Is There a Link Between Soy Products and Breast Cancer?

Breast cancer is one of the most common cancers in women in the United States, affecting 1 in 8 women. Misconceptions about soy food products have led to the myth that there is a link between soy and breast cancer risk. The latest scientific evidence shows that soy foods do not increase breast cancer risk and may even help lower it. 

Key Takeaways

• Soy products and cancer: Current human research shows that eating whole soy foods does not increase breast cancer risk and may even offer protective benefits.
• Does soy milk cause cancer? No. Drinking 1–2 servings of soy milk a day is safe for healthy women and breast cancer survivors, according to major cancer organizations.
• Phytoestrogens and cancer: Plant-based phytoestrogens in soy act differently than human estrogen and do not cause cancer; some studies suggest they may help regulate hormone activity.
• Soy cancer myth: The long-standing belief that “soy is bad for breast cancer” is based on outdated animal research and is not supported by human clinical evidence.

Does Soy Milk Cause Cancer?

No. High-quality human studies have consistently shown that soy intake, including soy milk was not associated with causing cancer. Observational research and clinical trials have not found any link between soy intake, including soy milk and breast cancer growth or recurrence.

What Caused the Soy Cancer Myth?
Misconceptions about the role of soy in breast cancer can be traced back to a series of animal studies published in the 1990s. Since then, scientists have discovered that rodent studies are not an accurate method for studying soy consumption due to mice having different metabolism processes than humans.

Human Evidence to Disprove the Link Between Soy Products and Cancer
Since the animal studies in the 1990’s, researchers have investigated the potential protective effects of soy foods against breast cancer via human clinical trials. This is in large part due to historically low breast cancer mortality rates in soy food-consuming Asian countries and the potential for soybean isoflavones to function as anti-estrogens, which are compounds that may block the effect of estrogen in breast cancer cells and slow growth.

Soy and Breast Cancer Risk: Scientific Evidence

Population studies and randomized clinical trials have concluded that eating whole soy foods does not raise breast cancer risk. Human research indicates that isoflavones found in soy foods may be protective against breast cancer.

What Are Phytoestrogens?
Soy foods contain isoflavones, which are a type of phytoestrogen, a naturally occurring compound in many plants that shares some similarities with estrogen. While there are structural similarities, isoflavones are not the same as human estrogen. Isoflavones bind to estrogen receptors in cells in a manner that differs from the way estrogen does.

Do Phytoestrogens Cause Cancer?
No. The controversy around soy and breast cancer is based almost exclusively on rodent studies that were published in the late 1990s suggesting that isoflavones may stimulate the growth of existing estrogen-sensitive mammary tumors in mice. However, it’s since been concluded that rodents appear to be of limited value for concrete evidence as they metabolize isoflavones differently than humans.

Is Consuming Soy Bad for Breast Cancer Patients?

Evidence does not indicate a relationship between soy intake and an increased risk of breast cancer. Meta-analyses of observational studies in soy food-consuming countries suggest soy intake early in life actually may reduce the risk of developing breast cancer.

Major health organizations, including the American Cancer Society and the American Institute for Cancer Research, have agreed that soy consumption is safe for breast cancer patients and survivors. Any concerns about phytoestrogens and their effect on breast tissue have also been debunked by multi-year studies by the European Food Safety Authority and the Permanent Senate Commission on Food Safety of the German Research Foundation (SKLM) who concluded that “available human studies do not indicate that an isoflavone exposure as reported in the thoroughly studied Asian population or as investigated in clinical studies (i.e., about 100 mg/day) negatively influences breast cancer risk or the thyroid hormone system in healthy women.”

Soy Cancer Myths: Debunking Common Misconceptions

Despite the research, people still have questions about soy and its relationship to breast cancer.

Myth 1: Soy foods increase estrogen in the body.
Fact: Phytoestrogens bind to estrogen receptors differently than human estrogen, often exerting weaker or even potential protective effects.

Myth 2: All soy, especially processed soy, is harmful.
Fact: Soy foods such as soy milk, tofu, and tempeh are a good source of protein, nutrient-dense and safe. Processed foods like soy milk, soy-based meat alternatives, and soy protein bars are accessible, convenient, and often cost-effective choices for many people and can support an overall healthy eating pattern. Depending on the form, soy foods may offer protein, unsaturated fat, fiber, and probiotics which are valuable for a healthy diet.

Myth 3: Supplements of soy isoflavones are unsafe.
Fact: Moderate intake from food is well-studied and safe. High-dose supplements are less studied and should be discussed with a healthcare provider.

How Much Soy Should I Consume Per Day?

According to the American Institute for Cancer Research, moderate consumption of soy is about 1 to 2 servings daily of whole soy foods, like tofu, edamame, soy milk and more. One serving of soy foods averages to about 7 grams of protein and 25 milligrams (mg) of isoflavones. Research shows that up to 3 servings a day of soy (up to 100 mg of isoflavones a day) consumed in Asian populations long-term is not linked to increased breast cancer risk.

As always, it is best to consult a physician or registered dietitian regarding dietary changes.

Click here to dive deeper into the research on soy and breast cancer and download our FAQ for further reading.

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