Does Running Cause Colon Cancer? A Guide to Not Losing Your Mind on the Internet

If you've been anywhere near running social media in the last few months, you've probably seen the headlines. "Marathon Runners Face Unexpected Colon Cancer Risk." "Could Extreme Running Fuel Colon Cancer?" And my personal favorite, the TikTok accounts confidently informing you that ultramarathons "cause" colon cancer. So, does running cause colon cancer? That is not what anyone found. But sure, let's go ahead and terrify people who move their bodies for fun.

A listener recently wrote in asking about this, and I've gotten enough DMs and texts about it that I figured it was worth a longer breakdown. What's happening here is a pattern we see constantly on the podcast: genuinely interesting preliminary science gets processed through the content machine and comes out the other side as fear. This one is landing particularly hard in our community, because it's targeting something that isn't just a health behavior for most of us. It's identity.

So let's talk about what the research actually says, what it doesn't say, and why your feed is making it sound worse than it is.

What the marathon runners colon cancer study actually found

In the summer of 2025, an oncologist named Dr. Timothy Cannon presented findings at the American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago. Here's the backstory: over the course of about a year, Dr. Cannon had seen multiple ultramarathoners under 40 show up at his cancer center with advanced-stage colorectal cancer. These were otherwise healthy people with no genetic predispositions or inflammatory bowel disease. That's an unusual cluster, and it caught his attention.

So he designed a study. He recruited 100 long-distance runners between the ages of 35 and 50 (people who had completed at least five marathons or two ultramarathons) and gave them all colonoscopies. What he found was striking: 15% had advanced adenomas, which are precancerous growths that have a meaningful chance of eventually becoming cancerous. In the general population of adults that age, you'd expect to see that in roughly 4.5 to 6% of people. About 41% of the runners had at least one polyp of any kind (Cannon et al., 2025).

That is a notable signal. It's the kind of finding that makes a good scientist say, "Huh, we should look into this further." Which is exactly what Dr. Cannon said. His own characterization of the results was "hypothesis-generating more than proving anything."

But that's not how it played on your phone.

Is the ultramarathon colon cancer study peer-reviewed?

I don't want to totally discount this. The signal Dr. Cannon found is real, it's notable, and it deserves follow-up. But how we talk about it, and how much weight we give it, matters too. So let's get specific about what kind of evidence we're actually looking at.

Here's where language does more work than most people realize. When you see something described as "a study" in a headline or a social media post, your brain probably files it somewhere in the neighborhood of "settled science": peer-reviewed, replicated, authoritative. That's not what this is.

What Dr. Cannon presented at ASCO is a conference abstract. That means it's a summary of preliminary findings that was accepted for presentation at a major oncology conference. ASCO does have a selection process, so it's not nothing, but it is categorically different from a peer-reviewed publication. The full dataset, the complete statistical analysis, the detailed methodology, the limitations discussion: none of that has been independently scrutinized by other scientists yet. No one has formally come back and said, "Wait, why didn't you have a control group?" in a way that required a published response.

And speaking of that control group: there wasn't one. The 15% advanced adenoma rate in runners is being compared against historical baseline rates from entirely different studies, conducted on different populations, at different times. That's a significant methodological limitation. It doesn't make the finding meaningless, but it means we genuinely do not know whether a comparable group of non-runners from the same region, demographic, and time period would have shown similar rates. The sample was also small (100 people, mostly from the Washington, D.C. area), and potential confounders like diet, alcohol use, stress, environmental exposures, and family history weren't fully controlled for.

None of this means the research is bad or that we should wave it away. It means the research is early. There's a massive difference between "here's a signal worth investigating" and "running gives you cancer," and that difference evaporates somewhere between the press release and your For You page.

Does running cause colon cancer, or does it hide it?

Here's where it gets genuinely interesting, and where the online conversation has been least sophisticated.

Most of the social media discourse has treated this as a straightforward causal claim: extreme running damages the gut, damaged gut produces polyps, polyps become cancer. Running → cancer. Neat, scary, shareable. But the researchers themselves were much more careful than that, and the most important alternative explanation is one that should actually make runners more attentive, not less.

Dr. David Rubin, chief of gastroenterology at the University of Chicago, made an observation that got almost no play in the viral coverage: "It's possible exercising didn't cause the problem but was in fact the reason they became long-distance runners; because someone dear to them had cancer." That's a reverse-causality question, one of many that a controlled study would help answer and this one can't.

