The three patients should have been portraits of health. They were young, lean and physically active. Unusually active, in fact: Two regularly ran 100-mile ultramarathons, and one had completed 13 half-marathons in a single year.

By the time they came to see Dr. Timothy Cannon, all three had advanced colon cancer. He was mystified; the oldest of them was 40, and none had any known risk factors. The doctor couldn’t help wondering if extreme running might have played a role.

So Cannon, an oncologist with Inova Schar Cancer in Fairfax, Virginia, launched a study, recruiting 100 marathon and ultramarathon runners ages 35 to 50 to undergo a colonoscopy.

The results were staggering. Almost half the participants had polyps, and 15% had advanced adenomas likely to become cancerous.

The rate

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