Soy May Reduce Risk of Death, Breast Cancer Recurrence


Many women who have breast cancer have avoided soy because there is concern about it causing breast cancer recurrence. A new study, however, shows that women in China who had breast cancer and who consumed a higher amount of soy foods had a lower risk of breast cancer recurrence and death.

For years there has been a love-hate relationship with soy foods, and more specifically, the isoflavones that they contain. Isoflavones are phytoestrogens, which are substances found in plants such as soybeans and flaxseeds that have weak estrogen-like properties. Experts have hypothesized that phytoestrogens can reduce the risk of breast cancer because phytoestrogens compete with estrogen in the body to bind to estrogen receptors on cells, thus blocking the much more potent estrogen from reaching the receptors.

Other researchers claim that because estrogen triggers the reproduction of breast cells, a higher amount of estrogen in the body may increase a woman’s risk for breast cancer. Some investigators also propose that the interaction between isoflavones and tamoxifen, a chemotherapy drug taken by many women who have breast cancer, may increase the risk of cancer recurrence.


In the new study, which was conducted at Vanderbilt University Medical Center, Ziao Ou Shu, MD, PhD and colleagues evaluated the association between soy isoflavone intake with the recurrence and survival rate of women who had breast cancer. Their study population included 5,033 surgically treated breast cancer survivors in China ages 20 to 75 who had been diagnosed between March 2002 and April 2006 and followed up through June 2009.

After a midpoint follow-up of 3.9 years, 444 deaths and 534 recurrences or breast cancer-related deaths were documented. Women who had the highest intake of soy protein had a 29 percent lower risk of death during the study period and a 32 percent lower risk of breast cancer recurrence compared with patients who had the lowest intake of soy protein. When looking at the adjusted four-year mortality rates, they were 10.3 percent for women with the highest and 7.4 percent for women with the lowest intake of soy. The four-year recurrence rates were 11.2 percent and 8.0 percent, respectively.

The study found that soy intake is safe and associated with lower risk of death and breast cancer recurrence, and that the benefits were seen in women who did or did not use tamoxifen, and those who had either estrogen receptor-positive or estrogen receptor-negative breast cancer. The maximum amount of soy intake needed to achieve both benefits of lower risk of death and breast cancer recurrence was 11 grams daily of soy protein.

JAMA news release Dec. 8, 2009/JAMA 2009; 302(22): 2437-43