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Marathon Runners May Have Higher Colon Cancer Risk
Intensive long-distance running could be a risk for advanced adenomas (AAs) for the colon, a small prospective study reported this summer at the American Society of Clinical Oncology (ASCO) 2025.
Refined screening strategies for this running population are therefore warranted, and pathologic and epidemiologic evaluations should explore causation and ancillary risk factors in this unique population, according to Timothy L. Cannon, MD, oncologist at Inova Schar Cancer Institute in Fairfax, Virginia, and colleagues.
The full study (NCT 05419531), which is currently being reviewed for publication, looked at colonoscopy results from 100 marathon and ultramarathon runners and found that almost half had polyps, and 15% (95% CI, 7.9-22.4) had confirmed AAs).
The AA rate was higher than the 4.5% to 6% seen in adults in their late 40s in the general population and was higher even than the 12% found in Alaska Natives, who are at heightened risk for colon cancer.
"After meeting 3 extreme endurance athletes — 2 who ran 100-mile ultramarathons and 1 lady who ran dozens of triathlons — with stage IV colon cancer before age 40, I began to be suspicious of a link," Cannon told Medscape Medical News. At least 2 of them said they were told that bleeding after long runs was common, which they took to mean as normal. "I could imagine multiple reasons that endurance runners would be predisposed to cancer, with my initial focus on the inflammation and cell turnover incited by the well-described ischemia and runner's colitis."
Study Details
From October 2022 to December 2024, 100 eligible participants aged 35 to 50 years had colonoscopies. The median age was 42.5 years; 55 participants were female and 45 were male. In terms of endurance eligibility, all had completed at ≥ 2 registered ultramarathons (50 km or longer) or 5 registered marathons (26.2 miles). Patients were excluded if they were known or suspected to have inflammatory bowel disease, familial adenomatous polyposis, or Lynch syndrome (hereditary nonpolyposis colorectal cancer).
The historical 1.2% in average-risk individuals aged 40-49 years was used for the expected rate of AAs, defined as lesions > 10 mm, lesions with 25% tubulovillous features, or high-grade dysplasia.
In other findings, 39 had ≥ 1 adenoma and had ≥ 3 adenomas but did not meet AA criteria and were not included in the 15% with AA.
While no colon cancer was detected in the cohort, Cannon said 30% experienced rectal bleeding after exercise, especially those with AAs compared with those without: 53% vs 22%. "While rectal bleeding had a significant association with finding advanced adenomas on the colonoscopy, there were still many with advanced adenomas who reported no bleeding," he said.
Runner's colitis, or trots, is a common condition thought to be related to ischemia, mechanical stress, or adverse impact on the gut microbiome. "Mechanism is the huge question that I certainly can't answer at this point," Cannon said. "At some distance, blood flow gets diverted from the splanchnic circulation to the legs, and gut ischemia seems to ensue. I envision high rates of disorderly cell turnover and more opportunities for mutagenesis. This needs to be studied, and what I am describing is certainly either an oversimplification or simply not related at all."
The authors noted that exercise-induced gastrointestinal injury is likely associated with reduced blood flow to the intestines during long-distance running, but not evidence has linked this bowel ischemia to carcinogenesis.
Diet could be another factor. "I am fascinated with runners' diets. They seem to consume, on average, a huge amount of ultraprocessed bars and goos. They also may drink from plastic bottles far more than the average person. These are just 2 of many possibilities," Cannon said. "Nearly a third of our participants were vegan or vegetarian. We are planning a second, more detailed, survey or our participants. We will really dig down on these questions as well as specifics regarding their training regimens."
Commenting on the study but not involved in it, Thomas F. Imperiale, MD, professor of gastroenterology and hepatology at Indiana University Indianapolis, said that while the findings are provocative, several methodological issues require consideration in subsequent research.
"First, the comparative benchmark of advanced adenoma prevalence of 1.2% is based on screening colonoscopy data from 25 years ago. At the very least, a concurrent benchmark should be used," he told Medscape Medical News. The second issue is the absence of a control group of persons who may exercise but who do not run marathons. "This addition would strengthen study validity more than using a concurrent comparison."
