If you're a breast-cancer survivor, you may have been told to avoid eating soy foods. The concern: isoflavones in soybeans, natural compounds that act like very weak forms of the female hormone estrogen, could increase the chance of the cancer recurring. (It's thought that the body's own estrogen can promote the growth of breast-cancer cells.)
But according to the largest study to date on the influence of soy foods and breast cancer, a regular intake of soy significantly reduces the risk of the cancer returning.
The study, published online in the American Journal of Clinical Nutrition, included 9,514 breast-cancer survivors from the United States and China. After a followup period of almost eight years, 1,171 women died (78 per cent from breast cancer) and 1,348 women had a recurrence of breast cancer.
Women whose daily diets provided 10 milligrams or more of soy isoflavones had a 25-per-cent lower risk of breast cancer recurrence compared to those who consumed less than four milligrams daily.
Two tablespoons of soybeans, two teaspoons of soy nuts, ¼ cup firm tofu, and ½ cup of soy beverage have roughly 10 milligrams of soy isoflavones.
The higher the soy isoflavone intake, the stronger the protective effect. Women who consumed the most – versus least – isoflavones each day were 36 per cent less likely to have their breast cancer recur over the course of the study.
The protective effect of soy was more apparent for women with estrogen-receptor (ER) negative breast cancer, possibly because they have a poorer prognosis than women with ER-positive breast cancer. (Doctors test breast-cancer cells to see if they have hormone receptors. If breast cancer cells have estrogen receptors, the cancer is said to be ER-positive. If cancer cells do not have estrogen receptors, the cancer is called ER-negative.)
Some experts have also worried that soy foods might diminish the effectiveness of tamoxifen, a drug used to reduce breast-cancer recurrence in women with certain types of the disease.
However, the researchers found that tamoxifen users with higher soy-isoflavone intakes had a lower risk of breast-cancer recurrence than women who did not take the drug and who consumed the lowest amount of isoflavones.
The study also found that a regular intake of soy isoflavones reduced the risk of dying from breast cancer, although this finding was not statistically significant. (It could have been a chance finding.)
This isn't the first study to link soy to improved breast-cancer survival. Two previous studies, published in 2009 and 2010, revealed that a regular soy intake protected from breast-cancer recurrence in women with early and late-stage breast cancer, women with ER-positive and ER-negative breast cancer and tamoxifen users and non-users.
Soy may protect from breast cancer in a few ways. Isoflavones, being weak estrogens, can bind to estrogen receptors on breast cells, preventing normal estrogens from doing so. In this way, isoflavones may minimize the effect of estrogen on breast-cancer risk.
Soy contains folate, calcium, fibre, protein and many phytochemicals that individually, or together, could help guard against cancer.
Researchers also speculate that tamoxifen and soy isoflavones work synergistically to block estrogen production.
While mounting evidence suggests that a moderate soy intake is safe for breast-cancer patients and survivors – and likely beneficial – these findings are confined to soy foods, not isoflavone supplements. Data is lacking on the long-term safety of concentrated forms of isoflavones.
Leslie Beck, a registered dietitian, is the National Director of Nutrition for BodyScience Medical and appears on CTV News Channel's Direct every Thursday. www.bodysciencemedical.com