One of the most controllable factors in lowering cancer risk is your eating habits: Of the American Institute for Cancer Research’s 10 cancer prevention recommendations, six are based on diet. But while following a healthy dietary pattern is key, experts say that most Americans are missing out on one vital nutrient that could drastically impact your risk of developing cancer: fiber.

“If people are thinking about, ‘How can I optimize my health and reduce my risk of cancer,’ fiber is what we should be focusing on,” says Dr. Amy Comander, Medical Director of Mass General Cancer Center-Waltham.

The U.S. Dietary Guidelines recommend that women consume 22 to 28 grams of fiber per day, and men consume 28 to 34 grams of fiber per day. The American Society for Nutrition estimates that only 5% of men and 9% of women are hitting that target, with Americans overall only getting 10 to 15 grams of fiber per day on average.

“Diet has a big impact on our wellness in general,” says Dr. Andrea Tufano-Sugarman, gynecologic medical oncologist at Memorial Sloan Kettering Cancer Center. “I tell my patients, food is medicine.”

How can fiber help lower cancer risk?

A high-fiber diet has a direct correlation to better gut health, Tufano-Sugarman says. 

“The more fiber you eat—the more fruits, vegetables, legumes, whole grains—the more diverse your microbiome is,” says Tufano-Sugarman. That helps lower your risk of cancer—especially colorectal cancers—by reducing the risk of chronic diseases and inflammatory illnesses in your gastrointestinal tract, she adds.

Improving your gut health can also strengthen your immune system, says Dr. Nigel Brockton, Vice President of Research at the American Institute for Cancer Research (AICR). A stronger immune system will give your body the tools to fight off potentially cancerous cells, he says.

Plant-based foods like whole grains, fruits, vegetables, beans, nuts, and seeds, are rich in fiber and can also help lower your risk of obesity another major risk factor for cancer, Brockton points out.

That’s because plant-based foods are naturally lower in calories, while the fiber in them helps keep you fuller longer. That lessens the chances of you eating too many calories, Brockton says, while allowing you to fill up on whole foods over ultra-processed foods, such as deli meat and packaged snacks, which the AICR has linked to increased risk of several cancers.

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How to add more fiber to your diet

Experts Fortune spoke to agreed that a plant-based, fiber-rich diet that limits or cuts out alcohol, red meat, and processed meat is your best bet to help prevent cancer.

Here are expert-backed tips for easily upping your fiber intake:

• Swap refined grains like pasta, white rice, and white bread for high-fiber, whole-grain alternatives, Brockton advises.
• Fill half your plate with vegetables, Comander recommends.
• Eat the least-processed versions of plants—like a baked potato over French fries, Tufano-Sugarman says. 
• Opt for foods rich in the compound sulforaphane, which has been shown to have anticancer properties, Tufano-Sugarman says. Those include raw or minimally cooked or steamed cruciferous veggies like arugula, brussels sprouts, broccoli, cauliflower, and kale, she adds.

An easy mantra to follow that Comander tells her patients is coined by Food Rules author Michael Pollan: “Eat food, not too much, mostly plants.”

For more on lowering cancer risk:

• 5 lifestyle changes can significantly reduce your cancer risk, from giving up drinking to wearing sunscreen
• Working out at these times of day could reduce your risk of colorectal cancer by 11%, new study says
• 6 ways your brain and body benefit when you stop drinking alcohol