The case group of long-distance runners and a control group of nonmarathon runners could be compared for prevalence of AAs with adjustment for age, sex, race or ethnicity, family history of colorectal cancer, diet, other physical activity, tobacco use history, BMI or waist circumference, ethanol use, and perhaps other early-life exposures and indication for colonoscopy. "Last, it would be interesting to know whether and how often the 100 participants developed symptoms possibly consistent with colonic ischemia either during or after long-distance runs, which might provide indirect support for the presumptive mechanism of action."
In other comments, Hamed Khalili, MD, MPH, gastroenterologist at Massachusetts General Hospital and associate professor at Harvard Medical School, both in Boston, called the results very preliminary. "The sample size is small, and the comparator group is a historical control, so it's unclear whether the observed differences are just a sampling issue," he said.
Cannon has this advice for physicians: "Please don't dismiss symptoms of runner's colitis as benign. This condition requires investigation," he said. While he hasn't seen any expert recommendation to treat postrunning bleeding any differently from other causes of melena or hematochezia, both of which would normally merit a colonoscopy, in practice many gastroenterologists dismiss this type of bleeding as benign. "If larger studies confirm our findings, I don't think it's out of the question that marathoners will have unique screening recommendations. But this study is not robust enough, of course, to merit such a recommendation."
His group is planning a study on the runner's microbiome and on the proteome of the colonic tissue in this group.
Cannon reported having no relevant conflicts of interest to disclose. Imperiale and Khalili reported having no conflicts of interest relevant to their comments on the study.
A version of this article first appeared on Medscape.com.
Intensive long-distance running could be a risk for advanced adenomas (AAs) for the colon, a small prospective study reported this summer at the American Society of Clinical Oncology (ASCO) 2025.
Refined screening strategies for this running population are therefore warranted, and pathologic and epidemiologic evaluations should explore causation and ancillary risk factors in this unique population, according to Timothy L. Cannon, MD, oncologist at Inova Schar Cancer Institute in Fairfax, Virginia, and colleagues.
The full study (NCT 05419531), which is currently being reviewed for publication, looked at colonoscopy results from 100 marathon and ultramarathon runners and found that almost half had polyps, and 15% (95% CI, 7.9-22.4) had confirmed AAs).
The AA rate was higher than the 4.5% to 6% seen in adults in their late 40s in the general population and was higher even than the 12% found in Alaska Natives, who are at heightened risk for colon cancer.
"After meeting 3 extreme endurance athletes — 2 who ran 100-mile ultramarathons and 1 lady who ran dozens of triathlons — with stage IV colon cancer before age 40, I began to be suspicious of a link," Cannon told Medscape Medical News. At least 2 of them said they were told that bleeding after long runs was common, which they took to mean as normal. "I could imagine multiple reasons that endurance runners would be predisposed to cancer, with my initial focus on the inflammation and cell turnover incited by the well-described ischemia and runner's colitis."
Study Details
From October 2022 to December 2024, 100 eligible participants aged 35 to 50 years had colonoscopies. The median age was 42.5 years; 55 participants were female and 45 were male. In terms of endurance eligibility, all had completed at ≥ 2 registered ultramarathons (50 km or longer) or 5 registered marathons (26.2 miles). Patients were excluded if they were known or suspected to have inflammatory bowel disease, familial adenomatous polyposis, or Lynch syndrome (hereditary nonpolyposis colorectal cancer).
The historical 1.2% in average-risk individuals aged 40-49 years was used for the expected rate of AAs, defined as lesions > 10 mm, lesions with 25% tubulovillous features, or high-grade dysplasia.
In other findings, 39 had ≥ 1 adenoma and had ≥ 3 adenomas but did not meet AA criteria and were not included in the 15% with AA.
While no colon cancer was detected in the cohort, Cannon said 30% experienced rectal bleeding after exercise, especially those with AAs compared with those without: 53% vs 22%. "While rectal bleeding had a significant association with finding advanced adenomas on the colonoscopy, there were still many with advanced adenomas who reported no bleeding," he said.
Runner's colitis, or trots, is a common condition thought to be related to ischemia, mechanical stress, or adverse impact on the gut microbiome. "Mechanism is the huge question that I certainly can't answer at this point," Cannon said. "At some distance, blood flow gets diverted from the splanchnic circulation to the legs, and gut ischemia seems to ensue. I envision high rates of disorderly cell turnover and more opportunities for mutagenesis. This needs to be studied, and what I am describing is certainly either an oversimplification or simply not related at all."
