Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein. You can eat soybeans in many forms, including tofu, the beans themselves (also called edamame), soy milk, miso, and soy powder.
Soy foods have a lot of isoflavones, which are weak estrogen-like compounds found in plants. Because estrogen can promote the development, growth, and spread of breast cancers, doctors have worried that eating a lot of soy foods or soy isoflavones (which can be taken as a dietary supplement) might worsen the prognosis of women diagnosed with breast cancer.
While past research results have been mixed, a small study done by researchers at the Memorial Sloan Kettering Cancer Center and Weill Cornell Medical College suggests that for some women, adding a medium amount of soy to their diets turns on genes that can cause cancer to grow.
The research was published in the Sept. 4, 2014 issue of the Journal of the National Cancer Institute. Read the abstract of “The Effects of Soy Supplementation on Gene Expression in Breast Cancer: A Randomized Placebo-Controlled Study.”
The study involved 140 women who were newly diagnosed with stage I or stage II breast cancer between 2003 and 2007. Each woman had recently had a breast biopsy and was scheduled to have either mastectomy or lumpectomy in 2 to 3 weeks.
During those 2 to 3 weeks, the women were randomly assigned to receive either:
- soy protein (70 women)
- a placebo that looked like the soy protein (70 women)
The women in the soy protein group were given about 52 grams of soy protein – equal to about 4 cups of soy milk – per day. The researchers chose this amount because it would be reasonable for people who regularly eat soy to eat that much in one day.
After the women had surgery to remove the breast cancer, the researchers compared tissue from the biopsy (before the women consumed the soy supplement) to tissue from the cancer (after the women consumed the soy).
The results showed that several genes that encourage cell growth were turned on in women in the soy protein group.
The study didn’t last long enough to know whether these genetic changes would cause cancer to grow.
The study also didn’t look at:
- whether soy does or doesn’t reduce the risk of breast cancer
- whether eating soy would have any effect on women who don’t have breast cancer or who have non-cancerous breast lesions
The researchers didn’t recommend that women avoid soy. But they did say that soy should be eaten in moderation.
“If you currently have early-stage breast cancer, don’t eat soy in large amounts,” said Jacqueline Bromberg, M.D., Ph.D., who was one of the study’s authors. “If you’ve had breast cancer, you can eat soy, but in moderation.”
So in general, it’s fine to eat moderate amounts of soy foods – about one to three servings per day as part of a balanced diet (a serving is about a half cup). If you’ve been diagnosed with breast cancer and are concerned about any isoflavone effects, ask your doctor or a registered dietitian about how much soy you can eat.
In the Breastcancer.org Nutrition section, you can learn more about healthy eating before, during, and after breast cancer treatment.