Soy and Cancer Risk: Our Expert’s Advice

There’s a lot of conflicting information going around about soy: Is it healthy? Is it dangerous? And if it’s OK to eat, why do some people say it isn’t?

Some of the misunderstandings come from the fact that studies in people and studies in animals may show different results. In some animal studies, rodents that were exposed to high doses of compounds found in soy called isoflavones showed an increased risk of breast cancer. This is thought to be because the isoflavones in soy can act like estrogen in the body, and increased estrogen has been linked to certain types of breast cancer.

But rodents process soy differently from people, and the same results have not been seen in people. Also, doses of isoflavones in the animal studies are much higher than in humans. In fact, in human studies, the estrogen effects of soy seem to either have no effect at all, or to reduce breast cancer risk (especially in Asian countries, where lifelong intake is higher than the US). This may be because the isoflavones can actually block the more potent natural estrogens in the blood.

So far, the evidence does not point to any dangers from eating soy in people, and the health benefits appear to outweigh any potential risk. In fact, there is growing evidence that eating traditional soy foods such as tofu, tempeh, edamame, miso, and soymilk may lower the risk of breast cancer, especially among Asian women. Soy foods are excellent sources of protein, especially when they replace other, less healthy foods such as animal fats and red or processed meats. Soy foods have been linked to lower rates of heart disease and may even help lower cholesterol.

According to Marji McCullough, ScD, RD, strategic director of nutritional epidemiology for the American Cancer Society, soy foods are healthy and safe. But she advises against taking soy supplements – which contain much higher isoflavone concentrations than food – until more research is done.