Don't let anyone tell you that rectal bleeding is normal for runners.
A pattern noticed in a cancer clinic — three young, elite runners diagnosed with advanced colon cancer in a single season — has grown into a small but striking study suggesting that the very discipline celebrated for its health benefits may, at its most extreme, carry hidden risks for the gut. Researchers in Virginia found that nearly half of ninety-four extreme runners between thirty-five and fifty harbored colon polyps, with advanced adenomas appearing at rates far exceeding historical norms for that age group. The findings do not indict running itself, which decades of research have shown to be broadly protective against colorectal cancer, but they invite a more nuanced question: whether the body, pushed relentlessly to its limits year after year, may pay a price in ways we are only beginning to see.
- A oncologist's unsettling observation — three young, fit runners with advanced colon cancer in six months — refused to be dismissed and became the seed of a formal investigation.
- The numbers that emerged from colonoscopies were jarring: fifteen percent of elite runners in their prime showed advanced adenomas, a rate up to twelve times higher than historical screening data for the same age group.
- The proposed mechanism is visceral — hours of intense running starve the intestines of blood, killing cells, inflaming tissue, and triggering rapid repair cycles that multiply the chances of a cancerous mutation.
- Experts urge calm rather than alarm, stressing the study was small, lacked a control group, and cannot yet distinguish cause from coincidence.
- The path forward is watchful: extreme runners are advised to take persistent gut symptoms seriously, seek colonoscopies when warranted, and await a larger follow-up study designed to untangle genetics, diet, and microbiome from the miles themselves.
In 2019, oncologist Timothy Cannon treated three young, fit patients with advanced colon cancer within six months. Two were in their thirties, one had just turned forty — none carried the usual risk factors. What they shared was a devotion to extreme running: ultramarathons, back-to-back marathons, years of relentless mileage. The coincidence was troubling enough that Cannon decided to look closer.
He and his colleagues recruited ninety-four extreme runners between thirty-five and fifty, each having completed at least five marathons or two ultramarathons, and sent them for colonoscopies. Nearly half had adenomas. Fifteen percent had advanced adenomas — compared to historical rates of just 1.2 to 6 percent in that age group. No cancer yet, but the signal was hard to ignore. The study, published this month in Cancer Epidemiology, drew both serious attention and measured skepticism from the medical community.
The hypothesis Cannon offers is grounded in physiology. During prolonged intense running, blood is diverted away from the intestines to fuel working muscles. Starved of oxygen, intestinal cells die. The lining becomes permeable, inflammation sets in, and when tissue repairs itself, cells replicate rapidly — each replication a small opportunity for mutation. Repeat this over years of extreme training, and a plausible mechanism begins to take shape. Most ultramarathoners are already familiar with the symptoms: nausea, cramping, diarrhea, rectal bleeding during long efforts.
Eric McGlinchey, a fifty-two-year-old professor who has completed fifty-seven ultramarathons, heard about the study through his running group and volunteered for a colonoscopy. Researchers found multiple polyps, though not yet advanced. He took the news in stride — literally. Days after the study made headlines, he finished the hundred-mile Massanutten Mountain Trails race in Virginia.
The study's central limitation is the absence of a control group, leaving open the question of whether extreme running caused the elevated polyp rates or whether the researchers simply found a group of younger adults who happened to have unusual polyp burdens and also happened to run. Cannon acknowledges the uncertainty but believes the signal is real. Other experts, including gastroenterologist Amy Oxentenko of the Mayo Clinic — herself a marathon runner — called the findings genuinely interesting while urging against panic.
The practical advice is clear: extreme runners experiencing persistent intestinal symptoms, especially rectal bleeding, should see a doctor and ask about a colonoscopy. A larger follow-up study enrolling three hundred runners and a matched control group is already being planned, with attention to genetics, diet, microbiome, and whether the risk extends to triathletes and cyclists. For now, everyone involved — researchers, subjects, and skeptics alike — intends to keep running. "I don't want people to be afraid of running," Cannon said. The evidence for exercise as a protector against cancer remains strong. But the gut, it seems, deserves a closer listen.
In 2019, oncologist Timothy Cannon noticed something that didn't fit the usual pattern. Within six months, he treated three young, fit patients with advanced colon cancer. Two were in their thirties, one had just turned forty. None had the typical risk factors doctors watch for. But they shared something unexpected: all three were extreme runners, the kind who log hundred-mile ultramarathons or string together multiple marathons year after year.
Cannon, who directs the gastrointestinal cancer program at Inova Schar Cancer in Fairfax, Virginia, couldn't shake the question: could there be a connection between their relentless running and their cancer? It seemed unlikely. Previous research had consistently shown that moderate exercise protects against colorectal cancer. But the pattern was unusual enough to warrant investigation.
He and his colleagues recruited ninety-four extreme runners between thirty-five and fifty years old, each of whom had completed at least five marathons or two ultramarathons. When these runners underwent colonoscopies, the results startled everyone. Nearly half had polyps—adenomas, in medical terms—in their colons. More striking: fifteen percent had advanced adenomas, the kind that sit further along the path toward cancer. For comparison, historical screening data in that age group showed advanced polyp rates between 1.2 and 6 percent. None of the runners had developed colon cancer yet, but the numbers suggested something was happening.
