Is soy linked to breast cancer?

Eating soy could pose risks to some women who begin consuming it as adults by making breast cancer tumors resistant to treatment, US researchers said on Monday.

A study on lab rats showed that those who were fed a soy compound all their lives responded well to a popular breast cancer drug, tamoxifen, but those who began eating it as adults, and after they developed breast cancer, grew resistant.

The research suggests a possible reason why tamoxifen stops working and allows tumors to grow again in some women, said scientists from Georgetown University who presented their findings at a medical conference in Chicago.

"These results suggest that Western women who started soy intake as adults, should stop if diagnosed with breast cancer," said senior author Leena Hilakivi-Clarke, professor of oncology at Georgetown.

Soy contains isoflavones that mimic the estrogen produced in the body, only at lower levels, and is considered a healthy protein source found in foods like tofu, miso, soy beans and soy milk.

Its potential benefits against breast cancer are often linked to the lower rates of hormone receptor positive types of breast cancer seen in Asian women who live in parts of the world where soy consumption is common.

Soy Foods Probably Safe

Once found only in health food stores and Asian markets, soy is now a fixture in the American diet, even among people who have never tried tofu, tempeh, or miso soup.

A cheap source of protein, soy is used in the manufacture of a wide range of highly processed foods, including breads, cookies, crackers, breakfast cereals, soy 'milk', non-dairy creamers, imitation cheeses, and even some yogurts.

The experts and researchers interviewed by WebMD say that breast cancer survivors probably have little to fear from eating soy foods in moderation.

They also said that soy supplements, a widely used alternative to hormone therapy for menopause-related hot flashes, should be avoided.

The studies have not found these compounds to be very helpful in terms of reducing hot flashes,” Dana Farber Cancer Center oncologist Wendy Chen, MD, tells WebMD. “And they have not been studied in women with breast cancer, so the risk is not known.”

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Since tamoxifen is typically given to breast cancer patients with estrogen receptor and/or progesterone receptor positive types of tumors, the finding suggests that late-life adoption of a soy diet may have rendered the drug impotent.

The Case for Soy

The most widely reported of these studies involved about 5,000 breast cancer survivors in Shanghai, China. Over almost four years, people who ate the most soy foods had the highest survival rate and lowest risk of recurrence.