Soy and Breast Cancer

Conflicting results from studies on soy’s connection to breast cancer often leave consumers confused. As more research is published, there is stronger support for the role of soyfoods in reducing the risk of breast cancer – especially when soy is consumed during childhood and adolescence. More and more research also suggests that women who are at risk for developing breast cancer or who are breast cancer survivors can safely consume moderate amounts of soyfoods – anywhere from a few servings a week to three servings a day. The American Cancer Society recommends that breast cancer survivors can safely consume moderate amounts of soyfoods – anywhere from a few servings a week to three servings a day. Learn more from the Eating Well Magazine article.

Soyfood Consumption and Breast Cancer Prevention

A growing body of research shows that eating a healthy diet that includes soyfoods protects against breast cancer (1, 2). In an article published December 8, 2009 in JAMA, Dr. Shu of the Vanderbilt University Medical Center reported results of a four year study following 5000 Chinese women with breast cancer that show higher intakes of soyfoods were related to decreases in breast cancer recurrences and death.(21). A 2008 study from the University of Southern California found that the more soyfoods a person consumed, the lower the risk of breast cancer. Women who consumed one serving of soyfoods per day (or at least 20 mg of soybean isoflavones, a bioactive compound found naturally in soy) had a significantly lower risk of breast cancer compared to women who consumed less than one-quarter of a serving per day (5 mg of soybean isoflavones). (3) These results confirmed earlier findings from a 2006 study that soyfood consumption reduced breast cancer risk by 14 percent. (1)

Increasing evidence also confirms that soy consumed during childhood and adolescence protects against breast cancer (3 – 7) Researchers from the National Cancer Institute found that eating soy during childhood, adolescence, and adulthood are all associated with a decreased risk of breast cancer and the effect was strongest when soyfoods were eaten during childhood. (7) Asian women who follow a traditional Asian diet from childhood tend to have lower breast cancer rates — estimated to be one-fifth that of Western women. (8)

Soyfood Consumption and Women at Risk of Developing Breast Cancer

Women who are at risk for developing breast cancer can safely eat moderate amounts of soyfoods. No evidence exists to suggest that eating soyfoods is unsafe – no human trials have demonstrated a link between eating soyfoods and tumor growth. The only negative data arose from select experiments with mice given unnaturally high levels of soy isoflavones. (9, 10) The levels of isoflavones tested in these studies were much higher than levels found in traditional soyfoods and people who eat a diet high in soyfoods would not consume levels that high. Many researchers argue that findings from rodent studies should not be applied to human health.

Soyfood Consumption and Breast Cancer Survival

The National Cancer Institute and the American Cancer Society recommend that breast cancer survivors can safely consume moderate amounts of soyfoods – anywhere from a few servings a week (11) to 3 servings a day. (12) Only a few studies have looked at the effects of soy intake among women diagnosed with breast cancer, and none showed a link between consumption of soy and breast cancer recurrence or tumor growth. A 2009 paper suggests that women diagnosed with breast cancer who regularly consume soy may actually have a reduced risk of recurrence. (13) Furthermore, no evidence exists that soy, or its bioactive compounds, cause changes associated with human breast cancer, such as thickened breast tissue. (14 – 17), increased number of breast tissue cells (18), or increased estrogen circulating in the blood. (19)

Some physicians and researchers initially raised concerns that soy may interfere with the effects of tamoxifen in treating breast cancer; however, two human studies from 2007 and 2009 have found no link between intake of soy or serum isoflavones and the effectiveness of tamoxifen. (15, 20) Additional research may be needed to confirm these findings. Women who are taking tamoxifen should always consult their oncologists for individual advice on soy and other matters concerning nutrition.


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