Eating soybean may reduce risk of breast cancer

Increased intake of soy foods which is high in phytoestrogens like soy isoflavones may reduce risk of breast cancer, according to a new study presented at the Ninth Annual AACR Frontiers in Cancer Prevention Research Conference, held Nov. 7-10, 2010.

Early studies have established an inverse association between soy consumption and risk of breast cancer.  The current study led by Anne Weaver, at the University at Buffalo and at Roswell Park Cancer Institute examined the effect of soy compounds on specific subtypes of breast cancer.

Weaver and colleagues compared dietary data from 683 women with breast cancer with data from 611 healthy women and found those with the highest isoflavone intake were 30 percent less likely to be diagnosed with an invasive breast cancer and 60 percent less likely to be diagnosed with grade 1 tumor.

Among premenopausal women, the highest intake of soy isoflavones was linked with 30 percent decreased risk of state I breast cancer, 70 percent decreased risk of having a tumor large than 2 centimeters and 60 percent reduced risk of having stage 2 breast cancer, compared with those with the lowest intake.

The associations were not found among postmenopausal women.

The study is not a trial, meaning a causal relation between eating soy foods and risk of breast cancer has not been established.

A number of other studies conducted recently also found eating soy beans may help cut the risk of breast cancer and help women with breast cancer have a longer survival.

Published on Oct 18, a study in Canadian Medical Association Journal suggests that soy intake was associated with lower recurrence of breast cancer in hormone sensitive cancers among postmenopausal women.

Soy isoflavones are similar to estrogens in chemical structure and it may interfere with the interaction between endogenous estrogen with its receptor or it may stimulate estrogen-like action in tissue.  Endogenous estrogen is known to promote breast cancer growth.

The study led by Dr. Qingyan Zhang at the Cancer Hospital of Harbin Medical University, Harbin, China and colleagues followed 524 women who had surgery for breast cancer in the fifth year and sixth year after surgery.

The researchers found postmenopausal patients in the lowest quartile of soy isoflavone intake were significantly less likely to have recurrence of breast cancer.

"The recurrence rate of estrogen- and progesterone- positive breast cancer was 12.9% lower among patients in the highest quartile of soy isoflavone intake than among those in the lowest quartile and was 18.7% lower for patients receiving anastrozole therapy in the highest quartile," Dr. Zhang and colleagues wrote.

However, there was no association between soy intake and survival in premenopausal women and eating soy did not seem to increase overall survival in postmenopausal women.

The researchers acknowledged more research is needed to confirm the findings.

In the United States, more than 80 percent soy crops are genetically modified and the safety of the genetically modified soybean has not been tested, critics say.

Breast cancer is diagnosed in more than 175,000 women and the disease kills about 50,000 women each year in the United States, according to the National Cancer Institute. The disease is expected in one in seven women in their lifetime in the country.

By David Liu