Marathons and ultramarathons may be linked to colon cancer. Here’s why.

For oncologist Timothy Cannon, alarm bells first sounded in 2019, when, within six months, he treated three fit, young patients with advanced colon cancer. Two were still in their 30s, one had just turned 40. None had any known risk factors.

But all three “were extremely prolific runners,” said Cannon, who co-directs the gastrointestinal cancer program at Inova Schar Cancer in Fairfax, Va. The runners regularly competed in 100-mile ultramarathons or multiple full and half-marathons every year.

Could there be some link between their extreme running and their colon cancer, Cannon wondered?

It seemed unlikely. In past studies, moderate exercise repeatedly had been shown to protect against colorectal cancer.

But the idea merited exploration, he decided.

So he and his colleagues recruited 94 extreme runners, ages 35 to 50. Each had completed at least five marathons or two ultras (meaning any race longer than a standard 26.2-mile marathon).

The runners underwent a colonoscopy.

The outcomes “were rather startling,” Cannon said. Nearly half of the runners had polyps, also known as adenomas, in their colons. Some of these polyps could develop into cancer, Cannon said, although many won’t. But 15% of the runners had large, advanced adenomas, “which are much farther along on the continuum to cancer,” he said. None had colon cancer.

The number of adenomas, especially advanced polyps, was much higher than what typically has been seen during screening colonoscopies of adults in their late 40s. Historically, the incidence in that age group has ranged from about 1.2 to 6%.

“There does seem to be something potentially going on here” with the extreme runners, Cannon said.

News of the study’s results, published online this month in Cancer Epidemiology and presented last summer at an oncology conference, have been sparking urgent questions, concerns, and skepticism among gastroenterologists and runners.

“It’s a super interesting study,” said Amy S. Oxentenko, a gastroenterologist, vice dean of practice at Mayo Clinic in Rochester, Minnesota, marathon runner, and former president of the American College of Gastroenterology. She wasn’t involved with the new study.

“But let’s be real,” she continued. “It’s also a very small study, and the findings are quite preliminary, at best. Runners shouldn’t panic. There’s still so much more to learn.”

In general, running and other forms of exercise are known to reduce the risk of developing colon and other cancers. In a large-scale 2016 study published in JAMA Internal Medicine, researchers pooled health data from nearly 1.5 million U.S. and European men and women and found that those who regularly exercised were as much as 20% less likely to develop colorectal or 12 other major cancers as sedentary people.

Exercise also guards against colon cancer return, according to a groundbreaking 2025 study. Colon cancer survivors in the study who completed a three-year, structured routine of brisk walking and similar workouts were 37% less likely to die from a subsequent recurrence than survivors who didn’t exercise.

But running ultras or multiple yearly marathons represents a different magnitude of exercise, Cannon said. The training and racing involve hours of exertion, much of it at a draining pace. (Full disclosure: I’ve finished two marathons, though years ago. I still run four or five miles most days and recently signed up for a fall half marathon.)

During such prolonged exertion, “the body starts to shunt blood away from the gut,” Cannon said, directing it instead to working muscles elsewhere, especially in the legs. Starved of blood and oxygen, cells in the gut can die, while the intestinal lining may become leaky.

Inflammation and intestinal irritation usually follow. Many marathoners and ultrarunners report nausea, vomiting, cramping, diarrhea, or rectal bleeding during and after lengthy training sessions and races, Cannon said. Then, during later tissue repair, cells in the gut may start replicating furiously and fast, increasing the chances of mutations and, down the road, possible polyps and cancer.

Together, these effects, repeated over and over during years of training, provide “a pretty clear, potential mechanism” for how extreme running theoretically could contribute to heightened risks for polyps and colon cancer, Cannon said.

Eric McGlinchey, 52, would agree. The associate professor of politics and government at George Mason University in Virginia has run, by his latest count, 57 ultras, along with various marathons and other races.

He was in his late 40s when he heard about the study through his local running group and was struck by its focus on the gut. “We’ve all been there,” he said delicately of intestinal upset related to prolonged running.

He volunteered to join the study and had a colonoscopy.

The researchers found multiple polyps in his colon, although they weren’t yet advanced.

In recent years, the incidence of colon cancer has been rising precipitously among people younger than 50, runners or not. It’s now the leading cause of cancer-related deaths in this age group, according to a January study in JAMA, a substantial leap from its fifth-place ranking a generation ago.

So one of the big, unanswered questions about the new study is whether Cannon and his colleagues somehow randomly recruited a group of adults under 50 with a worrying abundance of polyps who also happen to be extreme runners. Or was it really their extreme running that contributed to the disproportionate numbers 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.”

In particular, he said, the message of the findings, preliminary as they are, is that if you are a hard core runner, completing multiple marathons or ultras, and you have lingering intestinal symptoms, such as bloating, cramping or, especially, rectal bleeding, see a doctor. Request a colonoscopy. “Don’t let anyone tell you that rectal bleeding is normal for runners,” Cannon said. “Get it checked.”

Other experts both concur and remain cautious. “I 100% agree” with the advice to take symptoms like bleeding seriously, said Sara Campbell, a professor of kinesiology at Rutgers University in New Jersey who studies exercise and intestinal health and is a triathlete.

“But the worst outcome” from this study, she continued, “would be for people to start thinking that running is bad for you.” The overwhelming majority of research about exercise, including running, suggests “the opposite,” she said. “Running is good for you and, most of the time, good for your gut,” speeding up the movement of waste and often helping to alter gut microbiomes in beneficial ways.

“I certainly wouldn’t tell runners to ignore this study,” Oxentenko said. “I’m a runner. I’m paying attention. But at this point, it’s only hypothesis-generating,” meaning its results raise questions worth pursuing, she said, but don’t provide answers.

Among the many open issues are the possible effects of mileage, pace, gender, genetics, microbiomes, and diet on the gut health of extreme runners, Campbell said. The study was too small and its runners’ histories too diverse to produce useful data about those concerns. Some swift 100-mile racers had no polyps; other slower marathoners had several.

Cannon said he and his colleagues are in the early stages of planning a much larger follow-up study. They aim to enroll close to 300 extreme runners and a like-sized control group and look at other factors, such as people’s genes, genders, nutrition, and microbiomes, to see how they interact with running to affect gut health.

The researchers would also like to study triathletes and competitive cyclists, to tease out whether it’s extreme running, per se, or extreme exertion of any kind that may predispose people to polyps. And, of course, they want to follow study participants, to see whether any of them progress to colon cancer.

For now, though, everyone I spoke with about the study intends to keep running.

“I’m up-to-date on my colonoscopies,” Oxentenko said. She’ll continue to train and race.

As will Campbell and McGlinchey, who met his wife through running and proposed to her after they ran the Big Sur International Marathon together.

“I think this is a very important study, and runners should be aware of it,” he said. “But it doesn’t mean we should stop running.” Recently, he completed the famously tough Massanutten Mountain Trails 100 Mile Run in Virginia.

Even Cannon plans to start training for marathons. Almost 20 years ago, he completed the New York City Marathon during his medical training. He’d like to do another, he said. “I don’t want people to be scared” about running, he said. “There’s such good data about exercise’s protective effect on cancer.” But pay attention to your body, he said, and talk to your doctor if your gut keeps acting up.