Fairfax, VA – A groundbreaking new study led by Tim Cannon, MD, Sheridan Director, Molecular Tumor Board and Co-Director of the Gastrointestinal Cancer Program at Inova Schar Cancer, has revealed a potential link between extreme endurance exercise and increased risk of advanced colon cancer in younger adults, raising new questions about screening for high-performance athletes.
The study was presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago and the New York Times published an article on the study on August, 19, 2025.
Background:
Several years ago, Dr. Cannon noticed a disturbing trend: multiple ultramarathoners under age 40 were presenting at Inova with advanced-stage colorectal cancer. Troubled by the pattern, he launched a prospective clinical study to investigate whether long-distance running could be associated with an elevated risk of developing colon cancer. Recruitment material was made with the assistance of the wives of two of the deceased runners.
“The participant athletes were otherwise healthy and had no known genetic predisposition or inflammatory conditions,” said Dr. Cannon. “Given that many runners describe bleeding after running, and runner’s colitis is understood to be related to colonic ischemia and recurrent inflammation, it made us wonder whether the intense physical stress of endurance training could be contributing to a higher likelihood of mutagenesis causing precancerous polyps.”
The study enrolled 100 participants between the ages of 35 and 50 who had completed at least five marathons or two ultramarathons and had never undergone a colonoscopy. Importantly, individuals with hereditary cancer syndromes or inflammatory bowel disease were excluded to isolate the potential impact of endurance exercise alone.
Each participant underwent a screening colonoscopy as part of the study. Prior to the procedure, each subject also completed a questionnaire covering dietary habits, bowel history, and long-distance running patterns. All polyps discovered during colonoscopy were reviewed by a panel of gastroenterologists, pathologists, and oncologists to determine if they met the criteria for advanced adenomas.
The findings were striking:
“This is the first prospective study we know of to explore whether exercise-induced intestinal stress – specifically bowel ischemia from long-distance running – might increase cancer risk,” said Dr. Cannon. “While the results don’t prove causation, they strongly suggest that intense endurance exercise could be a meaningful risk factor for colorectal cancer and warrants further study.”
The study was conducted from October 2022 through December 2024 and received Institutional Review Board (IRB) approval. The team now hopes these findings will spark discussion about earlier screening for high-risk athletes and lead to life-saving detection strategies.
“We believe this research may help define a new, at-risk population for earlier colorectal cancer screening,” said Dr. Cannon. “If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives. I feel strongly that young runners who have blood in their stool after long runs, a problem that has historically been dismissed as ‘normal’ or inconsequential, should receive screening. The good news is that screening can prevent advanced cancers.”
Further research is underway to analyze dietary factors, training regimens, and other lifestyle habits from the study group to better understand what’s driving this trend. Inova Schar Cancer continues to lead national efforts in early detection, personalized cancer care, and clinical research that shapes the future of cancer prevention.
We are Inova, Northern Virginia and the Washington, D.C. metropolitan area’s leading nonprofit healthcare provider. With expertise and compassion, we partner with our patients to help them stay healthy. We treat illness, heal injury and look at a patient’s whole health to help them flourish. Through our expansive network of hospitals, primary and specialty care practices, emergency and urgent care centers, and outpatient services, Inova provides care for more than one million unique patients every year. Total patient visits exceed four million annually, demonstrating our ability to deliver the best clinical care and ensuring a seamless experience for all who rely on us for their healthcare needs.
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Inova is home to Northern Virginia’s only Level 1 Trauma Center and Level 4 Neonatal Intensive Care Unit and provides high-quality healthcare to each person in every community we are privileged to serve – regardless of ability to pay – every day of their life. More information about Inova can be found at www.inova.org.
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I was training for a marathon when I was diagnosed with stage 3 colon cancer with Braff mutation back in 2022. I was 43 years-old at the time and fit as a fiddle. I'm in full remission now and only running half-marathons, but this article gives my pause. Thank you for this important research.
If you have a braf mutation it does not have nothing To do with marathons :) you can keep runing.
Fascinating research. I commend Dr. Cannon and your research group for spotting an interesting problem and investigating it further.
As an ultracyclist, self-interest prompts me to wonder whether the causal hinge(s) for this are rooted primarily in the physiologic mechanics of running OR are there factors in play that are common across other (ultra) endurance sports? Diet/fueling, for one.
I'm confident the team involved in this research has mapped out a number of areas of study. I'd nevertheless like to lobby here for comparison between long distance endurance sport athletes.