The authors noted that exercise-induced gastrointestinal injury is likely associated with reduced blood flow to the intestines during long-distance running, but not evidence has linked this bowel ischemia to carcinogenesis.
Diet could be another factor. "I am fascinated with runners' diets. They seem to consume, on average, a huge amount of ultraprocessed bars and goos. They also may drink from plastic bottles far more than the average person. These are just 2 of many possibilities," Cannon said. "Nearly a third of our participants were vegan or vegetarian. We are planning a second, more detailed, survey or our participants. We will really dig down on these questions as well as specifics regarding their training regimens."
Commenting on the study but not involved in it, Thomas F. Imperiale, MD, professor of gastroenterology and hepatology at Indiana University Indianapolis, said that while the findings are provocative, several methodological issues require consideration in subsequent research.
"First, the comparative benchmark of advanced adenoma prevalence of 1.2% is based on screening colonoscopy data from 25 years ago. At the very least, a concurrent benchmark should be used," he told Medscape Medical News. The second issue is the absence of a control group of persons who may exercise but who do not run marathons. "This addition would strengthen study validity more than using a concurrent comparison."
The case group of long-distance runners and a control group of nonmarathon runners could be compared for prevalence of AAs with adjustment for age, sex, race or ethnicity, family history of colorectal cancer, diet, other physical activity, tobacco use history, BMI or waist circumference, ethanol use, and perhaps other early-life exposures and indication for colonoscopy. "Last, it would be interesting to know whether and how often the 100 participants developed symptoms possibly consistent with colonic ischemia either during or after long-distance runs, which might provide indirect support for the presumptive mechanism of action."
In other comments, Hamed Khalili, MD, MPH, gastroenterologist at Massachusetts General Hospital and associate professor at Harvard Medical School, both in Boston, called the results very preliminary. "The sample size is small, and the comparator group is a historical control, so it's unclear whether the observed differences are just a sampling issue," he said.
Cannon has this advice for physicians: "Please don't dismiss symptoms of runner's colitis as benign. This condition requires investigation," he said. While he hasn't seen any expert recommendation to treat postrunning bleeding any differently from other causes of melena or hematochezia, both of which would normally merit a colonoscopy, in practice many gastroenterologists dismiss this type of bleeding as benign. "If larger studies confirm our findings, I don't think it's out of the question that marathoners will have unique screening recommendations. But this study is not robust enough, of course, to merit such a recommendation."
His group is planning a study on the runner's microbiome and on the proteome of the colonic tissue in this group.
Cannon reported having no relevant conflicts of interest to disclose. Imperiale and Khalili reported having no conflicts of interest relevant to their comments on the study.
A version of this article first appeared on Medscape.com.
Intensive long-distance running could be a risk for advanced adenomas (AAs) for the colon, a small prospective study reported this summer at the American Society of Clinical Oncology (ASCO) 2025.
Refined screening strategies for this running population are therefore warranted, and pathologic and epidemiologic evaluations should explore causation and ancillary risk factors in this unique population, according to Timothy L. Cannon, MD, oncologist at Inova Schar Cancer Institute in Fairfax, Virginia, and colleagues.
The full study (NCT 05419531), which is currently being reviewed for publication, looked at colonoscopy results from 100 marathon and ultramarathon runners and found that almost half had polyps, and 15% (95% CI, 7.9-22.4) had confirmed AAs).
The AA rate was higher than the 4.5% to 6% seen in adults in their late 40s in the general population and was higher even than the 12% found in Alaska Natives, who are at heightened risk for colon cancer.
"After meeting 3 extreme endurance athletes — 2 who ran 100-mile ultramarathons and 1 lady who ran dozens of triathlons — with stage IV colon cancer before age 40, I began to be suspicious of a link," Cannon told Medscape Medical News. At least 2 of them said they were told that bleeding after long runs was common, which they took to mean as normal. "I could imagine multiple reasons that endurance runners would be predisposed to cancer, with my initial focus on the inflammation and cell turnover incited by the well-described ischemia and runner's colitis."
