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

Updated: Feb 7

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.


How to incorporate more soy into your diet


Have you been curious about soy and unsure where to start? We're here for you! But, before you get started, know that it’s important to choose organic whenever possible as more than 90 percent of soy in the United States is genetically modified (meaning it has genetically modified compounds along with lots of pesticides). For each gram of soy protein, there are approximately 3.5 milligrams of isoflavones. For example, one (100 grams) serving of soy food, such as tofu, has around 25 milligrams of isoflavones. The average isoflavone intake in Asian countries is 25-50 milligrams per day whereas the average intake in the United States is less than one milligram per day. That’s a big difference. The recommended amount of soy intake for adults is 15-25 grams or 2-4 servings of organic soy food per day.

Soy milk has the same protein and calcium as dairy milk

Here are a few ways you can easily meet isoflavone recommendations:

  1. Swap out cow’s milk for soy milk in your coffee, tea, cereal, or smoothie (Fun facts: soy milk has the same protein as cow's milk and calcium absorption from soy milk is the same as cow milk at approximately 35 percent.)

  2. Snack on roasted soybeans or steamed edamame instead of chips (bonus that you also get tons of fiber and other essential nutrients. Plus, they're not fried!).

  3. Swap out meat with tofu, tempeh, or edamame in your stir-fry (try Kung Pao Tofu and Broccoli or Ginger Carrot Edamame Noodles) or in a sandwich (try the Tofu Banh Mi).

  4. Make a delicious salad dressing with miso (and get healing probiotics to boot!). Try this Lemon Miso Farro Bowl.

  5. Add tempeh into soups, chili or stew like this Easy Vegan Chili. Or try tempeh in a salad like this Warm Tempeh Kale and Radish Salad.

P.S. If you missed it, here is part one of our three-part series on plant foods for breast cancer prevention, How a Fiber-Fueled Diet May Help to Prevent Breast Cancer.


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Hi! My name is Kara and I'm a student in Georgia State University's Coordinated Program for Dietetics. As a former D1 cross country & track athlete, I've seen firsthand the important role that nutrition can play in one's wellbeing. I'm so excited to be pursuing my passion and learning a lot on the way! When I'm not studying or working, you can find me rock climbing with my husband, hanging out with our two dogs, or running on some fun trails.