Can Soy Cause Breast Cancer?

What science says about the real risks of eating soy-based foods
tofu cubes or soy with woman's hand

If you’re wondering whether eating tofu or getting your latte with soy milk can increase your breast cancer risk — here’s what you should know.

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“Soy in its natural form does not rank high on the list of contributing factors for this disease,” says breast cancer specialist Erin Roesch, MD.

Why soy was first studied

The relationship between soy food intake and breast cancer has been researched and studied for over 25 years. Here’s why.

Soy products contain isoflavones, which are molecules that are similar to the hormone estrogen. It’s this similarity which has led to some theoretical concerns that soy could increase the risk of estrogen-sensitive cancers (including breast cancer).

But studies show that isoflavones are not in fact identical to estrogen. And this difference matters in a big way.

Isoflavones and estrogen do not have the same preference for estrogen receptor binding, and this distinction can lead to very different downstream effects,” Dr. Roesch says.

Clinical trials consistently show that the intake of isoflavone​ does not adversely affect the risk of breast cancer. These studies on humans have not confirmed a link between eating natural soy and developing breast cancer. In fact, some actually dispute it and even suggest a protective effect.

For example, one 2010 study found no association between phytoestrogen consumption (which includes soy products) and an increased risk for breast cancer. No data is decisive enough to tout soy’s effects on breast cancer risk.

Still, when studies like these suggest any initial possible correlation between any food and cancer, misinformation can spread quickly. Consumers might quickly assume these studies show a cause-effect relationship — when in reality long-term, meticulous research is still needed to prove any connection.

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Reading health behaviors in different countries correctly

Science tries to avoid correlating behaviors with risk factors too quickly. And it’s that very reason extensive research must first be done to take all factors into consideration.

Consider any correlation between soy and breast cancer world-wide. Rates of breast cancer in general are much higher in the United States than in many Asian countries for example, where soy products are a major diet staple, Dr. Roesch says. 

Those countries also typically feature an overall lower-fat diet and differences in birthrates, both of which affect cancer rates.

One possible reason breast cancer has been on the rise lately in these countries may be due to adoption of a Western diet and lifestyle, which may include higher intake of saturated fats — and not specifically the consumption of soy.

Obesity, smoking, lifestyle and genetics are riskier than soy

Unfortunately when people worry about something like soy intake when it may not be a risk for breast cancer they may not be worrying as much as they should about true risk factors.

“Removing attention from these can be the greater risk,” Dr. Roesch says.

These other behavioral risk factors for breast cancer — like obesity, smoking at an early age, a sedentary lifestyle or high saturated fat intake — are bigger concerns than consuming plant estrogens like soy, she says.

Genetics also play a major role in a person’s risk of developing certain types of breast cancer.

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Natural soy is the healthiest choice

When choosing soy-based products go for natural options rather than highly processed foods. And eat them in moderation, Dr. Roesch advises.

Plant estrogen-based sources such as soy milk, tofu and edamame are all good choices. But make them part of a balanced diet that’s high in fruits and vegetables and low in saturated fats.

Try to avoid isoflavone extracts

Dr. Roesch does advise women to avoid soy isoflavone extracts, especially in large doses. 

And as a general rule you’re better off getting your nutrition through food sources than through supplements.

“When you’re taking doses of isoflavones from a vitamin store that can be several hundred times higher than what you would ingest from eating tofu or drinking soy milk, that could be a potential problem,” she says.

“Whether you’re concerned about your risk or if you’re high-risk for breast cancer it’s always best to make sure to talk to your doctor about everything you put into your body,” she emphasizes. “Together you can cut through any misinformation and identify what works best to keep you healthy.”​

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