Is Soy Safe for Women with Breast Cancer? New Discovery

Soy--is it safe for women with breast cancer?
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The debate over whether it is safe for women with breast cancer to eat soy products has been a long and often heated one, yet no definitive conclusions have been reached. Now a new discovery suggests the secret may be related to when women with breast cancer first begin eating soy.

For soy, timing may be everything

Soybeans and soy foods are recognized as low-fat, cholesterol-free, high-protein food choices. However, the estrogen-like compound in soy called genistein is a topic of concern when it comes to the risk of hormone-driven cancers, such as breast cancer, ovarian cancer, and prostate cancer.

Some research indicates women who are breast cancer survivors or who are at risk for developing breast cancer can safely eat moderate amounts of soy foods. The American Cancer Society (ACS), for example, notes it is still uncertain how the weak estrogen-like activity of genistein and the other isoflavone in soy--daidzein--might affect the growth of breast cancers that are estrogen receptor-positive.

Until this issue is resolved, the ACS explains on its website that “many oncologists recommend that people taking tamoxifen or aromatase inhibitors and people with estrogen-sensitive breast tumors should avoid adding large amounts of soy, including soy supplements or isoflavones, to their diets.”

At the same time, a National Cancer Institute study found that eating soy during childhood, adolescence, and adulthood was associated with a reduced risk of breast cancer. The most protection seemed to be in females who ate soyfoods during childhood.

New discovery about soy and breast cancer
In this new research, which was conducted under the leadership of professors of oncology Leena Hilakivi-Clarke, PhD, and Robert Clarke, PhD, DSc, both at Georgetown Lombardi Comprehensive Cancer Center, mouse models were used to evaluate the impact of soy on breast tumors.

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Four groups of female rats were used in the study, all of which had chemically induced breast tumors that were subsequently treated with tamoxifen, a drug commonly used to treat women who have breast cancer after they have received primary treatment.

The four groups of rats were as follows: (1) fed genistein once tamoxifen was started; (2) fed genistein only early in life and not again until tamoxifen was started; (3) given genistein only as adults and continued after tamoxifen was started; and (4) fed genistein early in life and continued up through and after tamoxifen was given.

Hilakivi-Clarke explained that the rats given genistein as adults and during tamoxifen treatment appeared to have tumors that were resistant to tamoxifen. “However,” she noted, “if animals were fed genistein during childhood, and intake continues before and after tumors develop, the tumors are highly sensitive to tamoxifen.”

If the findings in the rats hold true for women with breast cancer, then females who started eating soy foods during childhood and who later develop breast cancer would have tumors that respond to tamoxifen.

However, Clarke concluded, their results “suggest that western women who started soy intake as adults should stop if diagnosed with breast cancer.” The answer to the question, “Is soy safe for women with breast cancer” now has yet another qualified answer, ensuring that research into the matter will continue.

SOURCES:
American Cancer Society
Georgetown Lombardi Comprehensive Cancer Center
Korde LA et al. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev 2009; 18(4): OF1-10

Image: Wikimedia Commons

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