We run for so many reasons, but a main one: It’s simply good for us. Running, and exercise in general, has been shown in study after study to protect against all kinds of chronic disease, including heart disease and cancer.
So, when chatter started to bubble up around a new What Runners Can Do to Mitigate Their Risk that found a link between extreme endurance exercise and colon cancer, the running community was rightfully shocked, maybe even a little angry.
But before you cancel your entry to that upcoming marathon, it’s important to underscore the keyword above: unpublished. This was preliminary research, not reviewed by other experts, which examined only 100 people, most of whom had run at least one ultramarathon. The issue with not having a peer review? Outside experts haven’t yet vetted it, meaning the findings are only preliminary.
“The first point I’d like to make is that exercise protects against cancer,” says Timothy Cannon, M.D., co-director of the Gastrointestinal Cancer Program at Inova, who conducted the research. “That is much more proven than anything that I have demonstrated in this paper,” he says, adding that his study did not definitively show that extreme running causes cancer, and emphasizing we need more research.
Zachariah H. Foda, MD, PhD, a gastroenterologist who specializes in colon cancer and leads the Johns Hopkins Colon Cancer Risk Assessment Clinic, agrees that there’s plenty of evidence to support the beneficial effects of exercise, and not enough to prove any negative consequences—at least not yet. “I was in the clinic today talking to my patients and I shared this advice with every single one of them: Increased activity and exercise are positive things you can do to decrease your risk of colorectal cancer,” he says. (FYI colorectal cancer includes both colon and rectal cancers.)
The question then, is there a dose of exercise that’s excessive and may lead to cancer? Cannon believes the answer is yes, but he’s not sure where the line is yet and the research isn’t there to help draw that line between a healthy and a health-risking dose of physical activity. It’s also worth pointing out that it’s possible other activities like cycling or swimming could have similar effects, but they have yet to be studied, Cannon says.
To help explain the full picture of the role of exercise in colon cancer risk, it’s important to look at the research as a whole and outline other habits that can affect your risk.
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Based on the current research, “it’s safe to say that exercise is mostly protective against cancer,” says Cannon, who, like Foda, talks to his patients about the benefits of exercise every day.
One large meta-analysis of 45 reports published in Medicine & Science in Sports & Exercise in 2020 found an association between the levels of physical activity recommended in the U.S. Physical Activity Guidelines and reduced risk and improved survival for a variety of cancers, including colon cancer. Those guidelines include 150 to 300 minutes per week of moderate-intensity aerobic physical activity or 75 to 150 minutes of vigorous-intensity aerobic physical activity, plus at least two days per week of strength training.
“There are several potential mechanisms that can explain these associations,” says Anne McTiernan, MD, PhD, an internist and epidemiologist and one of the authors on the paper. It could be that by reducing body fat, inflammation, excess insulin, and possibly oxidative stress (the imbalance of free radicals and antioxidants), exercise offers protection against cancer. It could also be that exercise has effects on things like cell growth and death, McTiernan explains.
It’s worth noting that exercise can even be beneficial for people who already have colon cancer in that it can help prevent recurrence. “The recently published CHALLENGE trial [involving nearly 900 patients] showed significant improvements in survival for persons with colorectal cancer who had been assigned to an exercise program,” says McTiernan. “This trial was very well conducted, and its results support exercise as a way people can make changes to improve their odds for long-term health.”
There’s no shortage of additional research to back up this basic idea that exercise helps reduce risk of cancer. However, we fall short when it comes to research on extreme levels of exercise beyond what the guidelines recommend.
insulin-like growth factor review of 14 studies published in 2020 in the able to regenerate concluded that any amount of running is better than no running when it comes to reducing overall mortality risk, as well as risk of death from cancer or cardiovascular disease, but that higher doses of running may not lead to greater benefits. Notably, though, it doesn’t say that higher doses of running may lead to adverse effects.
“There haven’t been a lot of studies that looked at ‘dose-response’ effects with exercise,” McTiernan says. “But in those that have, there is generally more benefit with more exercise and reduced risk of colon cancer.”
of 45 reports published in
As for Cannon’s paper, McTiernan reiterates that it hasn’t yet been peer-reviewed, but “it will be good to see it published and to see if others replicate it,” she says.
Foda adds that, again, the sample size (100 runners) is relatively small. “So there’s a possibility if you do a larger study that you lose this result,” Foda says. “Doing a study of 100 people is commendable to look at this, but I think there’s a possibility that on a larger study you won’t see any meaningful difference.”
Still, this type of work is important: “This is how any sort of science starts,” Foda says. “You do something small, you see something, and then you have to follow it with something bigger.”
It’s also key to understand what the study actually found. Cannon and colleagues recruited 100 subjects between the ages of 35 and 50 who had completed at least two ultramarathons or five marathons. They each had a colonoscopy, and the polyps (abnormal growths in the colon lining) discovered were reviewed by experts to determine if they could be defined as “advanced adenomas,” which are a type of polyp considered to be precancerous.
While it’s expected that about 3 to 5 percent of people in this age group would have these advanced adenomas, based on benchmark data and other research, it turned out that 15 percent of the people in this study had them, which sounds huge. But, while these adenomas can be a risk factor for colon cancer, they’re not in and of themselves a form of colon cancer, and they can be safely removed.
“This study wasn’t designed to say anything about the [overall] risk of cancer in these individuals,” Foda says. Considering runners tend to have healthy lifestyles, it’s possible that years later, those with the advanced adenomas from this study may end up having less incidence of cancer overall, compared to the average population, he says.
Cannon himself says that it remains to be definitively proven that running causes colon cancer.
