A small study of young endurance runners found unusually high rates of precancerous colon polyps, a result that has prompted researchers to plan a larger follow-up. The work grew out of what Timothy Cannon said he first noticed in 2019, after treating three unusually young patients with advanced colon cancer within six months.

Two of the patients Cannon described were in their 30s and one was in their 40s, and all three were extremely prolific runners who regularly competed in ultramarathons or multiple marathons despite lacking known risk factors. That pattern led his team to look more closely at a group of 94 runners ages 35 to 50 who had completed at least five marathons or two ultramarathons.

Colonoscopies found that nearly half of the runners had colonic polyps, while 15 percent had advanced adenomas, the kind of lesions more closely associated with the development of cancer. Cannon said the finding was intriguing but far from settled science. “We don’t know at this point,” he said. “But I do think there’s a signal here.”

The study did not include a non-runner control group, which means it cannot prove that endurance running caused the findings or even that it was responsible for the higher rates seen in this group. Amy S. Oxentenko called the work “a super interesting study” but said it was “quite preliminary.” Researchers are now planning a bigger follow-up study with about 300 runners and a non-running control group.

That next phase is expected to examine whether genetics, diet, gut microbiome and other endurance sports help explain the pattern. One idea under review is that prolonged endurance exercise may shunt blood away from the gut and contribute to repeated irritation or injury, though that remains unproven. The source says runners should not be alarmed and should keep running, because research consistently shows exercise is beneficial for overall health.

Oxentenko urged people not to overreact to the findings. “Runners shouldn’t panic,” she said. Sara Campbell warned that “the worst outcome would be for people to start thinking that running is bad for you,” adding that “running is good for you and, most of the time, good for your gut.” She also said, “We’ve all been there,” a reminder that many athletes normalize stomach or bowel issues instead of getting them checked.

Oxentenko offered one clear warning: “Don’t let anyone tell you that rectal bleeding is normal for runners. Get it checked.” For now, the study raises a question that is too specific to ignore and too small to answer on its own: whether a subset of extreme endurance athletes may face a risk that ordinary running never suggests.