Does Soy Increase Breast Cancer Risk? Research says, no.

Updated: May 30

Guest post by Kara Moore, dietetic intern, Georgia State University's Coordinated Program for Dietetics

Let's chat about soy—one of the most polarizing foods when it comes to discussions around health. Which side are you on—the "no soy" side or the "I love tofu!" side?

First, what makes a food soy-based?

Soy foods are made from soybeans that come from the legume family. Soy foods include miso, soy milk, tempeh, tofu, tamari, and natto. Soy can also be enjoyed whole in the form of cooked soybeans or edamame (an immature soybean). Soy foods can add a big contribution to a plant-based diet because they're packed with protein, including adequate amounts of all nine essential amino acids, which classifies them as a “complete protein.” Soy also contains calcium, iron, magnesium, B vitamins, vitamin K, and fiber. Bonus that soy is also readily accessible and inexpensive. Soybeans and foods made with soybeans can be a wonderful source of nutrition for anyone, vegan or not. In fact, substituting meat protein with organic soy protein has been shown to reduce the risk of heart disease.

The Isoflavone Conundrum

The nutrients in question, that often create the big soy divide, are isoflavones, compounds in soy also known as phytoestrogens or phytonutrients. They've been studied over the past 25 years. Research shows that these phytonutrients, or plant nutrients, may serve as chemo-protective agents. Conflicting reports emerged when animal studies showed that soy may increase the risk of breast cancer when rats were exposed to high amounts of isoflavones. The problem with using soy-consuming rats in a study (other than the fact that using any animal in a study is unethical) is that rats process soy very differently than humans. In fact, soy is toxic to rats, but not to humans.

The results shown in the rodent study have not been replicated in human studies. In human studies, soy has been found to either have no effect at all or to reduce the risk of breast cancer, and, in some cases, significantly reduce the risk. In addition to reducing the risk of breast cancer, soy intake also has been associated with lower rates of heart disease and lower cholesterol.

The link between soy's beneficial effects on breast cancer was sparked by the historically low incidence of cancer and mortality rates in people from Asian countries that consumed high amounts of soy foods. There is controversy, through, over whether or not soy would have the same effect in people from Western countries. It's possible that there is a protective effect with long-term soy consumption in individuals raised in Asian countries where soy is consumed from a young age. Among Asian women, higher soy consumption was associated with approximately a 30 percent reduction in the risk of developing breast cancer. What’s more, after diagnosis of cancer, soy consumption was associated with improved treatment outcomes and lower recurrence rates. The evidence also suggests that for soy to have the greatest effect on reducing breast cancer risk, consumption should ideally begin earlier in life (adolescence or childhood).

Note: Soy is listed by the American Institute for Cancer Research as one of the “foods that fight cancer.

How does soy reduce the risk of breast cancer?

First, soy has fiber. Visit our previous blog about fiber's effect on breast cancer here. Also, isoflavones are one of the factors that may positively influence the risk of breast cancer. Isoflavones can exert a weak estrogen effect when it's needed or block estrogen from attaching to cancer cells when excess estrogen is present. Isoflavones have been touted for their potential chemo-protective properties. Some studies show that soy consumption earlier in life, as mentioned above, has the ability to change developing cells in such a way that makes them less likely to develop into cancer cells. The anti-estrogen effects of isoflavones include the lowering of the biological availability of sex hormones, reduction of estrogen synthesis and increased clearance of steroid hormones from circulation. These have been proposed as the most likely mechanisms of why soy is linked with breast cancer reduction and better treatment outcomes. Of note, isoflavones may also alleviate hot flashes, improve arterial health, and increase bone calcium in postmenopausal women.