Can Too Much Running Increase Colon Cancer Risk?

Just when I'm reading about Ultras

If you've read a previous post of mine you know that there is evidence showing that people who are physically active have a lower incidence of cancer. A 2016 meta-analysis of 52 prospective studies found physically active adults were 24 percent less likely to have colon cancer compared to sedentary peers.

So I was very surprised when I came across a poster presentation that suggested that those who do high volume ultra marathon and marathon running may actually have an increased risk of advanced pre-cancerous colon lesions. The New York Times wrote about this too.

Researchers had 100 long distance runners ranging from 35 to 50 years who completed at least 2 ultramarathons (> than 50 km) or 5 marathons. These runners underwent screening colonoscopies, with all polyps reviewed by a team of oncologists, pathologists and gastroenterologists. 

They were found to have polyps (> 10mm with  tubulovillous features) that were more likely to turn into cancer compared to the general population (40-49 years old). 15 percent of the 100 runners had advanced adenomas. This was greater than 10 times higher than the benchmark. 39 runners had at least 1 adenoma of any type.  Prevalence in the matched general population was just 1.2 percent.

Most cases happened in runners with very high training exposure (pictured above), multiple ultramarathons or completed more than 15 races. There were even a few cases of high grade dysplasia, just 1 step away from colorectal cancer.

The researchers' hypothesis was that due to extremely high volume distance running, this leads to repeated low blood flow to the gut leading to chronic injury and inflammation. This can repeatedly injure the stomach lining. This recurring pattern may then accelerate carcinogenic changes that lead to the development of pre-cancerous adenomas.

Moderate exercise does lower systemic inflammation, but ultra marathon training can increase inflammation in the gut made worse with frequent high volume racing. This worsens during intense exercise in the heat. Since chronic gastrointestinal stress impairs absorption of fibre and micronutrients that normally protects the colon, their findings may be reasonable. More so if recovery isn't adequate.

The difference compared to the normal population does cause some concern. However, do note that this is a relatively small study done only in 1 location. There is no proof of causation, genetics, nutrition and other lifestyle factors that may also play a role. 

For most of us runners, running is protective against colon cancer, and does not increase our risk to it. I do not have any patients who run >2 ultra marathons or more than 5 marathons a year. If they do, then their risk profile may (emphasis on may) be higher. 

So if you do log mega mileage, race often and are concerned about your cancer risk, it may be wise to get colon cancer screening particularly if you have persistent gastrointestinal symptoms.

Do wait for the full peer review study to be published before making any final conclusions.

Reference (for the abstract)

Cannon TL, Bonomelli S, SwainWR et al 92025). Risk Of Pre-Cancerous Advanced Adenomas Of The Colon In Long Distance Runners. J Clin Oncology. 2025 ASCO Annual Meeting. 43(16) suppl. DOI: 10.1200/JC0.2025.43.16_suppl.3619