Soy products, like tofu and soymilk, contain chemicals called phytoestrogens that may mimic how estrogen acts in the body. In laboratory studies, phytoestrogens sometimes increase breast cancer cell growth.1-3 On the other hand, lower rates of breast cancer are seen in many Asian countries, where diets are higher in soy. This seeming conflict has caused debate about the role of soy in breast cancer risk and survival.
Some studies in Asian women have suggested that soy lowers the risk of breast cancer development and recurrence and increases the chances for survival.4-6 Recent findings from the Shanghai Breast Cancer Survival Study of more than 5,000 Chinese women diagnosed with breast cancer found that a high soy diet lowered the risk of recurrence and breast cancer death.6 Women who ate more than 15 grams of soy protein per day had a 30 percent lower risk of recurrence compared to women who ate less than 5 grams of soy protein per day.6 Although the amount of soy protein varies by brand, typically, one cup of soymilk contains 6-8 grams of soy protein and a 4-ounce serving of regular tofu contains 9-10 grams.7-8
For a few reasons, it is difficult to know how findings from studies in Asia apply to women in the U.S. Women in Asia tend to eat much more soy than women elsewhere. In fact, among Caucasian women in the U.S the highest soy intake levels often fall into the lowest level of soy intake among women in Asia.9-10 Thus, the low levels of soy that U.S. women tend to eat may be too low to have an impact on breast cancer risk.
In addition, the sources of soy differ among women in Asia and U.S. Most women in the Shanghai study and other studies in Asia ate soy in the form of lightly processed whole foods. We do not know if these findings would apply to soy intake from soy supplements or pills or soy protein added to many processed foods commonly sold in the U.S. as soy products (such as soy chips, cereals and bars).
The timing of soy intake may also be important. Women growing up in Asian countries, such as Japan and China, tend to eat soy throughout their lives, beginning at a young age. Because few non-Asian women have a comparable lifetime exposure to soy in their diets, it is hard to know how these findings apply to non-Asian women. We do not know whether a breast cancer survivor who begins eating large amounts of soy later in life after a cancer diagnosis would get the same benefit from soy. However, the Shanghai study suggests that it is unlikely that dietary soy would be harmful among breast cancer survivors.
If you are a breast cancer survivor, talk to your health care provider about adding soy into your diet before making any major changes. Dr. Wendy Chen, a medical oncologist at the Dana Farber Cancer Institute often discusses this study with her patients when they ask about soy: “Although we don’t know if soy protein among U.S. women would be protective against breast cancer, this study shows that soy in food form is probably safe for breast cancer survivors.”