Colorectal cancer, once thought of as mainly a disease of the aging, is rising in younger adults by nearly 3% per year and is now a leading cause of cancer-related death among individuals under the age of 50.1 According to the American Cancer Society, in 2026, nearly 159,000 individuals will be diagnosed with colorectal cancer and over 55,000 will die of the disease, including approximately 4,000 individuals under the age of 50.2
Although hereditary gene mutations, such as Lynch syndrome, and lifestyle factors, including obesity, diet, physical inactivity, smoking, and alcohol use, have been linked to the onset of early-onset colorectal cancer, only about 20% of young adults with the disease have an underlying genetic condition,3 and many also lack traditional risk factors, such as obesity or poor fitness.4
Now, the findings from a small, prospective study investigating the risk of advanced adenomas in long-distance runners between the ages of 35 and 50 may add another piece to the puzzle of potential causes of early-onset colorectal cancer. Presented during the 2025 ASCO Annual Meeting, and published in the Journal of Clinical Oncology, the study by Timothy L. Cannon, MD, Sheridan Director of the Molecular Tumor Board and Co-Director of the Gastrointestinal Cancer Program at Inova Schar Cancer Institute, and his colleagues found that “intensive” long-distance running is a risk factor for advanced adenomas of the colon.5
Dr. Cannon emphasized that this study was hypothesis-generating only and warrants further investigation.
The study enrolled 100 participants who had completed at least five marathons (26.2 miles) or two ultramarathons (31 miles or longer) and had never undergone a colonoscopy screening examination. The study excluded individuals with hereditary cancer syndromes or inflammatory bowel disease to isolate the potential impact of endurance running alone.
Our study is just a hypothesis-generating study. I don’t know if there is an association between extreme exercise and the development of early-onset colorectal cancer. …The study started with my clinical observation, which is that I’m seeing many ultramarathoners who are mostly in their 30s with colorectal cancer. And I thought there could be a potential mechanism for cancer initiation because exercise-induced gastrointestinal injury is believed to be linked with reduced blood flow to the intestines during long-distance running.— TIMOTHY L. CANNON, MD
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Dr. Cannon’s results show that 15% of the study participants had confirmed advanced adenomas, compared to the expected 1% to 2% in average-risk individuals in the 35 to 50 age range, and 41% had at least one adenoma.5 The median age of the participants was 42.5 years, 2.5 years younger than the recommended age to start routine colorectal cancer screening in individuals at average risk for the cancer.6
In this wide-ranging interview with The ASCO Post, Dr. Cannon discussed what led him to launch this study, why long-distance running may be a potential risk factor for early-onset colorectal cancer, and whether young long-distance runners should be screened for colorectal cancer before the age of 45.
Observing Clinical Trends
Your study showing a potential risk of early-onset colorectal cancer in marathon and ultramarathon runners is troubling because exercise is known to reduce the development of colorectal cancer, as well as other cancers. While none of the 100 participants in your study had colorectal cancer, nearly half had adenomas, and 15% had large, advanced adenomas. Please talk about what led you to conduct this study.
My concern started in 2019, when, within 6 months, I treated three marathon runners, two in their 30s and one who had just turned 40, for colorectal cancer. And that alarming trend continues to this day. It was just that clinical observation and the thought that perhaps recurrent exercise-induced bowel ischemia could cause more cell turnover and opportunities for cancer-causing gene mutations to develop.
Understanding the Potential Cause of Colorectal Cancer in Young Long-Distance Runners
Why would long-distance running be a potential cause of early-onset colorectal cancer in these individuals?
To be clear, our study is just a hypothesis-generating study. I don’t know if there is an association between extreme exercise and the development of early-onset colorectal cancer. As I mentioned, the study started with my clinical observation, which is that I’m seeing ultramarathoners who are mostly in their 30s with colorectal cancer. And I thought there could be a potential mechanism for cancer initiation because exercise-induced gastrointestinal injury is believed to be linked with reduced blood flow to the intestines during long-distance running.
Ischemic colitis is a known condition in long-distance runners who divert all of their blood flow from the gut to the legs and experience small infarcts in their gut. If that happens repeatedly, there could be a lot of cell turnover and opportunities for gene mutations to occur. We need more research to understand the potential causation and ancillary risk factors in this unique population.
Detecting Colon Polyps in Younger Adults
Colon polyps typically begin forming in an individual’s 40s and 50s. Were you surprised to see so many of the runners in your study, who were mostly in their early 40s, diagnosed with colon polyps?
We thought the study would have more power if we focused on a younger age group that matched the patients I was seeing in the clinic. We don’t have a lot of data on how often people under the age of 45 start developing colon polyps because that is before the recommended age of colonoscopy screening.
