Soy and Breast Cancer: How Phytoestrogens Affect the Body (2024)

Past animal research indicated soy may increase breast cancer risk, but that research has been debunked. Research suggests soy can be part of a healthy diet that may even help reduce cancer risk.

Is there a link between soy and breast cancer risk?

The big concern is that a diet rich in soy may lead to breast cancer development. This is because soy contains estrogen-like compounds. Since estrogen has a connection to breast cancer, this is where it gets confusing.

It’s completely normal to feel overwhelmed by conflicting information, so let’s sort the facts from the fiction.

Read on for an overview of the newest research, plus a breakdown from an oncology dietitian on exactly what the story is with soy and breast cancer.

Why do some medical professionals say to avoid soy?

Misconceptions about soy stem from its phytoestrogen content, also known as isoflavones.

“Some doctors are careful about soy because it has phytoestrogens, which can act like estrogen in the body,” says oncology nutritionist Taylor Janulewicz.

The outdated concern that these compounds might stimulate estrogen-sensitive cancer cells largely stems from older animal research, which showed that mice who ate soy experienced a boost in breast cancer cells in their mammary glands.

“Health professionals originally told breast cancer survivors to avoid soy foods,” said breast cancer dietitian Tamar Rothenberg. “It was based on flawed research in mice. In the past 25 years of research, we’ve learned it’s not only safe but also protective.”

As Nichole Andrews, RDN, explains, phytoestrogens are plant-derived compounds that resemble estrogen but are different in structure and function from human estrogen.

Since we’re people and not mice, we process phytoestrogens differently. We do so by switching them into two isoflavones called genistein and daidzein. These are considered highly weak compared to human estrogen.

“Unlike human estrogen, which is produced by the adrenal glands, ovaries, and fat tissue, phytoestrogens from soy do not act the same way in the body,” Andrews said.

What does the newest research say about soy and breast cancer?

“Soy is not only safe, but it’s been shown to reduce the risk of cancer,” Andrews said.

A 2024 review found that eating more soy, especially tofu and soymilk, may lower overall cancer risk. Researchers found that consuming 61 grams of tofu daily may reduce cancer risk by 12%, while 23 grams of soymilk may reduce it by 28%.

A 2022 meta-analysis, reported that consuming soybean products containing isoflavones can actually lower the risk of breast cancer in premenopausal and postmenopausal people.

According to a 2020 study that monitored soy consumption in more than 300,000 participants in China, eating moderate amounts of soy didn’t increase breast cancer rates. Instead, participants who consumed large amounts of soy saw a decreased risk.

One meta-analysis from 2020 looked at 18 individual studies on soy consumption and breast cancer risk. The results showed that diets high in soy resulted in decreased breast cancer risk, especially for those who hadn’t started menopause.

In another example, a 2017 review showed that soy blocks estrogen action in tissues. This action likely reduces breast cancer risk.

Is there a difference between whole soy foods and other forms of soy?

“Absolutely, there are important distinctions,” Andrews said.

Some types of soy include:

• Whole soy foods (edamame, tofu, tempeh, soy milk): These are nutrient-dense options packed with high quality protein, fiber, vitamins, and minerals.
• Soy supplements and soy protein isolate: These processed forms of soy are more concentrated sources of isoflavones, aren’t as well understood, and lack the beneficial fiber and nutrients found in whole soy foods.
• Soy lecithin: Used as an emulsifier in various foods, soy lecithin contains only small amounts of phytoestrogens and is considered safe.

Whole soy foods “contain natural phytoestrogens that may actually help reduce cancer risk by providing a balanced nutrient profile and beneficial plant compounds,” Andrews said.

Rothenberg agrees.

“Whole soy foods have breast cancer protective compounds, such as isoflavones, protein, and fiber,” she said. They are also among the least processed foods in our food supply.”

On the other hand, it’s best to avoid soy protein isolates.

“Soy protein isolates are not recommended for now, as they haven’t been studied enough in that high of a concentration,” said Rothenberg.

“Soy protein isolate is a processed form with most fat and carbohydrates removed, offering concentrated protein but lacking the full spectrum of nutrients found in whole soy,” Janulewicz said.

Soy lecithin is considered safe as a binder and emulsifier. However, it doesn’t offer any health benefits.

What about soy when it comes to estrogen-receptor-positive breast cancer (ER/PR+)?

For those with ER/PR+ breast cancer, whole soy foods can be a healthy addition to the diet.

“Research suggests that the phytoestrogens in soy do not increase the risk of cancer recurrence and may even offer protective benefits,” Andrews said. “These compounds can weakly bind to estrogen receptors, potentially blocking the stronger effects of human estrogen and helping to regulate estrogen activity in the body.”

What foods should you avoid to reduce cancer risk?

There is some consensus about foods that increase the risk of breast cancer.

“The only foods to avoid for breast cancer risk are alcohol, processed meats, and sugar-sweetened beverages, such as soft drinks,” said Rothenberg.

• Alcohol: Strong evidence shows alcohol consumption is related to an increased risk of a slew of cancer types.
• Processed meats: Processed meats are known for an increased risk of certain cancers, including breast cancer. Andrews suggests limiting or avoiding these foods altogether.
• Sugar and refined carbs: Diets high in sugar and refined carbohydrates can lead to obesity, a well-known risk factor for 13 types of cancer.

Your best bet is to eat a varied diet balanced with lots of whole foods, like whole grains, fruits, and vegetables.

What should we include in our diet?

Here are top tips to stay informed and eat well.

• Balance: Nourish yourself with a diet loaded with whole grains, fruits, vegetables, healthy fats, and lean proteins. Keep sugar, alcohol, processed meat, and refined carbs to a minimum.
• Individualized advice: Work with a registered dietitian who has specialized training in oncology to create a personalized plan that considers your specific diagnosis, treatment, and overall health.
• Research updates: Stay informed about the latest dietary recommendations and scientific findings.

“Regular consultations with healthcare [professionals] can help patients navigate these updates effectively,” Andrews said. “This approach supports overall health and may help in cancer prevention and management.”

Bonus myth-bust: flaxseed

Like soy, flaxseed contains beneficial phytoestrogens (lignans) and omega-3 fatty acids that may help lower inflammation and potentially decrease cancer risk.

“Including flaxseed in the diet in moderation can be beneficial,” Andrews said.

“There have been concerns about potential interactions betweenflaxseed and Tamoxifen, a common breast cancer treatment,” said Janulewicz. “However, preclinical studies suggest that no harmful interactions exist.”

The bottom line

If you’ve taken anything away from this deep dive about soy and breast cancer, we hope it’s peace of mind.

“Soy is an excellent choice for many, offering numerous health benefits,” said Andrews.“It’s a nutrient powerhouse that supports overall health and can be part of a balanced diet for cancer prevention and management.”