When the study appeared online this month in Cancer Epidemiology, it triggered urgent questions, genuine concern, and considerable skepticism. Amy Oxentenko, a gastroenterologist at the Mayo Clinic in Rochester, Minnesota, and herself a marathon runner, called it "a super interesting study." But she added the necessary caution: it was small, the findings were preliminary at best, and runners shouldn't panic. "There's still a lot to learn," she said.
Cannon's hypothesis rests on the physiology of extreme endurance. During hours of intense running, the body redirects blood away from the intestines toward the working muscles, particularly in the legs. Starved of oxygen, intestinal cells begin to die. The intestinal lining becomes permeable. Inflammation and irritation follow—something most ultramarathoners know intimately. Many report nausea, vomiting, cramping, diarrhea, or rectal bleeding during and after long training sessions and races. When the tissue repairs itself, intestinal cells replicate rapidly, and rapid replication creates opportunities for mutations. Repeat this cycle year after year, and you have what Cannon describes as "a potentially quite clear mechanism" for how extreme running might contribute to higher polyp and cancer risk.
Eric McGlinchey, a fifty-two-year-old associate professor of policy and government at George Mason University in Virginia, has run fifty-seven ultramarathons, plus numerous marathons and other races. When he heard about the study through his local running group, he volunteered for a colonoscopy. Researchers found multiple polyps in his colon, though not yet advanced ones. "We've all been through that," he said gently about the intestinal distress that comes with prolonged running.
One major unanswered question hangs over the research: did Cannon and his colleagues happen to recruit a random group of younger adults who happened to have an unusual abundance of polyps and also happen to be extreme runners? Or did the extreme running itself cause the elevated polyp rates? "We don't know at this point," Cannon acknowledged, because the study lacked a control group of non-runners. "But I do think there's a signal here," he added, "and it's worth paying attention to."
Cannon's immediate advice is straightforward: if you're a serious runner completing multiple marathons or ultramarathons and you experience persistent intestinal symptoms—bloating, cramping, or especially rectal bleeding—see a doctor and ask for a colonoscopy. "Don't let anyone tell you that rectal bleeding is normal for runners. Get it checked," he said. Other experts agree with this caution while resisting alarm. Sara Campbell, a kinesiologist at Rutgers University who studies exercise and intestinal health and is herself a triathlete, emphasized that the worst outcome would be if people concluded that running is bad for you. The overwhelming body of research suggests the opposite: running is good for you and usually good for your gut, accelerating waste movement and often improving intestinal microbiomes in beneficial ways.
Cannon and his team are already planning a much larger follow-up study, aiming to enroll about three hundred extreme runners and a similar control group. They want to examine genetics, gender, nutrition, and microbiomes to understand how these factors interact with running to affect intestinal health. They're also interested in studying triathletes and competitive cyclists to determine whether it's extreme running specifically or extreme effort of any kind that might predispose people to polyps. And they plan to follow participants over time to see if any progress to colon cancer.
For now, everyone involved in the study intends to keep running. Oxentenko stays current with her colonoscopies and continues training and competing. McGlinchey, who met his wife through running and proposed to her after they ran Big Sur together, completed the grueling hundred-mile Massanutten Mountain Trails race in Virginia just days after the study made news. "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." Even Cannon plans to train for marathons again. He completed the New York City Marathon two decades ago during his medical training and wants to do another. "I don't want people to be afraid of running," he said. "There's very good data on the protective effect of exercise against cancer." But pay attention to your body, he concluded, and talk to your doctor if your gut keeps misbehaving.
Citas Notables
It's a super interesting study, but it's also very small and the findings are preliminary at best. Runners shouldn't panic.— Amy Oxentenko, gastroenterologist at Mayo Clinic
There's a signal here related to the effects of extreme running, and it's worth paying attention to.— Timothy Cannon, oncologist
La Conversación del Hearth Otra perspectiva de la historia
What made Cannon notice this pattern in the first place? Three young patients in six months seems like it could be coincidence.
It would be, except they were all extreme runners with no other risk factors. That's what stopped him. He'd never seen that clustering before, and it bothered him enough to investigate rather than dismiss it.
But moderate exercise protects against colon cancer. Everyone knows that. So why would extreme exercise be different?
Because extreme is a different animal entirely. We're talking about hours of intense effort, year after year. The body diverts blood from the intestines to feed the working muscles. The intestinal lining gets damaged, inflamed. Then it repairs itself with rapid cell division. Do that repeatedly, and you create conditions where mutations become more likely.
The study found polyps in nearly half the runners. That's a lot. Why are experts still cautious?
Because the study is small—ninety-four runners—and it has no control group. You don't know if those polyp rates are actually unusual or if you just happened to recruit a group that would have had polyps anyway. You need a bigger study with non-runners to compare against.
So should runners be worried?
Not panicked. But aware. If you're running ultramarathons and you're bleeding or having persistent gut problems, that's worth getting checked. The research isn't saying running is bad. It's saying extreme running might need closer monitoring.
What happens next?
Cannon's planning a much larger study with three hundred runners and a control group. They want to look at genetics, diet, microbiomes—all the variables that might explain why some runners develop polyps and others don't. That will take time, but it's the right next step.