The average age of the participants in our study was 42. I certainly had a suspicion that we would see a higher level of colon polyps in our study cohort, but I was surprised to find that 15% had advanced adenomas, and that 41% had at least one adenoma. Those percentages were higher than I had expected.
Evaluating the Microbiome of Marathoners
Is there a connection between changes in the gut microbiome in young adults, which has been linked to the development of early-onset colorectal cancer, and the inflammation and intestinal irritation that long-distance runners often experience? What is your theory about why these individuals have higher rates of colon polyps and advanced adenomas than nonrunners?
I can’t speak yet to a specific link between changes in the gut microbiome and the development of young-onset colorectal cancer in long-distance runners. We have a study underway in collaboration with researchers at Johns Hopkins Medical School to evaluate the microbiome of the runners who participated in our study, so we’ll have more information about a possible correlation in the near future.
What we know right now is that there are certain types of bacteria that are more prevalent in extreme endurance athletes, for example, prevotella. We also know that there are certain bacteria in the gut, most notably strains of E. coli, that produce the toxin colibactin, which affects the risk of colorectal cancer, but we have found no matching cancer-causing bacteria in the gut that are known to be in higher abundance in long-distance runners at this point.
Assessing Disease Risk of Polyps Found on the Left vs Right Side of the Colon
Were most of the polyps found in your study participants on the right or left side of the colon? Left-sided polyps raise the risk of the development of early-onset colorectal cancer.
Yes, the early-onset colorectal cancer epidemic is predominately a left-sided colon phenomenon. But a slight majority of the polyps found in our study participants were on the right side of the colon.
Investigating How Much Exercise Is Cancer-Protective
Based on your study results, should young adult long-distance runners be screened for colon cancer before the age of 45?
We don’t have enough data yet to make that recommendation.
Again, I want to emphasize that our study was a hypothesis-generating study only, so I wouldn’t make a broad screening recommendation for long-distance runners based on a single-arm, prospective study that may have some selection bias. We did the best we could with the limitations of our study.
Guest Editor
Brandon Hayes-Lattin, MD, FACP
Brandon Hayes-Lattin, MD, FACP, is Professor of Medicine and Deputy Division Head of Hematology and Medical Oncology at Oregon Health and Science University, and Medical Director of the Adolescent and Young Adult Oncology Program at the Knight Cancer Institute at Oregon Health and Science University, Portland.
What I will say regarding colorectal cancer screening in younger adults, based on our study results and on my own clinical observations of now dozens of marathon runners with early-onset colorectal cancer, is that many physicians are familiar with ischemic colitis and rectal bleeding that sometimes occurs in runners, which is often dismissed as a normal side effect of running. But any bleeding for any reason, whether it happens after a run or not, merits an investigation by a gastroenterologist and possibly a colonoscopy screening even if the patient is under the age of 45.
We are planning a larger study of marathon and ultramarathon runners in the future, which hopefully will provide more answers to these questions. What is proven is that exercise reduces the risk of colon cancer, and that message should not be lost in the findings from our hypothesis-generating study. We are just investigating whether there is a certain amount of exercise that goes beyond cancer-protective and into the realm of cancer-causing.
DISCLOSURE: Dr. Cannon reported no conflicts of interest.
REFERENCES
1. Regal K: Colorectal Cancer Rates Are Skyrocketing in Young Adults—Is Your Lifestyle Putting You at Risk? Cancer Research Institute, March 2, 2026. Available at www.cancerresearch.org/blog/colorectal-cancer-awareness-month.
2. Siegel RL, Wagle NS, Star J, et al: Colorectal cancer statistics, 2026. CA Cancer J Clin 76(2):e70067, 2026.
3. Early-onset colon cancer. July 9, 2025. Available at www.mayoclinic.org/diseases-conditions/colon-cancer/in-depth/early-onset-colon-cancer/art-20583551.
4. Brueck H, de Graaf M: Young, fit—and hit with a deadly cancer. Inside the mysterious rise in young colon cancer—and what might be driving it. Business Insider. Available at www.businessinsider.com/why-so-many-young-people-getting-colon-cancer-answer-infancy-2025-12.
5. Cannon TL, Bonomelli S, Swain WR, et al: Risk of precancerous advanced adenomas of the colon in long-distance runners. J Clin Oncol 43:3619, 2025.
6. American Cancer Society: American Cancer Society Updates Colorectal Cancer Screening Guideline: Major Changes Emphasize Blood-Based and At-Home Stool Testing. Available at https://pressroom.cancer.org/colorectal-cancer-screening-guideline-update-2026.
