A possible link has been found between colon cancer and marathoners – here’s what to know

A small study has raised concerns after finding unusually high rates of precancerous colon polyps in young extreme endurance runners.

Oncologist Timothy Cannon told The Washington Post he first observed a potential link in 2019 after treating three unusually young patients with advanced colon cancer within six months. Two were in their 30s and one was in their 40s. All were “extremely prolific runners” regularly competing in ultramarathons or multiple marathons despite lacking known risk factors, Cannon said.

Cannon, who co-directs the gastrointestinal cancer program at Inova Schar Cancer Institute, said the pattern prompted a study of 94 runners aged 35 to 50 who had completed at least five marathons or two ultramarathons. Colonoscopies found nearly half had colonic polyps, and 15 percent had advanced adenomas more closely associated with cancer development.

The study’s biggest question is whether the researchers accidentally sampled unusually high-risk people who also happen to be extreme runners, or whether intense long-distance running itself may be linked to the higher rate of polyps.

“We don’t know at this point,” Cannon said, because the study had no non-runner control group. “But I do think there’s a signal here” related to the effects of extreme running, he added, “and it’s worth paying attention to.”

Experts cautioned against drawing conclusions. Amy S. Oxentenko called it “a super interesting study,” but said it is “quite preliminary” and warned, “Runners shouldn’t panic.”

Researchers are also exploring possible biological explanations. Cannon said prolonged endurance exercise may “shunt blood away from the gut,” and repeated stress, inflammation, and irritation could affect the intestinal lining. He linked this to common symptoms in endurance athletes such as nausea, cramping, and rectal bleeding, suggesting that repeated injury and repair could, in theory, increase mutation risk, though he called it an unproven mechanism.

Eric McGlinchey, a study participant, said digestive distress is common in the sport, said, “We’ve all been there,” and reported having multiple non-advanced polyps himself.

Sara Campbell of Rutgers University urged caution, saying while symptoms like bleeding should be evaluated, “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.”

Oxentenko also described the work as “only hypothesis-generating,” and Cannon said the small sample limits conclusions, though he still urged athletes to seek care for persistent symptoms.

“Don’t let anyone tell you that rectal bleeding is normal for runners. Get it checked.” Oxentenko said.

Researchers are now planning a bigger follow-up study with about 300 runners and a non-running control group, looking at things like genetics, diet, gut microbiome, and other endurance sports to figure out whether the effect is specific to running or just intense long-term exercise in general.