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.
There are a lot of health-related phytochemicals in soy. Protein kinase inhibitors help keep cell growth and activity normal. Phytosterols and saponins help regulate cholesterol. Phenolic acid and phytates are antioxidants.
Then there are the isoflavones, which are weak phytoestrogens (estrogen-like compounds found in plants). Isoflavone levels vary in different types of tofu and soy milk products. Your body's estrogen is much, much stronger than the estrogen-like isoflavones in soy. So if the weak soy substance replaces the natural high-strength estrogen in cells, then maybe the soy will protect against cancers that would prefer a stronger estrogen signal.
Soy is often promoted as a healthy protein alternative for people who would rather not eat meat. Research about the effects of soy on cancer risk reduction has been mixed. Much of it has come from observing population groups such as the Japanese, who eat much more soy and begin eating it at a much younger age than people in Western countries.
In fact, the average woman living in East Asia eats about 10 times the quantity of soy foods as the average woman in the United States. Yet East Asian women have lower rates of hormone-receptor-positive breast cancer than women in the United States.
The "soy story" and its possible connection to breast cancer 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 significant amounts of alcohol
All of these other factors add up to produce a healthier lifestyle and a lower overall risk of breast cancer in Asian women living in Asia.
It's not clear if soy isoflavones affect breast cancer, especially hormone-receptor-positive breast cancers. Isoflavones may affect hormonal therapy's ability to do its job if both of these molecules compete to get into the same estrogen receptors. If isoflavones deliver a weaker estrogen signal to the receptor compared to tamoxifen (and your body's estrogen), then the isoflavones might be able to decrease breast cell growth that's estrogen-dependent. But if the isoflavones give breast cells a stronger estrogen signal than tamoxifen, that's a problem.
One 2009 study found that isoflavones DIDN'T increase breast density in postmenopausal women. Denser breast tissue is linked to a higher risk of breast cancer. Still, the study didn't show that isoflavones helped lower breast cancer risk. Another 2009 study, conducted in more than 5,000 Chinese women diagnosed with breast cancer, suggests that a diet rich in soy doesn't worsen prognosis in women diagnosed with breast cancer and may offer some protection against recurrence.
Until the issue becomes clearer, many doctors recommend that women who take hormonal therapy or who have estrogen-receptor-positive breast cancer avoid soy supplements because they contain high concentrations of isoflavones. But in general, it's fine to eat moderate amounts of soy foods as part of a balanced diet. One to 3 servings of soy a day (a serving is about a half cup) is similar to an average Japanese woman's daily soy intake. If you are taking hormonal therapy to fight off a hormone-receptor-positive breast cancer, and you are concerned about any phytoestrogen effects, ask your doctor or registered dietitian about how much soy you can eat.
While you may see web sites or magazine articles recommending fermented soy instead of unfermented soy, research is limited and more studies are needed.