Study Details
From October 2022 to December 2024, 100 eligible participants aged 35 to 50 years had colonoscopies. The median age was 42.5 years; 55 participants were female and 45 were male. In terms of endurance eligibility, all had completed at ≥ 2 registered ultramarathons (50 km or longer) or 5 registered marathons (26.2 miles). Patients were excluded if they were known or suspected to have inflammatory bowel disease, familial adenomatous polyposis, or Lynch syndrome (hereditary nonpolyposis colorectal cancer).
The historical 1.2% in average-risk individuals aged 40-49 years was used for the expected rate of AAs, defined as lesions > 10 mm, lesions with 25% tubulovillous features, or high-grade dysplasia.
In other findings, 39 had ≥ 1 adenoma and had ≥ 3 adenomas but did not meet AA criteria and were not included in the 15% with AA.
While no colon cancer was detected in the cohort, Cannon said 30% experienced rectal bleeding after exercise, especially those with AAs compared with those without: 53% vs 22%. "While rectal bleeding had a significant association with finding advanced adenomas on the colonoscopy, there were still many with advanced adenomas who reported no bleeding," he said.
Runner's colitis, or trots, is a common condition thought to be related to ischemia, mechanical stress, or adverse impact on the gut microbiome. "Mechanism is the huge question that I certainly can't answer at this point," Cannon said. "At some distance, blood flow gets diverted from the splanchnic circulation to the legs, and gut ischemia seems to ensue. I envision high rates of disorderly cell turnover and more opportunities for mutagenesis. This needs to be studied, and what I am describing is certainly either an oversimplification or simply not related at all."
The authors noted that exercise-induced gastrointestinal injury is likely associated with reduced blood flow to the intestines during long-distance running, but not evidence has linked this bowel ischemia to carcinogenesis.
Diet could be another factor. "I am fascinated with runners' diets. They seem to consume, on average, a huge amount of ultraprocessed bars and goos. They also may drink from plastic bottles far more than the average person. These are just 2 of many possibilities," Cannon said. "Nearly a third of our participants were vegan or vegetarian. We are planning a second, more detailed, survey or our participants. We will really dig down on these questions as well as specifics regarding their training regimens."
Commenting on the study but not involved in it, Thomas F. Imperiale, MD, professor of gastroenterology and hepatology at Indiana University Indianapolis, said that while the findings are provocative, several methodological issues require consideration in subsequent research.
"First, the comparative benchmark of advanced adenoma prevalence of 1.2% is based on screening colonoscopy data from 25 years ago. At the very least, a concurrent benchmark should be used," he told Medscape Medical News. The second issue is the absence of a control group of persons who may exercise but who do not run marathons. "This addition would strengthen study validity more than using a concurrent comparison."
The case group of long-distance runners and a control group of nonmarathon runners could be compared for prevalence of AAs with adjustment for age, sex, race or ethnicity, family history of colorectal cancer, diet, other physical activity, tobacco use history, BMI or waist circumference, ethanol use, and perhaps other early-life exposures and indication for colonoscopy. "Last, it would be interesting to know whether and how often the 100 participants developed symptoms possibly consistent with colonic ischemia either during or after long-distance runs, which might provide indirect support for the presumptive mechanism of action."
In other comments, Hamed Khalili, MD, MPH, gastroenterologist at Massachusetts General Hospital and associate professor at Harvard Medical School, both in Boston, called the results very preliminary. "The sample size is small, and the comparator group is a historical control, so it's unclear whether the observed differences are just a sampling issue," he said.
Cannon has this advice for physicians: "Please don't dismiss symptoms of runner's colitis as benign. This condition requires investigation," he said. While he hasn't seen any expert recommendation to treat postrunning bleeding any differently from other causes of melena or hematochezia, both of which would normally merit a colonoscopy, in practice many gastroenterologists dismiss this type of bleeding as benign. "If larger studies confirm our findings, I don't think it's out of the question that marathoners will have unique screening recommendations. But this study is not robust enough, of course, to merit such a recommendation."
His group is planning a study on the runner's microbiome and on the proteome of the colonic tissue in this group.
Cannon reported having no relevant conflicts of interest to disclose. Imperiale and Khalili reported having no conflicts of interest relevant to their comments on the study.
A version of this article first appeared on Medscape.com.
Marathon Runners May Have Higher Colon Cancer Risk
Marathon Runners May Have Higher Colon Cancer Risk