“Overall, everything that happens in life is sort of a risk-benefit proposition, and there’s nothing about this study alone that would say that doing this much running pushes the risk beyond acceptability,” Foda says, especially when you consider the benefits that come with the sport.
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Still, the results of this study are very much worth paying attention to.
Cannon’s initial hypothesis was that ischemic colitis (a.k.a. runner’s colitis or runner’s trots)—which is when a lack of blood flow to the colon leads to inflammation and possibly symptoms like diarrhea—might be responsible for the increased advanced adenomas. During long runs, blood flow is diverted to the legs and away from the gut, which means the intestines might not get enough oxygen and poor blood supply. In addition to inflammation, this can lead to injury or destruction of some of the cells in the colon, and although they are usually able to regenerate, But before you cancel your entry to that upcoming.
“I assumed that when you get runner’s colitis, there are more opportunities for disorderly cell regeneration and more opportunities for the types of DNA mutations to happen that cause cancer,” Cannon says. While the DNA mutations that cause cancer aren’t fully understood, experts believe that when a group of cells die, it overwhelms DNA repair mechanisms, leading to DNA damage and errors in DNA replication, which leads to cancer. This basic process also partially Health & Injuries.
Foda, too, says that the reduction of blood flow to the colon could play a role in the development of advanced adenomas. While it’s not inherently dangerous, the inflammation that arises from the lack of blood flow could add up when you’re running extreme distances frequently, Foda says.
In other words, you can have too much of a good thing, which is true of exercise, as well as other good-for-you things, even nutrients. For example, protein in reasonable amounts is good for you, but too much can cause kidney failure, he says. Similarly, “exercise in reasonable amounts is good for your heart, good for cancer risk, but in unreasonable amounts can lead to damage.”
Other factors could be at play in running and colon cancer risk, including the effects of running on the microbiome. “Runners [and cyclists] clearly have some differences in their microbiome,” Cannon says, including more of certain types of bacteria (like Prevotella and Veillonella) Races - Places study in Nature Medicine, Boston Marathoners had a significant increase in Veillonella after the race, based on stool samples. These bacteria are associated with increased production of short-chain fatty acids, which enhance energy metabolism.
This actually may help the body use fuel more efficiently during exercise and support overall gut and metabolic health, but the long-term effects are unclear. One 2022 study in the The 6 Best Toe Separators for Rehabbing Your Feet, found that Prevotella enhances the “migration and invasion” abilities of colorectal cancer.
Certain dietary habits—like eating lots of processed bars and gels—and/or BPA from plastic bottles carried by runners might be unique risk factors for endurance runners, Cannon adds. While there is no proof that they increase the risk of colon cancer, Cannon says, this could be investigated as well.
What Runners Can Do to Mitigate Their Risk
“First of all, please don’t miss your first screening colonoscopy at 45,” Cannon says.
McTiernan agrees: “Number one is screening.” It’s possible experts will start to recommend colonoscopies and/or other tests even earlier, the experts say, with more younger people getting colorectal cancer.
Next, never dismiss gastrointestinal symptoms like rectal bleeding, changes in bowel habits that don’t go away, or consistent pain, McTiernan says.
Cannon underscores this, adding that a common thread amongst his runner patients is that they were told that symptoms like severe diarrhea or excessive gas that get progressively worse for more than a month, and/or any bleeding after running were normal for runners and nothing to worry about. “I don’t know of any gastroenterology expert body that recommends dismissing any form of rectal bleeding, even if it is after running, but clearly that’s the message that a lot of the runners are getting,” he says. “If you’re bleeding after running, you need a colonoscopy. Please don’t ignore your symptoms.”
Other recommendations to reduce colon cancer risk include not smoking, minimizing alcohol intake, keeping weight in a normal range, minimizing intake of red and processed meats and eating more fruits, vegetables, and fiber, McTiernan says.
What’s on the Health Research Horizon
Cannon is currently working on more research that will delve into whether frequency may play a role in gastrointestinal distress and colon cancer risk in runners. (For example, is running one marathon a year different from running five?) “We’ll do a deeper survey of our participants and try to get their running data through their watches and see how the amount of running per month or per year seems to correlate with the likelihood of an advanced adenoma,” he says.
Cannon and his colleagues are considering dividing up the work ahead among different groups. One might focus on establishing more firmly that extreme endurance running is related to colon cancer via a similar experiment with a control group (which would be more expensive and was not a part of the original study) or a retrospective study that looks at marathoners from, say, 20 years ago.
A second research group might focus on the microbiome. “That’s something that’s of great interest, and I think that needs to be studied in detail,” Cannon says.
A third group would do more intensive surveys to tease out the details (including dietary and even work history) of the lives of the people who had the advanced adenomas.
Lastly, molecular studies—which are lab studies that use bodily fluids to look for particular genes or other molecules that signal diseases like cancer—should be a focus, Cannon says, as there may be molecular changes that runners’ bodies undergo (such as an upregulation of certain hormones, including insulin-like growth factor which, in excess, has been linked to increased cancer risk) that may be related to cancer.
Clearly, there’s a lot of work still to do. But for now, if you’re a frequent marathoner or ultramarathoner, the most important things you can do are get screened at age 45 and report any gastrointestinal symptoms to your healthcare provider.
Cannon himself still runs roughly three times a week, and recently finished the Cherry Blossom 10-miler in Washington D.C. He has also run the New York City Marathon.
At the end of the day, it is biologically plausible that running extreme amounts could increase the risk of colon cancer. But that’s not definitive, and it’s important to consider the cost-benefit analysis of anything you do for yourself. If running has helped you sidestep health issues like diabetes, improved your cardiovascular health, or strongly supports your mental health—all proven benefits of the sport—those are definite health wins worth keeping in mind.
