I think the data presented at ASCO this year showed an increase in adenomas (benign, potential precursors of colon cancer) among marathoners. It’s unknown yet whether the that translates into an increase incidence of colon cancer in this population, though it would be prudent to assume so for now. The study was smallish n=100 IIRC so more research is warranted. Interesting to consider the biological mechanisms if this relationship holds up.