Some of the most physically fit people in the world may have a unique health risk. New research uncovers a possible link between marathon running and colorectal cancer.

Oncologists at the Inova Schar Cancer Institute in Virginia conducted the study, which examined the colons of relatively young people who had run several long-distance races. They found these runners had a much higher rate of having potentially dangerous adenomas (a type of polyp) than would be expected for their age. Though the findings are preliminary and require more confirmation, they may point to a real connection between colorectal cancer and extreme physical activity.

“It tells us there’s a signal here,” David Lieberman, a gastroenterologist and professor emeritus at Oregon Health and Science University not affiliated with the study, told the New York Times Tuesday. “We wouldn’t have expected these rates of high-risk adenomas, which are cancer precursor lesions, in an age group like this.”

A mysterious trend

Lead researcher Timothy Cannon was inspired to perform the study after he treated three young patients with colorectal cancer, all of whom had run ultramarathons (defined as any race longer than 26.2 miles). Not only were his patients fit, but they were also much younger than the typical case, the oldest being 40.

In 2022, Cannon and his colleagues began recruiting endurance athletes for their prospective study. The volunteers had all run at least two ultramarathons or five regular marathons; they also had no family history of colorectal cancer or other apparent risk factors. All told, 100 athletes between the ages of 35 and 50 took part and were given colonoscopies.

The researchers went looking for advanced adenomas in the colons of their volunteers, relatively large or otherwise unusual polyps. Though these growths are themselves benign, they have a higher risk of turning cancerous than other polyps. Then they compared the rate of finding these polyps in their athletes to historical trends.

About 1.2% of people in their 40s at average risk for colorectal cancer would be expected to have advanced adenomas, according to the researchers. By sharp contrast, 15% of the runners they studied had them, while nearly half had polyps in general.

“Consideration of refined screening strategies for this population is warranted,” the researchers wrote in their study.

Much left to understand

The team presented its results earlier this year at the annual conference of the American Society of Clinical Oncology. That means this study hasn’t yet undergone the formal peer-review process. The authors are also quick to note their work isn’t definitive proof that endurance running can cause colorectal cancer.

Assuming this link is causative, there remains the burning question of why. As even weekend 5k joggers will know, running can occasionally trigger bouts of gastrointestinal distress (the namesake runner’s diarrhea). These injuries are sometimes caused by temporarily restricted blood flow to the intestines that damages nearby cells. It’s possible, the researchers speculate, that extreme runners who regularly experience this blood flow loss can develop the sort of chronic inflammation that makes cancer more likely to emerge.

At this point, though, that’s only one hypothesis for what may be happening here. The researchers say future studies should try to confirm their findings as well as untangle the causes and risk factors that could explain this potential higher risk.

All that said, this research shouldn’t scare anyone away from running or any other form of cardio. The many health benefits of regular physical activity—which importantly include a lower risk of at least eight different types of cancer—still far outweigh the risks for the average person.