Breast cancer is one of the most common forms of cancer in women, affecting approximately 220,000 women in the United States every year. The disease can also affect men, with 2,000 male patients being diagnosed yearly with breast cancer in the U.S.
Previous research has suggested that the consumption of soy products may increase the risk of developing breast cancer. One study showed that adding a medium amount of soy to one's food may activate genes that make cancer grow and spread.
A possible explanation for the results has been that soy contains a lot of isoflavones, which are plant-based compounds that resemble estrogen. Estrogen has been shown by some studies to help cancer cells multiply and spread - especially in hormone receptor-positive cancer, which is the most common form of the disease. Researchers have therefore been worried about the adverse health effect of soy on breast cancer patients.
However, a new study may now settle the controversy, as researchers from Tufts University in Massachusetts investigate the link between a dietary intake of isoflavones and breast cancer mortality.
Examining the link between soy intake and breast cancer mortality risk
The new findings were published in the journal of the American Cancer Society, Cancer.
The lead author of the study, Dr. Fang Fang Zhang, of the Friedman School of Nutrition Science and Policy at Tufts University, explains the motivation behind the research:
"Isoflavones - the component of soy that has estrogen-like properties - have been shown to slow the growth of breast cancer cells in laboratory studies, and epidemiological analyses in East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality," Dr. Zhang says.
"However, other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer. Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients," Dr. Zhang explains.
Therefore, Dr. Zhang and her team set out to examine isoflavone intake in 6,235 women diagnosed with breast cancer from the U.S. and Canada. The women were followed over a median period of 9 years, and the study examined the isoflavones that occur naturally in foods, not supplementary isoflavones.
High amount of soy consumption linked to 21 percent mortality drop
Overall, researchers found dietary soy intake to be safe, and noticed a correlation between high soy consumption and a decrease in the mortality risk for some breast cancer patients.
During the follow-up period, women with breast cancer who consumed isoflavones in large amounts were 21 percent less likely to die than their counterparts who consumed small amounts.
This drop in the mortality risk was noticed only in women who had hormone receptor-negative cancer and women who had not been taking anti-estrogen therapy such as tamoxifen. However, high amounts of isoflavone did not associate with higher mortality in women who did receive hormonal therapy.
"Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy. For women with hormone receptor-negative breast cancer, soy food products may potentially have a protective effect. Women who did not receive endocrine therapy as a treatment for their breast cancer had a weaker, but still statistically significant, association."
Dr. Fang Fang Zhang, lead author
The study's senior author, Esther John, Ph.D., of the Cancer Prevention Institute of California, also weighs in on the findings. Referring to receptor-negative breast cancer - which makes up 20 percent of all recently diagnosed cases of cancer - she says that "whether lifestyle factors can improve survival after diagnosis is an important question for women diagnosed with this more aggressive type of breast cancer."
"Our findings suggest that survival may be better in patients with a higher consumption of isoflavones," Dr. John adds.
The study was observational, and the mechanism by which soy consumption may improve survival rates is not yet known. However, isoflavones have been shown to have antioxidant and anti-inflammatory properties, which might affect the growth and survival of breast cancer tumors.