But there's an even more important possibility to sit with: running might not cause colorectal cancer, but it might mask the early signs of it.

Think about it. Runners, especially endurance and ultra runners, experience GI distress constantly. Cramping, loose stools, urgency, and yes, blood in the stool are so common in our community that they have their own casual nicknames: runner's trots, runner's colitis. These symptoms are normalized, joked about, bonded over. If you've ever stood in a porta-potty line at a race, you know the drill. And doctors are in on it too. According to Dr. Cannon, several of the cancer patients who inspired his research had reported GI bleeding that their physicians dismissed as a normal byproduct of endurance training.

Now here's the thing: rectal bleeding, changes in bowel habits, cramping, and unexplained GI distress are also the most common early warning signs of colorectal cancer. They're the exact symptoms that, in a non-runner, would prompt a referral for a colonoscopy. In a runner, they get a shrug.

Why is early-onset colorectal cancer rising?

So you have two things happening at once.

First, there's a broader generational trend: early-onset colorectal cancer is rising in everyone under 50, across all demographics and fitness levels. The incidence has been increasing by about 2% per year, and people under 50 now account for roughly 12% of new colorectal cancer cases. Colorectal cancer recently became the leading cause of cancer death in Americans under 50. Let that sink in. Researchers believe this is a birth cohort effect: people born from the 1950s onward have been exposed to a constellation of factors (ultra-processed food, microbiome disruption from childhood antibiotic use, environmental exposures like microplastics, sedentary behavior in adolescence) that appear to be accelerating colorectal cancer development across the board. The honest answer from the scientific community is that we don't fully know yet. It's almost certainly multifactorial, and it is emphatically not a running problem (Sung et al., 2025).

Second, you have a population of endurance runners embedded in a culture that treats early warning signs of cancer as the normal background noise of the sport.

Combine those two things and you don't need a causal link between running and colon cancer to explain what Dr. Cannon found. You just need more young people developing precancerous lesions (which is happening everywhere) plus a community that's uniquely primed to ignore the symptoms that would catch them early. The running might not be creating the problem. It might be hiding it.

That's a less dramatic story than "your ultra is giving you cancer." But it's a more honest one, and frankly, it's a more useful one, because it points toward something we can actually change.

What the research says about exercise and colon cancer risk

While one small, preliminary conference abstract has dominated your feed, here's what decades of research across hundreds of thousands of participants has consistently shown: physical activity is one of the most well-supported protective factors against colon cancer that exists.

A meta-analysis of 52 studies found that physical activity was associated with a 24% overall reduction in colon cancer risk (Wolin et al., 2009). The World Cancer Research Fund's Continuous Update Project, analyzing 12 prospective studies, found a 20% decreased risk of colon cancer when comparing the most and least active people. These findings hold across sexes, study designs, and types of physical activity.

And here's the part that really puts the Cannon abstract in context: at the exact same ASCO 2025 conference where his findings were presented, a massive phase III randomized controlled trial called CHALLENGE, enrolling 889 colon cancer patients and following them for nearly 8 years, showed that a structured exercise program after treatment improved disease-free survival by 28% and overall survival by 37% (Booth et al., 2025). This was published in the NEJM, fully peer-reviewed, with a control group, and it's one of the strongest pieces of evidence we've ever had that exercise is medicine for colon cancer.

One of these studies made TikTok. The other didn't. I'll let you guess which one.

Why the running and colon cancer headlines went viral

Okay, let's talk about the information ecosystem for a second, because the why of this is just as important as the what.

There's a whole genre of health content that runs on the formula: the thing you thought was good for you is actually bad. It's the same engine that drives "is oatmeal actually unhealthy?" and "your morning run is destroying your cortisol." It works because it lets the sharer feel like they've got insider knowledge. They're not just consuming wellness content, they're debunking it. It's contrarian signaling dressed up as health literacy. And running is a particularly juicy target because runners are culturally coded as virtuous and disciplined, maybe a little self-righteous about it. There's a schadenfreude element that makes people who don't run feel validated in sharing it, and people who do run feel compelled to engage with it.

Then there's the identity layer. For endurance athletes, running isn't just exercise. It's how a lot of us manage stress, build community, and structure our lives. When a headline suggests the thing that makes you you might also be harming you, it doesn't register as a neutral piece of health information. It feels existential. And when something feels existential, people don't scroll past it. They screenshot it, send it to their running group chat, tag their training partners. That anxiety-driven sharing is what keeps the cycle going.

