Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. Soy is the main source of protein for billions of people around the world.
Some doctors are concerned about the safety of soy foods for women who’ve been diagnosed with breast cancer. That’s because soy contains a protein, called isoflavone, which can act like a weak estrogen. Soy supplements, such as powders, pills, and capsules, contain more isoflavones than soy eaten as food, such as tofu, soy milk, and the beans themselves (also called edamame). Some doctors are concerned that the growth of hormone-receptor-positive breast cancers may be turned on by isoflavones.
Other doctors think soy might protect breast health because the hormone-like strength of isoflavones is much weaker than the estrogen your body naturally makes. So it might be healthier if soy’s weak isoflavones wash out or replace some of your body’s stronger estrogen.
A study suggests that eating soy foods may help improve survival in women diagnosed with hormone-receptor-negative breast cancer.
The research was published online on March 6, 2017 by the journal Cancer. Read the abstract of “Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry.”
"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; however, other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer," explained Fang Fang Zhang, MD, PhD, of the Friedman School of Nutrition Science and Policy at Tufts University and lead author of the study. "Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients."
To make things clearer, the researchers looked at the relationship between eating isoflavones and dying from any cause in 6,235 American and Canadian women who had been diagnosed with breast cancer. The women were all part of the Breast Cancer Family Registry, a large ongoing study looking at families with a higher-than-average risk of breast and ovarian cancer based on family history or genetic mutations.
To determine the quantity of isoflavones the women ate, they filled out a food questionnaire.
Women in Asia eat about one to two servings per day of soy, mostly as tofu. The women in this study ate considerably less than that.
Half the women were followed for more than 9 years and half the women were followed for less time.
The researchers found that women who had been diagnosed with hormone-receptor-negative breast cancer who ate the most isoflavones were 21% less likely to die than women diagnosed with the same type of breast cancer who ate the least isoflavones.
Women diagnosed with hormone-receptor-positive disease who were taking hormonal therapy and ate the most isoflavones were no more likely to die than women diagnosed with the same type of breast cancer who at the least isoflavones.
"Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy," said Zhang. "For women with hormone-receptor-negative breast cancer, soy food products may potentially have a protective effect.
"It’s encouraging that despite the overall low consumption [of soy], women living in North America still benefit from increasing isoflavones," she added.
It’s VERY important to know that this study looked only at isoflavones that occur naturally in soy food products. It did NOT look at isoflavone supplements, such as pills, powders, and capsules. If you’ve been diagnosed with hormone-receptor-negative breast cancer, please do not start taking large amounts of isoflavone supplements.
"I feel comfortable recommending moderate amounts of soy foods as part of a balanced diet for healthy women and breast cancer survivors," said Marisa Weiss, MD, Breastcancer.org founder and chief medical officer. "This means two to three half-cup servings of soy per day, similar to a Japanese diet."
Still, it’s important to:
- Avoid highly concentrated soy products or protein supplements (they come in powders and capsules).
- Avoid hidden soy, found in many packaged foods in various forms (protein isolate and soybean oil). Real, whole foods provide the best nutrients. These packaged foods may affect your body differently than whole soy. Eating hidden soy may mean that you’re eating more soy than you planned.
- Stick to certified organic soy foods. More than 90% of conventional soybeans come from genetically modified seeds. So far, no research shows that genetically modified foods affect cancer risk or cause long-term health problems. But many crops, including soy, are engineered to withstand spraying with certain pesticides, and pesticide residues can cause unhealthy cell changes. Certified organic products don’t use genetically modified organisms (GMOs).
Of course, anyone with soy allergies should avoid soy. Limit how much soy you eat if you’re being treated for low thyroid hormone levels, and don’t take your medicine with soy foods. And if you have problems absorbing minerals, you may want to avoid soy because compounds in them called phytates can slow or block absorption of important nutrients including iron, calcium, and zinc.
For more information on diet and breast cancer, including foods to consider, visit the Breastcancer.org Nutrition and Breast Cancer Risk Reduction pages.