Soy and Where It Comes From

Soy foods are made from soybeans, which are part of the legume family. Soybeans grow in pods, like peas, and contain natural compounds called isoflavones. Isoflavones are part of soybean plants that have a similar structure to estrogen, but it’s important to know three things: 

• Isoflavones are much weaker than human estrogen.

• Isoflavones in soy foods do not turn into estrogen when you eat them. 

• Isoflavones don’t work the same way in the body as estrogen. 

For many years now researchers have been studying whether or not isoflavones (from plants like soybeans) can stimulate the growth of cancer cells. As a result, we have ample data explaining how isoflavones function, and the consensus is clear: soy foods do not fuel the growth of cancer cells.  

Estrogen-Receptor Positive (ER+) Breast Cancer and Soy

As with most research, the earliest work on isoflavones happened in test tubes and mice. The studies conducted in mice led to a belief that high concentrations of isoflavones from soy foods might fuel cancers that rely on estrogen. The problem was that back then, it wasn’t understood (yet) that mice metabolize isoflavones differently than humans. We now know that mice build up much higher levels of isoflavones than humans do when eating soy, and the animal research on isoflavones’ impacting cancer isn’t very relevant to human health.  

In the past few decades, researchers have been exploring how soy foods affect cancer risk in humans, including cancers that need estrogen to grow, such as ER+ breast cancer.  

If you’ve heard or read things like, “plant-versions of estrogen stimulate an estrogen-like response in the body,” keep reading to get the latest on this topic.  

What Research Shows

Studies show soy foods do not increase the risk of any type of cancer. In fact, there is reliable research linking soy with lower rates of some cancers.

For example, research in Asia finds that 1-2 servings of soy foods daily lower the risk of developing breast cancer. One possible explanation for this is that isoflavones may block estrogen from binding to cancer cells. Other research shows lower rates of lung cancer in non-smokers who eat soy foods.  

The same holds true for soy intake after a diagnosis of cancer. Research finds no increased risk of cancer progression with moderate intakes of soy, even for people with ER+ breast cancer. A moderate intake of soy foods is defined as 1-2 servings/day (which we cover in more detail below).  

Things You May Be Wondering 

• What’s a serving of soy food?

• How many servings are okay to eat? 

• If breast cancer runs in my family, should I avoid soy foods? 

The Bottom Line

Soy foods, in moderation (1-2 servings/day) are safe to consume in terms of cancer risk.  

These are examples of serving sizes for some soy foods to help you gauge how much and how often you might include soy foods:  

• Tofu: extra or super firm –3 oz. 

• Silken tofu – 1/3 cup 

• Soymilk – 8 oz. (1 cup) 

• Edamame – ½ cup 

• Soynuts – ¼ cup 

Action Steps to Take

If you enjoy soy products like tofu, edamame, and soy milk there is no reason to avoid them for cancer prevention or survivorship reasons. This is true for people experiencing cancer presently and for family members who worry about the familial risk for certain cancers.  

Soy condiments like soy sauce or soy lecithin do not contain phytoestrogens and do not need to be avoided.  

If you have additional questions about soy, our Iris Care Team is available to you.