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Soy and Breast Cancer: Is Soy Safe To Eat?

Posted on May 26, 2021
Medically reviewed by
Todd Gersten, M.D.
Article written by
Ryan Chiu, M.D.

Thinking about factors that might increase your risk of breast cancer — which affects 1 in every 8 women in the United States at some point in their lives — is important to being mindful of your overall health. People concerned about breast cancer sometimes wonder whether consuming soy products can increase their risk of developing the condition or relapsing after treatment.

A MyBCTeam member recently asked whether it was advisable to use a supplement for preventing osteoporosis because it contained soy. “Does anyone have information on soybean products after having had breast cancer?” they asked, noting that the supplement “has several different vitamins and minerals in it, one of which is fermented soybean oil.”

Concerns about soy intake in relation to breast cancer are often based on a confusing association between soy and the female sex hormone estrogen — which, at high levels, can contribute to some types of breast cancer. Let’s take a closer look at the claims that soy is unsafe and what the science actually says about its link to breast cancer.

Soy and Its Sources

Soy is derived from soybeans, which are native to Asian countries and used to produce soy milk and common soy foods, such as soy sauce, bean paste, miso, and tempeh. Soybeans themselves are also cooked in many foods. Soy protein isolates are also an inexpensive source of protein used in animal feeds and many packaged foods, meaning that soy works its way into meat and other food products that aren't obviously associated with it.

Soy proteins are thought to provide many health benefits, particularly in heart health. Studies have shown that consuming soy proteins (versus typical animal proteins) might lower blood-cholesterol levels and reduce your risk of heart disease and stroke. This finding is especially true for people who have high cholesterol.

Soy can also be found and packaged as soy supplements, which are manufactured from concentrated soy extracts.

Soy Estrogens and Breast Cancer Risk

Some of the active compounds found in soy are called isoflavones. Isoflavones are a type of phytoestrogen, a compound found in legumes. Phytoestrogen is similar (but not the same) in structure to the human version of estrogen. Phytoestrogens and isoflavones are thought to have the same heart-healthy effects as soy proteins. However, the close resemblance between estrogen and phytoestrogen (and other isoflavones) has caused much concern in the breast cancer community. Soy, in fact, does not contain estrogen.

Breast Cancer and Estrogen

Many breast cancers are estrogen receptor positive (ER-positive), which means that the cancer cells have estrogen receptors that respond to increased levels of estrogen in the body. Too much estrogen can cause these cells to expand and divide, causing cancer growth.

This effect is why estrogen-based hormone therapies — commonly used during menopause and postmenopause to prevent symptoms like hot flashes — have been linked to an increased risk of breast cancer. This link between estrogen levels and cancer explains why some cancer treatment drugs — such as tamoxifen, which blocks estrogen receptors — are used to treat breast cancers that are ER-positive.

What About Soy Estrogens?

Older studies involving animals are at the root of the myth that soy can contribute to breast cancer the same way estrogen does. Research found that animals given highly concentrated amounts of soy developed breast cancer at higher rates than other animals. However, results from animal studies do not necessarily translate into the same results for people. In this case, animals and humans do not process soy in the same way.

In fact, researchers over the years have conducted population studies and found that the results from these animal studies were not applicable to humans. When comparing countries where soybean consumption was much higher than others — such as East Asian countries versus the U.S. — researchers found soybean intake was actually linked to the same or even lower risk of breast cancer. This decreased risk may result from isoflavone’s ability to block the more potent natural estrogens in the blood, as noted by the American Cancer Society.

Avoid Soy Supplements

These human studies are largely based on natural sources of soy (in foods). However, the evidence surrounding soy supplements — which may be far more concentrated and a lack the “good parts” of natural food-based soy — is still mixed. Many recent studies have suggested that taking soy supplements, not natural soy, may lead to increased breast cancer risk — the opposite effect of eating soy foods. Dietitians currently suggest avoiding soy supplements until more concrete evidence comes out, as their higher concentration of soy estrogens may raise your risk of cancer developing or recurring.

Is Soy Right for You?

Moderate amounts of soy consumption — particularly whole soy foods — is likely safe for most people with breast cancer, breast cancer survivors, and people in general. Soy foods may actually be more likely to lower breast cancer risk than to increase it. Moreover, soy offers other health benefits, including reducing the risk of heart problems — which make it a great addition to your diet.

You should talk to your health care team about your individual diet, including soy products, and before starting any supplement or other over-the-counter products.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 52,000 members come together to ask questions, give advice, and share their stories with others who understand life with and after breast cancer.

What other foods are you concerned about? Share your thoughts in the comments below, or start a conversation by posting on MyBCTeam.

A MyBCTeam Member said:

My oncologist says no soy for me at all.

posted about 2 months ago

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Todd Gersten, M.D. is a hematologist-oncologist at the Florida Cancer Specialists & Research Institute in Wellington, Florida. Review provided by VeriMed Healthcare Network. Learn more about him here.
Ryan Chiu, M.D. obtained his medical degree from the University of Illinois College of Medicine in 2021. Learn more about him here.

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