Can Soy Cause Breast Cancer?

Rumors about soy and breast cancer have been around for a long time. But is there any truth to them?

Two breast cancer specialists, oncologists Erin Roesch, MD, and Tiffany Onger, MD, tell you what you need to know.

Does soy cause breast cancer?

Let’s get right to the point: Eating soy does not increase your risk for breast cancer.

“Studies in humans have not shown that soy-based products necessarily increase the risk for cancer,” Dr. Onger confirms.

So, why does misinformation linger? Let’s break down what the research shows.

Understanding concerns about soy and breast cancer

“The concern about soy has long been about whether, when consumed, it can act like phytoestrogens in the body,” Dr. Onger explains. To better understand what that means, you first need to understand a bit about breast cancer and the chemical makeup of soy.

The surface of your cells includes protein molecules called receptors, which can attach to certain substances in your blood. The receptors on breast cancer cells specifically can attach to the hormone estrogen — and that can fuel the growth of cancer. That’s one reason why higher levels of estrogen in the body are a risk factor for developing breast cancer.

And then there’s soy. Soy products contain phytoestrogens — plant-based compounds that act similarly to estrogen when you consume them. That led to the theory that eating soy and soy-based products could raise the risk of developing estrogen-sensitive cancers, including breast cancer.

In short, people worried that eating soy could increase estrogen in the body, which could, in turn, cause breast cancer.

But here’s the thing: “Phytoestrogens, like those found in soy, do resemble estrogen. But they don’t make the same bond with those receptors to fuel cancer cell growth,” Dr. Roesch clarifies. “And that’s a big distinction.”

What do studies say?

This belief that soy causes breast cancer couldn’t have come from nowhere, right? Surely there’s some science behind it? Well, yes and no.

In lab studies, high doses of isoflavones (the type of phytoestrogens in soy) have been shown to increase the risk of breast cancer. But human clinical trials consistently show that isoflavones don’t increase breast cancer risk in humans. In fact, some studies suggest that soy may even lower the risk of breast cancer recurrence and improve survival.

“Issues have only come up in lab models, not in human studies,” Dr. Onger reports. “Still, when studies suggest any possible correlation between food and cancer, misinformation and worry can spread quickly. That’s why it’s important to take a close look at the data and understand exactly who was studied.”

What about soy supplements?

There’s no need to clear your kitchen of foods like soy milk, tofu, edamame and other soy-based items. But Dr. Onger and Dr. Roesch both advise against turning to soy supplements.

“I encourage people to get their soy through diet, as opposed to taking supplements,” Dr. Onger says. “We don’t yet know as much about what effects soy supplements might have.”

Plus, there’s a universal concern that applies to supplements: In the U.S., supplements aren’t regulated the way that pharmaceutical medications are. And that means there’s no way to know exactly what’s in them or how precise your dosage is.

“Doses of isoflavones from a vitamin store can be much higher than what you would ingest from eating tofu or drinking soy milk,” Dr. Roesch notes. “Therefore, the best thing for your health is for most vitamins and nutrients to come from your diet.”

So, stick to soy through the foods and drinks you consume, and go for more natural options over ultra-processed foods. (That’s a tip that applies to all facets of your diet, really!)

The bottom line on breast cancer risk

There are many known risk factors for breast cancer, and they should take priority over worrying whether it’s OK to put soy milk in your latte or tofu in your salad.

“It’s important to know what the risk factors for breast cancer are so that you can try to minimize your overall risk as much as possible,” Dr. Onger encourages.

Some breast cancer risk factors are non-modifiable, meaning you can’t change them (like age, genetics or breast density). But there are lifestyle-related risk factors that you may be able to change. These healthy habits help stack the odds in your favor:

• Be physically active. Staying sedentary raises your risk — so find some type of exercise or activity you love, and get moving. Aim for 30 minutes of brisk movement at least five days a week.
• Focus on a balanced diet. Moderation is key! Embrace lean protein, whole grains, and fruits and veggies, while limiting saturated fat and sugar.
• Curb substance use. Smoking and excessive drinking both raise your risk of breast cancer, as well as other cancers. Talk to a healthcare provider if you need help quitting.
• Maintain a healthy weight. Obesity, particularly after menopause, is a breast cancer risk factor. But focusing on the other tips in this list can help you achieve or maintain a weight that’s healthy for you.

“Ignoring known risk factors for breast cancer is a much bigger risk than consuming soy,” Dr. Roesch reiterates. “So, don’t hesitate to talk to your healthcare provider if you need any additional guidance to make lifestyle changes that will help lower your risk.”