Studies show that a lifelong diet rich in soy foods reduces the risk of breast cancer in women. This protective effect is less dramatic for women who eat less soy or who start eating soy later in life. Soy contains protein, isoflavones and fiber, all of which provide health benefits.
It was once thought that soy foods increase the risk of breast cancer. However, eating a moderate amount of soy foods does not increase risk of breast cancer — or other types of cancer. A moderate amount is one to two servings a day of whole-soy foods, such as tofu, soy milk and edamame.
So where did the idea come from that soy increases breast cancer risk? Isoflavones, which are found in soy, are plant estrogens. High levels of estrogen have been linked to an increased risk of breast cancer. However, food sources of soy don't contain high enough levels of isoflavones to increase the risk of breast cancer.
Soy or isoflavone supplements, on the other hand, generally contain higher levels of isoflavones. Some studies have suggested a link between soy or isoflavone supplements and an increased risk of breast cancer in women who have a family or personal history of breast cancer or thyroid problems.
Talk with your doctor or dietitian before taking supplements.