Myth Busted: Soy Does Not Raise Cancer Risk

Soy foods’ isoflavones (naturally occurring plant compounds) were once believed to act like estrogen, raising the risk of cancer. We now know that we had it wrong! Studies have shown that soy consumption from foods, protein powders and isoflavone supplements show no increased risk of any type of cancer. 

The original studies were conducted on rodents and found that soy isoflavones promoted the growth of estrogen receptor-positive breast cancer. However, scientists have since found that humans metabolize isoflavones differently than rodents. In fact, these more recent studies suggest quite the opposite—that soy appears to be protective against both breast cancer and prostate cancer, may act as a tumor suppressor and is safe for breast cancer survivors to consume.

In a two-week study of people with breast cancer who consumed 200 mg per day of soy isoflavones, soy isoflavones did not seem to stimulate breast cancer cell growth. Also, taking a soy tablet containing 50 mg of soy isoflavones daily for 12 months did not alter mammographic or breast MRI tissue density in women at high risk of breast cancer, those with non-endocrine treated breast cancer, or those previously treated for breast cancer who had no evidence of recurrence. So, not only has research shown that soy doesn’t increase risk of breast cancer, it also is safe for breast cancer survivors to consume.

According to the American Institute for Cancer Research, “Observational studies also link moderate soy consumption (one to two servings a day) with lower breast cancer risk in Asia, where soy foods are commonly consumed throughout life.”

Soy foods can be a helpful and healthy option when trying to decrease animal products in the diet. When shopping for soy foods, like all foods, choose the least processed to minimize added salt and sugar. I encourage my patients to choose “whole food” forms of soy such as tofu, soy milk, edamame, soy nuts, tempeh and miso. Fermented soy has the added benefit of probiotics. 

However, soy is not essential to a cancer-preventing diet, if you prefer to avoid it, have a soy allergy or soy intolerance, just be sure to include plenty of other cancer-preventing plant foods in your diet. Aim to make at least two-thirds of your meal from plants—fruits, vegetables, whole grains and beans. Keep animal products—low-fat dairy, chicken, fish and occasional lean red meats—to one-third or less of the meal.

If you have further questions regarding nutrition and cancer prevention, please talk to your healthcare provider about a referral to a registered dietitian.