Do Soy Foods Cause Cancer?

— Alex Gayton

Spoiler alert! Soy foods do not cause cancer; keep reading to understand why.

As a community-focused registered dietitian, I very often hear people say they avoid soy because it causes cancer. I have lacked access to a simplified, science-based resource to provide education about this misconception, so I decided to write the resource I wish I’d had. I feel passionately about not villainizing food, so let’s change the narrative of soy by talking about what the current science says.

What Is Soy?

Soy foods derive from soybeans. Soybeans are a nutrient-dense legume (part of the bean family), and they are a great source of plant-based protein. Here are some common soy foods:

• Edamame: Young soybeans harvested when they are green and are often steamed and eaten out of the pod
• Soy milk: A liquid made by soaking, grinding, and boiling soybeans with water
• Tofu: Made by curdling soy milk and pressing the curds into blocks.
• Tempeh: Made from fermented whole soybeans, pressed into a dense, nutty-flavored block
• Miso: A thick savory paste made from fermented soybeans
• Soy sauce: A salty liquid condiment made from fermented soybeans and wheat

For more information about tofu and tempeh, read our blog Where Do You Get Your Protein? A Helpful Guide To Plant-Based Protein Options.

Why Are People Afraid of Soy?

The concern about soy started with early research in 1998 (PubMed) on mice. Scientists found that giving mice very large doses of soy isoflavones increased the growth of breast cancer cells. Because they already knew that human estrogen can drive certain breast cancers, researchers initially thought plant estrogen would do the same thing.

There are two major reasons why this early mouse research doesn’t apply to humans:

• Different metabolism: Mice break down and process isoflavones differently than humans.
• Unrealistic doses: The doses, or amounts, of isoflavones given to the mice in the studies were much higher than what a person would consume in a day.

What are Isoflavones?

Isoflavones are plant-based estrogens called phytoestrogens. Because their chemical structure looks very similar to the human estrogen our bodies make, they have the ability to attach to estrogen receptors in the tissue in our bodies.

To help visualize how this works, think of your body’s estrogen receptors as locks on a door, and the human and plant estrogen as the keys. There are two different keys competing for the same locks.

• Human estrogen is the “master key” that unlocks the door. When it attaches, it opens the door and activates cell growth. In a healthy body, this is normal and necessary. In certain hormone-sensitive breast cancers, human estrogen acts as a fuel to unlock and multiply cancer cells.
• Plant estrogen is like a “faulty key” – it fits into the lock, but it doesn’t turn. Instead, by the key sitting in the lock, it prevents the human estrogen from getting into the receptor and potentially fueling the cancer cells.

“Is plant estrogen blocking the human estrogen my body needs?”

The answer to that is no. Our bodies have two different types of estrogen receptor “locks,” alpha and beta (Binaytara).

• Estrogen receptor alpha (ERα): This lock is associated with cell multiplication in breast and uterine tissue. This is the receptor that certain hormone-sensitive breast cancers depend on to grow.
• Estrogen receptor beta (ERβ): This lock works like a brake pedal, slowing down and inhibiting cell multiplication.

Here is the main difference between these two locks:

• Human estrogen is reckless; it turns on all the locks, including the ones that can fuel certain cancers (ERα).
• Plant estrogen is selective; it skips the cancer-fueling locks and chooses the locks (ERβ) that help slow down cell growth.

What Does Science Say Now?

I’ve gathered multiple science-based resources for you to reference about what the science currently reports about soy.

• The American Cancer Society: Human studies show the estrogen effects of soy have no effect at all or may reduce breast cancer risk. They also note that soy foods are an excellent source of plant protein to choose over higher saturated fat options like red or processed meats. They do not currently recommend soy supplements for cancer prevention. Instead, the recommendation is to consume nutrients through whole foods. Soy supplements can contain much higher amounts of isoflavones than are naturally found in whole soy foods. More research on soy supplements is needed to understand their effect on health.
• The American Institute for Cancer Research: Population studies show no increased risk for breast cancer survivors who eat soy foods, studies actually show that soy may be protective against breast cancer recurrence.
• National Center for Complementary and Integrative Health: Studies among Asian and Asian-American women have shown that higher dietary intakes of soy are associated with a lower risk of breast cancer, and soy products in supplement form have not been shown to reduce breast cancer risk.
• Binaytara: Current evidence does not show that eating tofu or other soy foods increases breast cancer risk or worsens outcomes for most breast cancer patients.

Verdict – Soy Does Not Cause Cancer

Current research has conclusively found that soy does not cause cancer. If you choose to include soy foods in your diet, then it is recommended to choose whole soy foods most often, such as:

• Edamame
• Tempeh
• Tofu
• Soy milk

Whole soy foods are a good source of fiber, lean protein, and heart-healthy fats.

I hope you found this information helpful and that it serves as a reliable resource to help ease any fears about soy. Next time you hear that soy causes cancer, be sure to share this resource with them so we can stop villainizing soy foods.

This blog was reviewed by Allison Bruno MS RD CSO LD CNSC.

Resources

• Estrogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer (MCF-7) cells in vitro and in vivo (PubMed)
• Nutrition & Health Info Sheets for Health Professionals – Soy (UC Davis)
• Soy (National Center for Complementary and Integrative Health)
• Soy and Breast Cancer: Is It Safe to Eat Tofu? (Binaytara)
• Soy and Breast Cancer: Myths and Misconceptions (American Institute for Cancer Research)
• Soy and Cancer Risk: Our Expert’s Advice (American Cancer Society)
• Soy and Cancer, Studies Indicate No Increased Risk (American Institute for Cancer Research)
• Where Do You Get Your Protein? A Helpful Guide To Plant-Based Protein Options

Author

Alex Gayton, MS, RD is a registered dietitian (RD) and has been with University of Maine Cooperative Extension since January 2020. Alex is a supervisor and the social media coordinator for the Expanded Food and Nutrition Education Program’s (EFNEP) Facebook, Instagram, and TikTok pages. She also is the creator of the Mainely Dish recipe video series. Alex loves food photography, baking, and cooking plant-based meals!

This blog was reviewed by Allison Bruno MS RD CSO LD CNSC.