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 cancer, 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. A study found that Chinese women diagnosed with breast cancer who ate a diet rich in soy foods had a lower risk of dying of breast cancer and a lower risk of breast cancer coming back (recurrence) compared to women diagnosed with breast cancer who didn't eat a lot of soy.
In this study, researchers looked at the medical histories and soy consumption of 5,042 Chinese women (age 20 to 75) diagnosed with breast cancer. Over 4 years of follow-up, 444 of the women died and 534 had breast cancer come back. The researchers found:
- women who ate the most soy (more than 15.3 mg of soy protein or more than 62.3 mg of soy isoflavones per day) were 21% to 29% less likely to die from breast cancer compared to women who ate the least soy (less than 5.3 mg of soy protein or less than 20 mg of soy isoflavones per day)
- women who ate the most soy had a 23% to 32% lower risk of recurrence compared to women who ate the least soy
Some women in the study were taking tamoxifen, a hormonal therapy medicine that blocks the effect of estrogen on breast cancer cells. Tamoxifen is often prescribed to lower the risk of recurrence in women diagnosed with early-stage, hormone-receptor-positive breast cancer. Surprisingly, the women not taking tamoxifen who ate the most soy seemed to have a better prognosis than women who took tamoxifen and ate the least soy.
The results suggest that a diet rich in soy doesn't worsen prognosis in women diagnosed with breast cancer and may offer some protection against recurrence.
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. You can eat soybeans in many forms, including tofu, the beans themselves (also known as edamame), soy milk, miso, and soy powder. Asian women typically eat much more soy than women in Western countries (about 10 times more). Still, other research has shown that Asian women have lower rates of hormone-receptor-positive breast cancer than women in the United States.
The relationship between soy and breast cancer prognosis is complicated by other factors. Most women living in Asia depend on soy as their main source of protein. They consume only small amounts of beef, chicken, and pork -- which means less animal fat and other possibly unhealthy substances (such as growth hormones and antibiotics) in these animal protein sources. Also, compared to the average woman in the United States, the average Asian woman:
- eats more fresh vegetables
- is closer to her ideal body weight
- is more physically active
- is less likely to consume large amounts of alcohol
All of these other factors add up to produce a healthier lifestyle and may have contributed to the better breast cancer prognosis seen in the Chinese women in this study who ate the most soy.
In the Breastcancer.org pages on Nutrition and Breast Cancer Risk Reduction, you can learn more about foods that may keep you as healthy as you can be.