Is Soy Bad for You? What the Research Says About Soy and Breast Cancer

Soy foods are made from soybeans, a legume that's unusually rich in protein. The most common ones are:

• Edamame
• Tofu
• Tempeh
• Soy milk
• Miso
• Roasted soy nuts

These foods are high in protein and also contain fiber, unsaturated fats, vitamins, and minerals. They're popular with people eating more plant-based meals, but there's no reason they can't sit next to the chicken on anybody's plate.

The concern goes back to laboratory studies in mice and rats. When researchers gave rodents very large amounts of isoflavones, some studies showed effects on breast cancer cell growth.

Scientists later learned that rodents break down isoflavones differently than people do, so what happens in a mouse doesn't predict what happens in a woman eating a bowl of edamame. When researchers studied people, the results looked completely different.

Studies in people haven’t shown that soy foods raise breast cancer risk, and some have found the opposite. Women in East Asia, who eat soy regularly from childhood, tend to have lower rates of breast cancer than women in the United States, and researchers have looked hard at whether soy is part of the reason.

The evidence is also reassuring for breast cancer survivors. The American Cancer Society says soy foods are safe, and the American Institute for Cancer Research reports that population studies show no increased risk for survivors who eat soy. Some research has even linked eating soy after a breast cancer diagnosis with a lower chance of the cancer returning.

To understand why this may happen, it helps to understand how isoflavones behave. Rather than acting like a strong estrogen, they attach weakly to the same spots in the body, which may actually block stronger natural estrogens from doing so.

Once the myth about soy and breast cancer is out of the way, you’ll find that soy stands up well on its own merits.

• It's a complete protein. Unlike most plant foods, soy has all nine essential amino acids your body needs. That makes it a strong protein choice for vegetarians and meat eaters alike.
• It supports heart health. Swapping foods high in saturated fat for soy foods may help lower LDL, the "bad" cholesterol. Soy also supplies unsaturated fats that are good for your heart.
• It helps with bone health. Tofu made with calcium is a solid source of the mineral, and most soy milk is fortified with calcium and vitamin D.
• It may ease menopause symptoms. Some women notice fewer hot flashes when they eat soy regularly. Our guide to foods that may help with hot flashes covers that in more detail.
• It tastes like whatever you cook it with. Tofu and tempeh soak up marinades, spices, and sauces well, which makes them some of the most flexible proteins in the kitchen.

One serving of soy has roughly 7 grams of protein and about 25 mg of isoflavones. That works out to about a third of a cup of tofu, half a cup of tempeh, half a cup of edamame, or a cup of soy milk.

People in East Asia typically eat one to two servings a day, throughout their lifetime. That's the pattern the research is built on, and it's a very normal amount of food. Two or even three servings of soy a day is considered a reasonable range for most adults.

Whole and minimally processed soy foods like edamame, tofu, tempeh, miso, and unsweetened soy milk are what the reassuring research is based on. Heavily processed products such as soy protein bars and some soy-based meat substitutes are different, often containing more additives than you'd expect.

Isoflavone supplements are a separate matter entirely. They deliver far more concentrated doses than food does, and the American Cancer Society doesn’t recommend them. Unless your healthcare provider has suggested one, food is the better way to get your soy.

There are a few situations that warrant a discussion with a healthcare provider before you start eating soy on a regular basis. Check with your doctor or a registered dietitian if you’re being treated for breast cancer, take thyroid medication (soy can interfere with how it's absorbed when taken at the same time), or are considering an isoflavone supplement rather than food. For nearly everyone else, soy is a nutritious, affordable, versatile food that has been safely eaten for centuries.