Soy foods are the only commonly consumed foods that provide significant amounts of isoflavones.
Isoflavones are plant chemicals that are also called phytochemicals, have biological activity but are not nutrients. These compounds are referred to as plant estrogens or phytoestrogens. Foods that are rich in isoflavones include soy foods like tofu, soymilk, miso, and tempeh. Most of these soy foods contain about 3½ milligrams of isoflavones for every gram of protein.
For example, ½ cup of regular tofu has about 8 grams of protein and about 28 milligrams of isoflavones. Certain types of food processing reduce the amount of isoflavones in foods. Products such as soy-based meat analogs often have much lower amounts of isoflavones. Soy is also sometimes added to foods like breads, cereals, and meat products, and used as a meat substitute in vegetarian products such as soy burgers and soy hot dogs.
In Asia, women who eat the most soy have a lower risk of breast cancer compared to women who eat little soy. Evidence suggests that this is true only if they also ate soy early in life. Eating soy foods during childhood and the teen years may protect breast tissue from cancer. Beginning soy consumption later in life doesn’t appear to have any effect on risk of getting breast cancer.
However, women with breast cancer who eat soy foods are less likely to see their cancer return and are less likely to die from their cancer. The American Cancer Society states that women with breast cancer can safely consume soy foods.
Men may also benefit from eating soy foods. In Asia, men who eat the most soy have about one-half the risk of getting prostate cancer compared to men who eat little soy. For men who have prostate cancer, soy may be helpful as well. One small study found that soy isoflavones reduced some of the side effects of treatment for prostate cancer.
Human studies support the safety of both isoflavone supplements and soy foods. Isoflavones have no effect on estrogen levels in women or testosterone levels in men. Clinical studies show they also have no effect on sperm or semen.
Soyfoods have no effect on thyroid function in people with normal thyroids. However, for those who take thyroid pills, changes in soyfood intake may require changes in the amount of medicine needed. Your doctor can make these adjustments.