If you’ve ever trained for a marathon or ultramarathon and found yourself sprinting for the porta-potty afterward, you’re not alone. GI drama is a well-known companion of long-distance events. And now, a new study is raising eyebrows (and some colonoscopes) about whether extreme mileage might actually increase the risk of precancerous colon polyps.
At the 2025 ASCO Annual Meeting, oncologist Dr. Tim Cannon and colleagues at Inova Schar Cancer presented a small but striking study: among 100 elite runners - each with at least five marathons or two ultramarathons under their belts - 15% had advanced adenomas (precancerous growths) and 41% had at least one adenoma. That’s far higher than the ~1-2% rate typically seen in healthy adults of similar age.
The researchers suspect a link between exercise-induced gut stress - aka runner’s colitis - and these findings. Repeated blood shunting away from the intestines during endurance events may cause ischemia (temporary oxygen loss) and micro-injury to the colon. Over years of high-volume training, those tiny hits might add up.
This isn’t a new idea. The Science behind the Runner’s Runs is well documented.
Even in healthy young adults, intense exercise in extreme heat (even stop-and-go sprints over just 3-4 miles) can trigger ischemic colitis. Studies report exercise-induced gastrointestinal (GI) symptoms in up to 70% of endurance athletes.
So yes, that stomach cramp isn’t just your gel packet revolting; your colon might actually be gasping for air.
Wait, Isn’t Exercise Supposed to Be Good for You? Absolutely. Decades of evidence, including a large randomized trial in NEJM (Courneya et al., 2025), shows exercise improves survival in colon cancer survivors. Moderate, consistent activity is a cornerstone of cancer prevention.
The Inova study doesn’t suggest jogging a few miles a week will hurt you - it’s zeroing in on ultra-endurance lifestyles, where pounding out 40+ miles a week is normal and multiple marathons are a badge of honor. (Because apparently 26.2 miles once wasn’t enough of a flex.)
Exercise remains one of the best things you can do for your health, but like any powerful intervention, more isn’t always better. Science is still catching up on what happens when you treat your body like a mileage sponge. For now, lace up, enjoy your runs, and listen to your gut - literally. After all, nobody brags about a Personal Record in their colonoscopy prep.
REFERENCES and Notes
Timothy Lewis Cannon, Bonomelli S, Swain WR, Kaltman RD, Mani H, Xia M, Randall J, Wang H, Harnden I, Donet JA, Nguyen VA. Risk of pre-cancerous advanced adenomas of the colon in long distance runners.et al. Risk of pre-cancerous advanced adenomas of the colon in long distance runners. JCO 43, 3619-3619(2025). DOI:10.1200/JCO.2025.43.16_suppl.3619
Vujasinovic M, Omae M, Panic N, Fjellgren E, Bloch N, Kikec Z, Grasselli M, Lindberg G, Löhr JM, Baldaque-Silva F. Gastrointestinal bleeding in long-distance runners: a systematic review. Eur J Gastroenterol Hepatol. 2025 Jun 1;37(6):691-701. doi: 10.1097/MEG.0000000000002931. Epub 2025 Apr 29. PMID: 39976001.
Courneya KS, Vardy JL, O'Callaghan CJ, Gill S, Friedenreich CM, Wong RKS, Dhillon HM, Coyle V, Chua NS, Jonker DJ, Beale PJ, Haider K, Tang PA, Bonaventura T, Wong R, Lim HJ, Burge ME, Hubay S, Sanatani M, Campbell KL, Arthuso FZ, Turner J, Meyer RM, Brundage M, O'Brien P, Tu D, Booth CM; CHALLENGE Investigators. Structured Exercise after Adjuvant Chemotherapy for Colon Cancer. N Engl J Med. 2025 Jul 3;393(1):13-25. doi: 10.1056/NEJMoa2502760. Epub 2025 Jun 1. PMID: 40450658.
