Is soy safe for patients with breast cancer?

Soybeans are nutritious and versatile. They are used to make products ranging from tofu to soy milk, and contain protein, fiber, potassium, magnesium and iron.

But despite their nutritional profile and the many ways they can be enjoyed, there has been concern that soy products might be linked to increased cancer risk.

As a breast cancer survivor and senior clinical dietitian at MD Anderson, Christie Siebel is passionate about debunking misinformation and making sure patients get the nutrients they need to stay healthy. She stresses that soy is generally safe to eat, even for people with cancer.

“Soy is a great alternative to animal protein to include in your daily diet,” she says. “There’s no reason to avoid eating soy.”

Ahead, she talks more about soy’s link to cancer, its health benefits and the best ways to add it to your diet.

Soy foods may decrease overall cancer risk

If soy is safe to eat, why have soy products been linked to cancer risk?

It could be because soy contains phytoestrogen, the plant form of the hormone estrogen. Because phytoestrogen and estrogen sound similar, Siebel says there has been hesitation around eating phytoestrogen, especially among patients with breast cancer and other types of cancer that are hormone sensitive. In some breast cancer treatments, estrogen is even blocked to prevent cancer cells from forming.

However, phytoestrogens and estrogen aren’t the same. Eating phytoestrogens doesn’t affect the estrogen found naturally in your body.

Instead, research shows that eating soy products may reduce cancer risk.

“Soy intake is associated with reduced risk of lung, prostate and breast cancer,” Siebel says, adding that a 2022 analysis suggests that eating soy may reduce the risk of all types of new cancer diagnoses.

“The average lifetime risk for any cancer is about 40%,” she says. “Researchers found that people who eat a diet rich in soy can decrease their risk for cancer to 36%.”

Siebel is also particularly excited about research that shows soy consumption may lower lung cancer risk by over 2%.

“New research is showing exciting benefits on soy intake and lung cancer having even more of an impact on lowering risk for lung cancer than even breast or prostate cancer,” she says.

Soy foods may reduce the risk of breast cancer recurrence

Research also suggests that eating soy foods may reduce the risk of cancer recurrence – even in patients with estrogen receptor positive cancer.

“Soy isn’t bad for you, and it may actually be beneficial for cancer prevention,” Siebel says.

This may be especially true for patients who carry a BRCA mutation. These genes are responsible for DNA repair. It’s thought that isoflavones, a type of phytoestrogen found in soy, may restore tumor suppression in BRCA1 and BRCA2 genes.

“That’s why I try to eat soy every day,” says Siebel, who carries the BRCA gene.

There are multiple theories on why soy reduces cancer risk

Wondering how soy can reduce the risk of breast and other cancers? Here are some possibilities researchers are exploring.

Isoflavones

Isoflavones may be particularly helpful in decreasing cancer risk and preventing cancer recurrence. The 2022 analysis writes this may be because isoflavones reduce inflammation and oxidative stress.

In addition to possibly reducing cancer risk, isoflavones may have other benefits, including bone health.

“Evidence suggests that soy isoflavones may help reduce bone loss and improve bone health in menopausal women,” Siebel says.

Fiber

Whole soy foods like soybeans, soy nuts and edamame contain fiber.

Fiber has many health benefits, including supporting digestion and stabilizing blood sugar levels. That’s why our experts recommend women consume 25 grams of fiber per day and men consume 38 grams of fiber per day.

Fiber may also play a role in reducing cancer risk. For example, Siebel says fiber might lower colorectal cancer risk.

Phytochemicals

As a plant-based food, soy products are also a good source of phytochemicals. These phytochemicals may offer benefits like boosting your immune system and lowering inflammation.

How to add soy to your diet

Although soy is a plant-based product, the way it is prepared can impact its health benefits. Here, Siebel shares her tips for incorporating soy foods into your diet.

Choose whole soy foods when possible

The healthiest soy products are whole soy foods. These options are minimally processed and contain more plant-based protein, Siebel says.

Examples of whole soy foods include:

• Edamame
• Unsweetened soy milk
• Tofu

Depending on your dietary needs, Siebel says two servings of soy foods a day can be part of a healthy diet. This might look like half a cup of edamame and an eight-ounce cup of soy milk.

Curious about soy-based protein bars and powders? These products are often made using a form of processed soy called soy protein isolate. While processed soy products can be a helpful way to add plant-based protein to your diet, Siebel says it's always best to get nutrients from real foods whenever possible. She adds that your care team can help you determine how much protein you need to reach your health goals.

Use soy condiments sparingly

Soy condiments include soy sauce, soybean oil and soy lecithin. Since these products don’t contain the beneficial phytoestrogens found in whole soy foods, they offer few, if any, health benefits. They can also be high in sodium which can increase your risk of high blood pressure and other health issues. However, they’re not necessarily harmful if you eat them in small amounts, Siebel says.

Be cautious about soy supplements

After hearing about the possible health benefits of soy isoflavones, it might seem like a good idea to take a soy supplement. But Siebel recommends proceeding with caution. “Supplements are not well regulated by the FDA and may contain harmful impurities or additives, contain much larger or smaller doses than the label lists or interact with other medications,” she says.

Instead, get the health benefits of soy through whole soy foods like those listed above.

Talk to your care team about your diet needs

If you have any questions about your diet during treatment, Siebel says to ask your care team.

“Work with your dietitian and care team to make the best decisions for you,” she says. “But there’s no reason to avoid eating soy.”

Request an appointment at MD Anderson online or call 1-877-632-6789.