December 9, 2009.

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