Soy and Cancer Risk: Our Expert’s Advice

Soy and foods that comes from soy are excellent sources of protein. Soy foods can also be a good source of dietary fiber, healthy fats, and many nutrients, including potassium and magnesium.

For people eating a plant-based diet or looking for healthier alternatives to meat, this can be a great thing. However, you may have heard some conflicting information about soy and cancer risk. You might have questions like, “Is soy healthy? Is it dangerous? And if it’s OK to eat soy foods, why do some people say it isn’t?”

Soy isoflavones and cancer risk

Soy foods are healthy and safe, according to Dr. Marissa Shams-White, Senior Principal Scientist in the American Cancer Society's Population Science department.

“A large body of evidence supports the safety of soy foods as part of a healthy diet. Research studies have found consuming soy foods is associated with a decreased risk or no change in risk for cancer. Some of the confusion around the safety of soy foods has been due to some earlier study findings in mice and rats compared to humans, and some misconceptions around a phytochemical naturally found in soy foods called isoflavones,” Dr. Shams-White said.

Isoflavones in soy can act like estrogen in the body, and increased estrogen has been linked to certain types of breast cancer.

Animal vs. human studies on soy risk

In some earlier studies in mice and rats, researchers found high doses of soy isoflavones could lead to an increased risk of breast cancer.

However, mice and rats break down and process isoflavones differently than humans. Also, the doses used in animal studies are much higher than in studies conducted in people.

Human studies have found the estrogen effects of soy seem to either have no effect at all or may reduce breast cancer risk (especially in Asian countries, where lifelong soy intake is higher than in the US). This may be because soy isoflavones can bind estrogen receptors in the body and actually block the more potent natural estrogens in the blood.

How does soy impact health including cancer risk?

For people eating soy, the health benefits appear to outweigh any potential risk. In fact, there’s growing evidence that eating soy foods such as tofu, tempeh, edamame, and miso and drinking soy milk may lower the risk of breast cancer, especially among Asian women. Soy foods are an excellent source of protein. Soy protein is especially good when it replaces other, less healthy foods, such as those high in animal fat and red or processed meats. Soy foods have also been linked to lower rates of heart disease and may even help lower cholesterol.

For cancer prevention, the American Cancer Society does not recommend soy supplements. Instead, the recommendation is to consume nutrients through whole foods. Soy supplements can contain much higher amounts of isoflavones than are naturally found in foods. More research on soy supplements is needed to understand their effect on health.

Learn more

Learn more about diet and cancer risk from the American Cancer Society:

• Common Questions About Diet, Activity, and Cancer Risk
• Guideline for Diet and Physical Activity for Cancer Prevention