Melissa Mogor, Public Health Advisor at WCRF, develops nutrition information for people living with and beyond cancer.
The main concern people have when it comes to soy is the link between circulating hormones (particularly oestrogen) and cancer development, especially breast cancer.
Soy beans are the main dietary source of isoflavones. Isoflavones are often termed as phytoestrogens as they have a chemical structure similar to the human hormone oestrogen.
Isoflavones have a similar chemical structure to human oestrogen, but they bind to the body’s oestrogen receptors differently and function differently.
Different kinds of oestrogen receptors are present in different parts of the body. Activation of some receptors seems to promote cell growth. But studies suggest that isoflavones more often bind to oestrogen receptors with other effects, potentially acting as a tumor suppressor.
Population studies don’t consistently link soy consumption with increased risk of any cancer, and limited evidence shows soy possibly protecting against lung cancer in people who have never smoked tobacco, and either no effect or decreased risk of prostate cancer.1 Observational studies also link moderate soy consumption (one to two servings a day) with lower breast cancer risk in Asia, where soy foods are commonly consumed throughout life.
Population studies do not indicate increased risk for breast cancer survivors who consume soy. In fact, limited evidence shows the potential for greater overall survival and perhaps decreased recurrence, among women a year or more after diagnosis who include moderate amounts of soy.
For now, there is no reason to steer clear of soy foods, and there is also no reason to consider them must-haves if people prefer not to include them in their diet.