Ultramarathons and Colon Cancer: Should Runners Be Concerned?

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Mia Hubert

Running is widely considered one of the healthiest and most accessible habits you can adopt. It has proven cardiovascular benefits, improves mood and cognition, and aids with weight and stress management. However, a recent cancer study raised an unsettling question for endurance athletes: does running increase the risk of colon cancer?

The preliminary study, released by Cannon et al. in May 2025, suggested that ultramarathon runners may be at higher risk of colon cancer. The authors performed colonoscopies on 100 adults that had run several marathons (26.2 miles/42 km) or ultramarathons (>50 km). They found that 15% of participants had precancerous growths on colonoscopy, called advanced adenomas (AAs). This was compared to a previously published estimate of how common these growths are in the general population (1.2%). These findings were presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting. The potential link between running and cancer triggered some panic in the long-distance running community and led to coverage by well-known news outlets, including the New York Times and the BBC. Some experts have suggested that the increased risk may be due to temporarily reduced blood flow to the gut that occurs during exercise, but the exact mechanism is still unclear.  

So, should you be panicking if you are an avid runner? Is this risk limited to ultramarathon distances, or does moderate-distance running also confer a similar risk? Before hanging up your running shoes for good, let’s unpack what these results mean and the established research on exercise and cancer. 

While this study highlights what would be a concerning link between running and cancer, there are several reasons to interpret these findings cautiously. Firstly, this study compared the rates of precancerous AAs discovered in runners compared to a historical benchmark in the general population. As a result, the study itself did not have a control group for comparison (participants that are not ultramarathon runners and have an average risk of colon cancer). Additionally, the historical benchmark rate AAs used in the study may be outdated. In fact, the incidence of colon cancer has been rising in young adults (<50 years), meaning that the baseline rates of precancerous AAs in the general population may actually be higher than this study assumed. Finally, while this study uncovered a potential correlation between intensive running and colon cancer, this does not mean that ultramarathons cause cancer. The authors themselves point out that further research is required to understand this phenomenon better. 

Additionally, the benefits of running in terms of cancer risk and outcomes are better established. Several studies have found that walking or running can reduce your risk of developing a wide range of cancer types. Interestingly, there does not appear to be a “dose-response relationship,” meaning that more running does not necessarily mean greater reduced risk of cancer. Regular aerobic exercise has also been demonstrated to reduce the risk of recurrence and death and improve overall outcomes in individuals with cancer. There is also evidence that physical activity such as running may improve treatment efficacy. Exercise improves blood and oxygen supply to tissues, including tumors. This might actually improve cancer treatment outcomes by enhancing chemotherapy delivery or radiotherapy response in cancerous tissues. Aerobic exercise improves baseline cardiorespiratory fitness, leading to improved surgical outcomes in individuals with cancer. Additionally, exercise may also reduce cancer treatment side effect severity and improve mental wellbeing. 

As with many things in life, there is likely a happy middle when it comes to the benefits of running. Low to moderate amounts of exercise have been shown to reduce cancer risk and improve outcomes. By contrast, the preliminary ASCO study raises the possibility that running extreme distances could carry risks that we do not yet fully understand. While this idea still needs to be explored further, it is well established that marathon runners are at a higher risk of skin cancer due to prolonged UV exposure—so next time you lace up your shoes, throw on a hat and some sunscreen as well.

Header Image Source: https://www.needpix.com/photo/1731412/marathon-run-sport-endurance-competition-record-sports-shoes-lifestyle-road

Edited by Cameron Van Cleave

References

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