After three otherwise healthy endurance runners younger than 40 showed up in his office in 2017 with metastatic Stage 4 colorectal cancer, Tim Cannon grew suspicious. Those suspicions would lead him to a groundbreaking discovery, published in the American Society of Clinical Oncology’s Journal of Clinical Oncology in May.
Cannon, an oncologist specializing in colon cancer at the Inova Schar Cancer Institute, says he began wondering whether long-distance runners are more prone to getting colon cancer—a hypothesis that contradicts established research showing that exercise protects against cancer.
“Because [these cases] happened so close together and they had described such similar stories, I was interested in knowing more about whether extreme running predisposes people to colon cancer,” says Cannon, who lives in the City of Falls Church.
With funding from the Inova Foundation, he conducted a study from 2022-2024 in which 100 long-distance runners between the ages of 35 and 50 underwent colonoscopies. The screenings showed that almost half of them had polyps—clumps of cells that are usually benign but can become cancerous—and 13% had advanced adenomas, which are precancerous polyps.
How significant is that? As a point of comparison, Alaska Natives, who experience colorectal cancer at twice the frequency of the general population, are diagnosed with adenomas at a rate of 12%. Cannon says he was expecting to see advanced adenomas in his study group at a rate of 1.5%
Possible Causes
Cannon says it’s unclear what’s causing the higher rate among extreme athletes. One potential explanation is that long-distance runners can experience runner’s ischemia, a condition in which blood flow to the intestines is temporarily restricted during or after strenuous exercises.
“What I believe to be the case is that, just like in inflammatory bowel disease, runners who get repeated ischemia get a lot of cell turnover, and there are a lot of opportunities for mistakes in DNA replication that cause cancer,” says Cannon, a co-director of the Gastrointestinal Cancer Program at Inova Schar. “That’s sort of a basic idea of what might be happening.”
Other contributing factors may be that endurance runners have a diet high in processed foods, such as protein bars and goos (energy gels), or high in carbohydrates for “carb loading.” (Eating a lot of carbs before a long run can stave off fatigue.)
To be eligible for the study, participants could not have had a colonoscopy before, and they had to have no history of inflammatory bowel disease, which increases the risk of developing colon cancer. They also could not have a genetic predisposition to the disease, such as Lynch syndrome or familial adenomatous polyposis. Additionally, they had to have run at least five marathons or two ultra marathons, defined as a 50-kilometer race. Of the 100 athletes in the study, 81 had run at least two ultra marathons.
What It Means for Runners
Arlington running phenom Michael Wardian, who has completed about 180 marathons and more than 130 ultramarathons, says the study has come up in his circle of fellow runners, “but no one is overly concerned.”
The findings have not affected how much mileage Wardian covers in a given week. “Studies like these are great. I think they ask intelligent questions and I love having as much data as possible when making a decision, “he says. “But life, to me, is all about weighing the risks and seeing if they are worth taking, I am very comfortable exposing myself to some risk to do something that I love as often and as much as possible.”
That’s the big takeaway Cannon says he wants people to draw from the study. “What’s much more established is that exercise protects against cancer,” he stresses, “so please don’t use this study as an excuse not to exercise. And once you have colon cancer, you’re less likely to have a recurrence if you exercise. The question is whether there is a dose of exercise that is too much…[and] I’m sure that it’s not going to be the same in every person.”
Rather than abandon running, he advises endurance runners to self-advocate when they feel like something in their bodies isn’t right. Warning signs to watch for are blood in the stool and painful cramping.
Wardian intends to be hyper-vigilant. “I am definitely not planning to give up running, but…if something feels off or out of place or I just feel meh, I will be 100% be…getting everything checked out,” he says.
Next Steps
Colon cancer is one of the most common cancers nationwide, and new diagnoses have doubled in people younger than 55 in the past 20 years. That’s why the American Cancer Society in 2018 lowered the recommended age to start getting colonoscopies from 50 down to 45. The U.S. Preventive Services Task Force did the same in 2021.
Colon cancer is also one of the most preventable forms of cancer. In addition to regular colonoscopy screenings, healthy habits, such as maintaining a well-balanced diet, can help protect against it.
Cannon says he is now planning follow-up research to do “a deeper dive on the runners we recruited for the study.” For instance, he’s planning to study their gut microbiome—the bacteria in their intestinal tracts.
“It’s known that runners and extreme athletes have a slightly different makeup of bacteria in their colon, he explains, “particularly that they have the types of bacteria that help them adapt to running long distances.”
Cannon says he also wants to do a bigger survey-based study that asks more details about their running habits.
“I’ve found that the runners tend to be a very meticulous group of people who keep a lot of data and keep track of a lot of health-related information about themselves,” he says. “I’ve never studied a group of people quite like this. I think it really is advantageous for us that they keep all this data and are so willing to share it.”
Meanwhile, Cannon has not let the findings affect his own workout regimen. He ran the Cherry Blossom 10-miler in April. “I’m exercising as much as ever,” he says.