[elric]

Can't read the full study, but it seems to be specifically about people who run ultramarathons. Which sounds like a very small subset of the general population. I doubt that tiny cohort is in any way responsible for the overall increase in bowel cancer in younger people.

[reedf1]

Actually this is the shocking part - it actually is enough to move the needle. Incidents of young people with cancer is small so a 100% increase can be driven by a small absolute number of patients. There has been an absolutely enormous increasing in high endurance sport something more like 20x (or 1900% increase) and ultramarathon/marathon runners are 7.5x more likely to get colon cancer, so this leads to a surprisingly large relative increase in colon cancer.