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Running Into Risk: Colon Cancer and Marathon Runners. My Interview with Dr Timothy Cannon
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Released: September 17, 2025

John Marshall
John Marshall, MD
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Running Into Risk—My Interview with Dr Timothy Cannon

 

John Marshall, MD: Hey, everybody out there—John Marshall for Oncology Unscripted. You're frittering away more of your time, but we've got something that is clearly worth your effort. You know what? I have run, myself, three half marathons. I hated every one of them. I did it 'cause I thought I was supposed to. I trained up for it—it took a whole season to train up for it. I didn’t hurt anything too bad, which is good. I wasn’t fast—let’s be clear. I was always glad to not be doing the other half of the marathon.

 

So why do you care about this? Well, there's been new evidence—it’s been building over time—and it's been really led by and championed by a very good friend and colleague of mine, Dr. Tim Cannon, where too much exercise—or too much strain on the body, maybe we should rephrase that—might, in fact, be bad for us, specifically around colorectal cancer. We already know the data—'cause we've talked about it—around a personal trainer improving your survival if you had colon cancer. But what about joining an ultramarathon team?

 

Dr. Tim Cannon has something to say about that. Tim, introduce yourself and tell the gang your science.

Timothy Cannon, MD: Sure. So, I'm Tim Cannon. Thank you so much for having me, John. This is a study that you cite, that we've just done in the DC area, on ultramarathoners. And I had seen, in the course of about a year, three different ultramarathoners—actually, two were ultramarathoners, one was a triathlete. They had both done dozens of those types of races. And they had stage IV cancer in their 30s.

 

And I thought, you know, there may be a connection here. They were all describing bleeding after they run. I had heard a lot about runner's trots, or bleeding when you run long distances. And I can see how there could be a mechanism—that this could cause cancer if you run so much that you're having repeated insults to your colon, and bleeding.

 

And so, we decided to start a study. We opened our Cancer Prevention Center here, and that's what we're here to talk about today. I'm really glad that you had an interest in this.

 

The study was of 100 long-distance runners. I would call them all extreme. They had all run at least five 26.2-mile marathons. Most of them had run ultramarathons. Many of them had run 100-mile ultramarathons. They had to be between the ages of 35 and 50, not have a known familial syndrome, and not have inflammatory bowel disease. And we screened them to see if they had precancerous polyps.

 

John Marshall, MD: And, as you found—they did. And some of them had an increased risk for cancer. So, fascinating work. When people were doing what they thought was gonna keep them outta trouble, they might've been getting themselves into trouble.

 

And I know you, you and I and others have had discussions about the “why” of this. And you just described sort of a trauma. I was always thinking like watershed—not enough blood flow, maybe hypoxia. There's also the other side that I’m, you know, obsessed with—and that's microbiome. These ultramarathoners eat all sorts of funky stuff. They do these protein gels, and they do all sorts of things that are not your classic Mediterranean diet.

 

If you had to put your quarter down, what do you think's the reason for it?

 

Timothy Cannon, MD: Yeah, and we haven’t proven anything quite yet. But I'm believing more and more that there is a connection here. And like you, I thought the watershed idea made the most sense to me initially.

 

Since this came out, everyone is emailing me with their own ideas about it. And some of them are pretty compelling. Microbiome, I think, may be among the most compelling. You know, I've been reading about differences in abundance in people who do endurance sports. There’s a bacteria called prevotella, for instance, that's more abundant in runners—and it may be related.

 

Sure, there's so much we don't know about this, but that's what we're hoping to explore in part two—analyzing the microbiome. And then, of course, there’s the lifestyle. Things that characterize long-distance runners—the goos, the... you know, I’ve worried about everything. They drink a lot of electrolyte drinks out of bottles, and maybe they have high exposure to BPA. Or maybe it’s the high-protein diet that was highlighted this week in The New York Times. Who knows? It could be any of these exposures that could cause it. It’s hard to know, hard to study. To isolate any one of these variables is tricky. But I think it’s important to try to get to the bottom of it.

 

John Marshall, MD: We're into prevention, right? We tell people not to smoke. We tell people to eat right. Should there be some sort of sign at the beginning of a marathon that says, “You're running at your own risk”?

 

Timothy Cannon, MD: Yeah, like the Surgeon General’s warning. I'm not sure we know enough quite to recommend that yet. And of course, I want to emphasize what you did first at the beginning here: that exercise is—by and large—going to be a good thing. We’ll have much bigger problems from there not being enough exercise. And I think we know fairly definitively that exercise reduces the risk of cancer recurrence. So, I want to emphasize that from the beginning.

 

But the question is whether there’s a dose of exercise that is too much. I believe there is. I’d like to get more evidence before we start putting signs on marathons or discouraging people too much. But I could see a future where there is something like that out there.

 

John Marshall, MD: Breakthrough work, in my opinion. Dr. Tim Cannon, thank you so much for, I’m sure, taking time out—when you're in The New York Times, you're much needed on the interview circuit. So, it's a real honor that you’ve taken some time to talk with us and our audience. Dr. Tim Cannon—

 

Timothy Cannon, MD: No way. This one means the most to me, John. Thank you.

 

John Marshall, MD: I love being lied to on a Wednesday. Hang in there everybody, and we'll see you next time on Oncology Unscripted.

 

This transcript has been edited for clarity.