This is a MYTH.
While certain types of soy (processed and unfermented) have been linked to an increased risk of breast cancer, whole, fermented soy is safe for consumption. In fact, soy first enjoyed the limelight as a possible preventative when researchers noticed that Asian women with diets high in soy had much lower incidences of breast cancer.
What the Studies Say
A review of studies made up of large populations of women refutes the link between soy and breast cancer. Studies indicate that soy consumption has either no association or a protective association with breast cancer. An analysis of 14 epidemiologic studies found that Asian women who ate the most soy had a 24% less chance of developing breast cancer. American women showed no such soy breast cancer link. Evaluating the health benefits of soy among the United States population, however, is difficult, as soy doesn’t feature prominently in diets. Asian women consume as much as 40mg of soy per day, whereas American women, on average, consume no more than one to two mg of soy per day.
Then there is the oft-debated question of whether soy causes cancer in breast cancer survivors. A recent review of three studies made up of 9000 breast cancer survivors found that 10mg of daily soy lowered the risk of breast cancer reoccurrence by 25%. In lieu of these studies, the American Cancer Society assures that moderate soy consumption is most likely safe.
The Science of Soy
Why did soy get such a bad reputation? Soy is made up of phytoestrogens called isoflavones that mimic human estrogen, which can have both a positive and negative impact on the body. Animal studies have shown that isoflavones increase breast cancer risk, but amount and type of soy used in animal and laboratory research isn’t reflective of the type of soy eaten in Asian populations.
It must be noted in any discussion of soy and breast cancer that isoflavones also have anti-estrogen properties that inhibit natural estrogens from binding to estrogen receptors and spurring breast tumor growth. Isoflavones also suppress the production of estrogens in fat tissue, and trigger the production of a protein that affixes estrogen to blood instead of to estrogen receptors. Rich in antioxidants and anti-inflammatories, the isoflavones in soy products can actually help lower the risk of cancer growth.
How Much and What Type is Safe?
One to two servings of whole, fermented soy a day (about 30mg of isoflavones) is certainly safe. Stay away from processed forms of soy like soy shakes, chips, bars, supplements, powders and fake meat products (the type of soy that has shown negative results in animal studies), and instead consume whole soybeans, roasted soybeans, tempeh, miso, tofu and edamame. Avoid products that contain the labels “soy protein concentrate,” “soy isolate” and “isolated soy protein.”