Ultracyclists share high-carb (sugar, sugar, sugar) fueling with ultrarunners -- as well as an avoidance of fiber around long training sessions and events. Albeit with a bit more ability to consume 'real food' during training and competition due to the more stable position of the stomach on the bicycle during the kind of sub-threshold efforts common to ultra distance riding. But comparatively precious little running.
Whereas long distance triathletes (full ironman, perhaps 70.3 distance too) likely have similar dietary fueling, but a running volume that sits somewhere between that of ultracylists and ultramarathoners.
Thanks for a stimulating preliminary study. Will look forward to future findings from this work.
I agree- will read to see what could be controlled for. There is also previous research on the link between complex trauma and engaging in ultra endurance sports, so the impact of stress can’t be ignored either.
When it notes that the expected positive result for advanced adenomas would have 1-2% hit rate for average-risk individuals, that gives me pause. When was that 1-2% measured? If it was 10-20 years ago, I have a feeling that isn't a valid reference anymore. They should have engaged a control group of both fit and average people in that age range.
I find it odd that this study is taking place amid covid. If this trend existed before covid, shouldn't it have been noticed much earlier? And what about professional cyclists, mountain bikers, ultra swimmers, and triathletes? These athletes' stomachs and intestines also take a beating... bumpy roads, twists and turns... I think it would be better to only release the results of such studies when there's more certainty. One could certainly recommend more screenings for ultra athletes, but don't go into detail. This is alarming. Period.
Conflicting evidence.
https://www.nejm.org/doi/full/10.1056/NEJMoa2502760
Thanks for sharing that study. It's interesting and I appreciated their design to try to isolate the effects of exercise vs. other factors.
It seems like the levels of exercise they were investigating are substantially different from this new Inova study, though. Part of the Discussion section says, "The exercise intervention met its goal of increasing moderate-to-vigorous physical activity from baseline by about 10 MET-hours per week throughout the entire 3-year intervention. This increase is the equivalent of adding about 45 to 60 minutes of brisk walking 3 or 4 times per week or 25 to 30 minutes of jogging 3 or 4 times per week."
I'm napking-mathing it out here so bear with me… I don't know how many hours of exercise the Inova study participants do, but my impression from my friends in the ultrarunning community is that 12–20 hours of running a week isn't unusual. That would be somewhere between 100–180 MET-hours per week (about 90–170 MET-hours per week above the baseline in the "Structured Exercise after Adjuvant Chemotherapy for Colon Cancer" study). That's 900%–1700% of the 10 MET-hours per week above the baseline. So even with fuzzy math, a pretty large difference.
Like P. R. in another comment, I'd be curious to know whether the the training volume itself might produce these different results, or whether it's related to the actual mechanics of running (particularly endurance running, where duration changes the game entirely) vs. other forms of exercise.
The level of exercise that the participants in the study you linked does not compare to ultra endurance athletes. This is not conflicting evidence.
Actually, Not the same cup of tea at all. And by no means conflicting. The study you site, is in patients who have had colon cancer and chemo and then started on STRUCTURED exercise and reported that they were currently exercising less than the equivalent of 150 minutes per week of moderate-to-vigorous intensity. The focus was on promoting aerobic exercise of at least moderate intensity, such as brisk walking. On the other hand, Dr Cannons study shows that patients who were ultramarathoners-, much more than a brisk walk, or multiple marathons had an increased risk. They could actually show similar things, low dose exercise may increase survival (the study end point was disease free survival), whereas intense exercise may increase your risk for initial cancer. The study with those after chemo discouraged more than 3 hours a week of jogging intensity exercise or 7 hours a week of brisk walking. Marathoners will do 7 hours of fast and long in one day.
Lots of studying needs to be done to truly elucidate any of it. Fascinating patterns to be seen in all aspects.
That is not conflicting evidence... You are linking a study which demonstrates improved outcomes post diagnosis in people who engage in a structured exercise program.
Exercise is healthy, that's not really up for debate.
The specifics here are that ultramarathoning and repeated episodes of bowel ischemia manifested by bleeding may be linked with increased risk of colonic ca. Of course other confounders may be present (drinking excessively from high risk plastic containers, waterproofing on clothing material, who knows).
Not necessarily conflicting. The article states "structured exercise" which most likely does not mean running ULTRA marathon distances. What instead I'm taking away is everything in moderation ;-)