Cha S, Kwon BS, Hong N, Park JS, Byun SK, Choi SC, Kim YS. Ischemic Colitis Associated with Rhabdomyolysis and Heat Stroke after an Intense Exercise in Young Adult. Korean J Gastroenterol. 2019 Aug 25;74(2):115-118. doi: 10.4166/kjg.2019.74.2.115. PMID: 31438663.
https://www.inovanewsroom.org/press-release/2025/08/groundbreaking-inova-study-finds-potential-link-between-long-distance-running-and-colon-cancer/
Summary of the press-release comment thread:
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Personal stories & thanks: A marathon trainee (stage III colon cancer with BRAF mutation) and others thank Inova for investigating; several endurance athletes are now reconsidering training or seeking screening.
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Scope beyond running: Ultracyclists and triathletes ask whether the signal is from running mechanics (impact/ischemia) or endurance-sport commonalities (high-sugar fueling, fiber restriction, long efforts).
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Methodology concerns: Readers question the cited “expected 1–2%” baseline for advanced adenomas (ask for contemporaneous controls), call out small sample, no control group, and urge comparisons with fit and average peers.
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Timing/context questions: One commenter wonders why this is being detected now (post-COVID era) and suggests caution in public messaging to avoid undue alarm while encouraging screening for ultra-athletes.
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Media amplification: Several aggregator articles echo the press release; one commenter links to potentially conflicting evidence in NEJM (no details discussed in-thread).
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Net sentiment: Interest and cautious concern, with strong support for earlier evaluation of symptomatic runners and for larger, controlled follow-up studies before drawing causal conclusions.
https://www.reddit.com/r/science/comments/1nf8uod/study_finds_potential_link_between_longdistance/
Summary of the r/science comment thread:
Headline numbers (from the press release/study summary): commenters highlight that 15% of the 100 endurance runners (ages ~35–50) had advanced adenomas and 41% had any adenoma; average age 42.5 (younger than routine screening). Many find this “shocking.”
Who counted as “long-distance”: people who’d done ≥5 marathons or ≥2 ultramarathons—i.e., very high-volume runners, not casual 5–10K folks.
Big caveats (most-upvoted critiques): tiny, single-center cohort; no control group; wide CIs (e.g., the 15% could be ~8–23%); likely selection/volunteer bias (e.g., ~30% reported blood in stool; among those with advanced adenomas, ~53% reported post-run bleeding). Several users note it appears to be an abstract/press release rather than a fully peer-reviewed paper yet.
Anecdotes abound: multiple stories of very fit runners (some in their 30s–40s) diagnosed with colon cancer; others say such cases are still uncommon in clinical practice.
Hypothesized mechanisms:
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Ischemia/blood-flow shunting during prolonged exertion (exercise-induced ischemic colitis), repeated micro-bleeds/micro-tears, and mechanical jostling/impact of the colon.
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Sympathetic surge (adrenaline/noradrenaline) diverting blood from the gut.
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Diet/exposures common to endurance sports: high sugar/gel use, artificial colors/sucralose, processed supplements, heavy traffic pollution/particulates, microplastics, asphalt dust.
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Genetic predisposition/selection for people who excel at endurance.
Practical notes repeated by clinicians/runners: rectal bleeding is not normal; screen at 45 (earlier if symptoms or risk factors). Don’t over-interpret a small, preliminary study; larger controlled studies are needed before drawing causal conclusions.
Evolution debates: back-and-forth on whether humans “evolved to run long distances” (persistence hunting) vs. arguments that extreme marathon/ultra training is not ancestral nor necessarily healthy.
Practical notes repeated by clinicians/runners: rectal bleeding is not normal; screen at 45 (earlier if symptoms or risk factors). Don’t over-interpret a small, preliminary study; larger controlled studies are needed before drawing causal conclusions.
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In fitness or sports, PR stands for personal record, PB (more commonly used in Canada) is personal best. Runners are using the terms PR and PB somewhat interchangeably.