And the ambiguity of the study itself actually helps it spread. A massive, peer-reviewed, well-controlled study with a boring, nuanced conclusion doesn't travel. A small, dramatic, not-yet-peer-reviewed finding with a scary signal and no definitive answer creates an information vacuum that every running coach, GI doc on Instagram, and wellness influencer rushes to fill. Everyone gets to have a take. The algorithm doesn't distinguish between "conference abstract with no control group" and "published, replicated research." It just sees engagement.

Blood in stool after running: when to see a doctor

If you've read this far, you already know this isn't a "stop running" situation. But there are a few things worth actually doing.

First: stop treating GI symptoms as a badge of endurance honor. I know I've been hammering this, but it bears repeating because it's the single most actionable thing in this entire conversation. Blood in your stool is not a normal cost of doing business. If it's persistent or recurring, after long runs or otherwise, talk to a doctor, and don't let them wave it off as runner's colitis without a real evaluation.

Second: know the screening timeline. Current guidelines recommend colorectal cancer screening starting at age 45 for average-risk adults, or earlier if you have a family history. That recommendation was updated in 2021 precisely because of the rise in early-onset CRC, and recent data shows it's working: more cancers in younger adults are being caught at earlier, more treatable stages. If you're approaching that age, or if you have risk factors, get screened. Colonoscopies aren't fun, but they're one of the most effective cancer prevention tools available. They can catch and remove precancerous polyps before they ever become cancer.

Third: keep the broader context in your back pocket. Early-onset colorectal cancer is rising in everyone, and the known modifiable risk factors are things like ultra-processed food consumption, heavy alcohol use, sedentary behavior, and obesity. Regular physical activity is consistently one of the strongest protective factors. If you're an active person who eats reasonably well, which describes most of the people reading this, you're already doing a lot of the things that reduce risk. Don't let a scary TikTok undo that.

Bottom line: does running cause colon cancer?

No. Here's what I want you to walk away with:

Exercise remains one of the most well-supported, evidence-based things you can do to reduce your risk of colon cancer and improve outcomes if you're ever diagnosed. Decades of research, hundreds of thousands of participants, multiple meta-analyses. This is not ambiguous.

One small, preliminary conference abstract found a higher-than-expected rate of precancerous polyps in a specific group of extreme endurance runners. That's worth investigating further. It is not worth rearranging your life over, and it is definitely not worth the panic you're seeing online.

Early-onset colorectal cancer IS rising, and it's rising in everyone, not just runners. The causes are complex, likely generational, and still being figured out. That's the story that actually deserves your attention and your advocacy.

The most important thing you can do is stop normalizing GI symptoms as just part of being a runner. Pay attention to your body. Talk to your doctor if something doesn't feel right. Get screened when it's time. And maybe, just maybe, be a little skeptical the next time a TikTok tells you that the thing keeping you healthy is secretly killing you. That's kind of the wellness industry's whole move, and you don't have to fall for it.

Your colon is going to be fine. Go run.

[If you and Kylee have covered this on an episode, drop a by-name link here. If not: "We're digging into this one on the podcast. Subscribe so you don't miss it."]

Have a question you want us to dig into? The weekly Nutrition Question Thread on Patreon is where Kylee actually answers these, and the discussion threads are where a lot of them start. And if this post was useful, send it to your running group chat. They've probably been doom-scrolling about this too.

Sources

Booth, C. M., et al. (2025). A randomized phase III trial of the impact of a structured exercise program on disease-free survival in stage 3 or high-risk stage 2 colon cancer: CCTG CO.21 (CHALLENGE). New England Journal of Medicine.

Cannon, T. L., et al. (2025). Risk of pre-cancerous advanced adenomas of the colon in long distance runners. Journal of Clinical Oncology, 43(16_suppl), 3619. https://doi.org/10.1200/JCO.2025.43.16_suppl.3619

Sung, H., et al. (2025). Colorectal cancer incidence trends in younger versus older adults: An analysis of population-based cancer registry data. The Lancet Oncology, 26(1), 51. https://doi.org/10.1016/S1470-2045(24)00600-4

Wolin, K. Y., et al. (2009). Physical activity and colon cancer prevention: A meta-analysis. British Journal of Cancer, 100(4), 611–616. https://doi.org/10.1038/sj.bjc.6604917

World Cancer Research Fund/American Institute for Cancer Research. (2018). Physical activity and the risk of cancer. Continuous Update Project Expert Report.