Do not stop running. Do pay attention.
A small but striking study out of Virginia has introduced a quiet question into the world of endurance running: whether the body's extraordinary adaptation to extreme distances might carry unintended consequences in the colon. An oncologist who noticed an unusual cluster of young, fit patients with advanced colorectal cancer—all serious distance runners—went on to find that nearly half of 94 extreme runners carried colon polyps, with 15 percent showing advanced lesions. The science remains preliminary and no causal link has been established, yet the finding invites a deeper reflection on the line between the body's resilience and its limits.
- A Virginia oncologist's unsettling pattern—three young, fit ultramarathoners with advanced colon cancer arriving within months of each other—set off a quiet but serious investigation.
- Colonoscopies of 94 extreme runners revealed nearly half had adenomas and 15% showed advanced lesions, results that surprised even the researchers conducting the study.
- The leading hypothesis points to a physiological trade-off: during prolonged intense running, blood is diverted from the intestines to working muscles, potentially causing repeated inflammation that may favor polyp formation over years.
- The study is small, has no control group, and proves no causal link—experts are urging runners not to panic, but not to ignore persistent symptoms like rectal bleeding or chronic abdominal discomfort either.
- A larger follow-up involving roughly 300 extreme runners and a matched comparison group is underway, aiming to determine whether the risk is tied to extreme racing, general endurance, genetics, diet, or some combination of factors.
In 2019, Timothy Cannon, an oncologist at Inova Schar Cancer Institute in Virginia, noticed something he could not easily set aside. Within a few months, three young patients in excellent physical shape—none with obvious risk factors—arrived at his clinic with advanced colon cancer. The common thread: all were serious distance runners who treated ultramarathons as routine training.
That pattern prompted a formal study. Cannon and his team recruited 94 extreme runners between 35 and 50 years old, each having completed at least five marathons or two ultramarathons, and gave every one of them a colonoscopy. The results were striking: nearly half had colon polyps, and 15 percent showed advanced adenomas—lesions that sit meaningfully closer to the path toward cancer. None had cancer itself.
The mechanism under investigation is rooted in basic physiology. During hours of intense running, blood is redirected from the intestines to the working muscles. Deprived of circulation and oxygen, the gut becomes irritated and inflamed—a reality most endurance runners know well through nausea, cramping, and rectal bleeding during long efforts. Whether this repeated stress, accumulated over years, creates conditions that favor polyp formation remains the central unanswered question.
Cannon is careful about the study's limits. It is small, lacks a control group, and establishes no causal link. It does not contradict the broader consensus that regular exercise lowers colorectal cancer risk. A larger follow-up involving around 300 extreme runners and a matched comparison group is already underway, designed to untangle whether the signal comes from extreme racing, prolonged endurance effort in general, or variables like genetics and diet.
The message from specialists is deliberate: do not stop running, but do not dismiss symptoms. Rectal bleeding, persistent abdominal pain, or chronic digestive discomfort deserve medical attention—not as causes for panic, but as signals that should not be written off as simply the price of the sport.
A Virginia oncologist's chance observation in 2019 has quietly opened a question that endurance runners are only beginning to grapple with: whether the extreme distances they chase—ultramarathons stretching past 100 miles, multiple marathons stacked into a single year—might be leaving marks on their colons that warrant attention.
Timothy Cannon, who directs the gastrointestinal cancer program at Inova Schar Cancer Institute, noticed something unsettling within a few months. Three young patients came through his clinic, all in excellent physical condition, none with obvious risk factors for colorectal cancer. Yet all three had advanced colon cancer. The thread connecting them: they were serious distance runners, the kind who log ultramarathon miles as routine training.
That pattern stuck with him. Cannon and his team recruited 94 extreme runners between 35 and 50 years old—each had completed at least five marathons or two ultramarathons. Every participant underwent a colonoscopy. The findings startled the researchers. Nearly half carried polyps, technically called adenomas, in their colons. More striking still: 15 percent showed advanced adenomas, the kind of lesion that sits closer to the path toward cancer. None of the runners had cancer itself.
The mechanism researchers are exploring is straightforward in theory. During hours of intense running, the body redirects blood flow to the muscles doing the work—primarily the legs. The intestines, starved of that circulation and oxygen, become irritated and inflamed. Endurance runners know this intimately: nausea, cramping, diarrhea, and rectal bleeding are almost routine complaints during and after long efforts. What remains unclear is whether this repeated stress, accumulated over years of training, creates conditions that favor polyp formation.
But Cannon is careful about what his study does and does not show. The research proves neither that distance running causes cancer nor that it contradicts the broader scientific consensus that regular exercise reduces colorectal cancer risk overall. The study is small, lacks a control group, and remains preliminary. Larger, more rigorous work is already underway—a follow-up involving roughly 300 extreme runners and a matched comparison group, designed to tease apart whether the signal comes from extreme racing itself, from prolonged endurance effort in general, or from other variables like genetics, diet, sex, gut microbiome, and training volume.
The message from specialists is measured: do not stop running. Do pay attention. If rectal bleeding appears, if abdominal pain persists, if bloating or intestinal discomfort becomes a pattern rather than an isolated incident, see a doctor. The concern is not that every marathoner should panic. It is that symptoms worth investigating should not be dismissed as the normal cost of the sport.
Notable Quotes
The study proves neither that distance running causes cancer nor that it contradicts the broader scientific consensus that regular exercise reduces colorectal cancer risk overall.— Research findings and expert consensus
The Hearth Conversation Another angle on the story
What made Cannon connect these three cancer cases to running in the first place?
He saw a pattern no one else had flagged. Three young, fit people with no family history or obvious risk factors—all serious ultramarathon runners. That's rare enough to stop and ask why.
But the study found polyps, not cancer. How big a leap is that?
It's not a leap to cancer. Polyps are precursors—some progress, many don't. The question is whether extreme runners are developing them at a higher rate than the general population, and we don't know that yet because there was no control group.
So why publish preliminary findings that might scare people away from running?
Because runners deserve to know what researchers are seeing. The alternative—staying quiet until perfect data exists—means people train in the dark. The study says: this is worth watching, not worth panicking about.
The blood-flow hypothesis makes sense, but couldn't that happen to any endurance athlete?
Exactly. That's what the next study will test. Is it specific to running? Does cycling cause the same thing? Does it matter how much you train? Those answers matter for how we interpret what Cannon found.
What should a runner actually do with this information?
Listen to your body. If you're bleeding or cramping persistently, don't write it off as normal. Get it checked. Keep running if you want to. Just don't ignore signals.