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Soy foods, including tofu, edamame, miso, soy sauce, and soy milk, are some of the most widely consumed foods on the planet. As more people embrace plant-based diets, their popularity is increasing.

Yet soy is known to produce phytoestrogens in the body, and estrogen has been linked to breast cancer. Is there any reason to be concerned about breast cancer if you eat foods containing soy?

Researchers say no. In fact, there’s solid evidence that eating foods rich in soy may actually lower your risk of developing breast cancer.

This article explores some of the research into soy foods and breast cancer. It also discusses some of the other benefits and risks of including soy in your diet.

Why was there a concern about the relationship between consuming soy and developing breast cancer? In some older animal studies, mice that ate soy had an increased number of breast cancer cells in their mammary glands. Those results led researchers to question whether soy might have similar effects on humans.

However, animal studies don’t always translate well to humans. In this case, it’s important to note that there are at least two reasons why these mice might have a different response to dietary soy than humans.

Differences in how soy is processed

First, mice process the soy differently than humans do. To understand how, a little background is necessary. Soy contains several kinds of phytoestrogens (isoflavones). Phytoestrogens are plant-based substances that act like estrogen in the body.

According to the American Cancer Society, certain types of breast cancer have been traced to increased estrogen in the body. That’s what gave researchers cause for concern about soy and breast cancer. However, in humans, phytoestrogens turn into genistein and daidzein, two isoflavones that are very different from and much weaker than human estrogen.

In fact, soy has been proven to block the action of estrogen in tissues. In tissues with breast cancer cells, estrogen stimulates the multiplication of cancer cells. When soy blocks this stronger form of estrogen, it is playing an active role in reducing the risk of breast cancer.

Higher concentrations of isoflavones in mouse studies

Because of the differences in how soy isoflavones are metabolized in mice and humans, the mice in these studies were exposed to much higher concentrations of isoflavones than the amounts human beings typically eat. Researchers have concluded that the higher concentrations mean the outcomes are likely to be different for the two species.

When researchers conducted similar soy studies on primates, whose biology is much closer to human biology, they found no increased risk of breast cancer among the primates who consumed soy.

What human studies show

A number of long-term studies involving human populations have shown that eating soy foods does not increase the risk of breast cancer. On the contrary: Studies show that diets rich in soy may actually help to protect you from developing breast cancer.

A 2020 study that tracked the soy consumption of over 300,000 women in China found that moderate soy consumption did not raise the risk of breast cancer for women in the study. Women in the study who reported eating higher amounts of soy products experienced a lower risk of breast cancer.

A 2020 meta-analysis evaluated the results of 18 separate studies. After evaluating the results of these studies, researchers concluded that higher amounts of soy in the diet lowered the breast cancer risk for women. The protective effect was highest for women who had not yet reached menopause.

If you have been diagnosed with breast cancer or are a survivor, you may be wondering whether you should be cautious about consuming soy. Experts at the American Cancer Society say soy foods are safe and healthy for people to eat. They caution, however, that more research needs to be done to see whether taking isoflavone supplements is equally safe, since these supplements may have higher concentrations of isoflavones than there are in soy foods.

If you have breast cancer

In 2017, the Breast Cancer Family Registry followed the intake of soy isoflavones for 6,235 women diagnosed with breast cancer and living in the U.S. and Canada. It was found that women who ate the highest amounts of soy isoflavones had a 21 percent lower risk of death compared with women with the lowest intakes.

If you are a breast cancer survivor

In 2019, researchers analyzed 12 studies that followed 37,275 women who had survived breast cancer. Their analysis found that eating soy foods both before and after their diagnosis was associated with a reduced risk of cancer recurrence in postmenopausal women.

The soy that we eat can impact our bodies in a multitude of ways. There are benefits of eating soy related both to fighting breast cancer and our general health.

Protein

Soy products are a great source of protein. As opposed to some other plant proteins, soy proteins contain all nine essential amino acids that the body cannot make, making it a complete protein. Replacing red meat with soy protein may also help reduce your risk of certain cancers, because eating red meat has been linked to higher cancer risk.

Bone health

A 2020 research review found that soy isoflavones helped improve bone mineral density and prevent bone loss in people with osteoporosis. Researchers noted that it may be necessary to consume the isoflavones for a year or longer to see the most benefit.

Beneficial bacteria

Fermented soy foods like miso, natto, and tempeh are cultured with beneficial bacteria. These bacteria boost the health of our microbiome, which can improve our heart health, brain health, and regulate weight.

It is worth noting that most of the studies regarding soy as a cancer-fighting food are observational, and more detailed studies need to be done. The relationship between soy consumption and breast health may additionally be related to the lifestyle and other dietary habits of people who eat soy products.

While there is no link believed to exist between soy and breast cancer, there might be other reasons for you to consider eating less soy.

Soy supplements

While soy foods are safe and have a number of health benefits, there is not enough data to say for sure that soy supplements are equally beneficial. At least one study conducted in 2019 recommended against using soy supplements until more research could be done. It’s important to note that this study did not find any association between the past use of soy supplements and breast cancer.

Migraine

One form of fermented soy, soy sauce, contains tyramine and monosodium glutamate (MSG). Some researchers once thought tyramine triggered migraine attacks by constricting and dilating blood vessels. However, blood vessel dilation is not considered the cause of most migraine.

While there is insufficient evidence linking MSG consumption to headaches, it is believed to be a migraine trigger for many people, and soy is sometimes found on lists of migraine triggers.

Genetically Modified Organisms (GMOs)

Many soy products may be created with genetically modified soybeans. Some people are concerned that GMO foods may be linked to cancer and allergies; however, there are no long-term human studies linking GMOs to cancer or allergies. More research is needed.

There is no known link between breast cancer and eating tofu, miso, edamame, soy milk, soy sauce, or any other soy food. In fact, researchers have found that consuming soy foods can actually lower your breast cancer risk.

Soy foods can also benefit your health in other ways: protecting you from bone loss, boosting the beneficial bacteria in your gut, and increasing healthy sources of plant protein in your diet. It’s important to note that soy isn’t entirely risk-free, however. Soy may be a migraine trigger in some people, and it may be genetically modified, which some people choose to avoid.

More research needs to be done before scientists can say with certainty that soy supplements are as healthful and safe as foods that contain soy.