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Why are distance runners experiencing high cancer rates recently?
Executive summary
A small, unpublished-at-the-time prospective study presented at the 2025 ASCO meeting found 41–39% of long‑distance runners aged ~35–50 had at least one colorectal polyp and about 15% had advanced adenomas—much higher than historical expectations of ~1–6% in that age band [1] [2] [3]. Reporting and experts stress this is an early signal that merits follow‑up, not proof that running causes more cancer; critics point to the study’s small size, lack of a matched control group and possible confounders such as diet, family history and outdoor sun exposure [4] [5] [6].
1. What the new study actually measured: surprising polyp rates in a specific group
Researchers at Inova Schar Cancer Institute recruited about 100 marathon and ultramarathon runners aged roughly 35–50 for screening colonoscopies and reported that nearly half had polyps and roughly 15% had advanced adenomas—numbers the investigators and media described as “staggering” compared with expected rates for that age [3] [2] [7]. Conference abstracts and news stories repeatedly cite the same headline figures: ~41% with any adenoma, ~15% with advanced lesions [2] [8] [9].
2. Why these results do not prove causation: study design limits
Multiple commentators and specialists note the study was small, presented as an abstract at ASCO and had no contemporaneous non‑runner control group—researchers compared rates to historical population data rather than to matched non‑runners—so you cannot infer a direct causal link from this single study [4] [5] [2]. The investigators also excluded people with known hereditary syndromes to focus on otherwise low‑risk athletes, but not all potential confounders (family history, diet, lifetime screening patterns) were fully accounted for in the reporting [4] [5].
3. Plausible biological and behavioural explanations under discussion
Authors and commentators propose hypotheses such as repeated gut stress and transient bowel ischemia during extreme endurance exercise (“runner’s colitis”) that could promote mucosal injury and, in theory, neoplasia; they also acknowledge alternative explanations like shared lifestyle factors (diet, supplements), selection bias, or that individuals with a family cancer history might self‑select into endurance sports [2] [10] [5]. Experts cited in coverage emphasize these are hypotheses to test, not established mechanisms [2] [10].
4. Other evidence points in a different direction—exercise’s broader cancer benefits
Large epidemiological reviews and meta‑analyses historically show that regular running and physical activity are associated with lower all‑cause and cancer mortality (e.g., running linked to ~23% lower cancer mortality in pooled studies), so the broader literature supports protective effects of exercise at population levels [11]. That creates a tension between a possible risk at extreme training volumes and overall known benefits of being active [11].
5. How the media covered it and why headlines amplified concern
Major outlets (New York Times, Gizmodo, ScienceAlert) and many niche sites framed the ASCO findings as surprising and worthy of attention; headlines often asked whether marathons are “linked” to colon cancer, which can conflate an early association with causation and raise alarm among runners [3] [8] [2]. Local and trade press amplified the numerical contrast (15% vs. expected ~1–6%), which explains public anxiety even as experts urged caution [7] [3].
6. Practical takeaways for runners and researchers
Clinicians quoted by news outlets urge runners not to stop exercising but to take symptoms seriously—any rectal bleeding or persistent change in bowel habits should prompt evaluation, and the study’s authors call for more research including larger, controlled studies and mechanistic work [10] [2] [5]. Researchers and commentators recommend investigating training dose‑response, dietary patterns, family history and objective control groups before changing screening guidelines solely on this study [5] [2].
7. Bottom line and remaining unknowns
The available reporting documents a clear, unexpected signal: a high proportion of advanced adenomas in a small cohort of serious distance runners [2] [3]. But because the work is preliminary, lacks a matched control group and has plausible alternative explanations, it does not answer “why” runners might have higher cancer rates; larger, peer‑reviewed studies are required [4] [5]. Available sources do not mention long‑term, population‑level causation proven by randomized or large prospective controlled studies [4] [11].