Studies have linked marathon running and colon cancer. So we should put out PSAs! Limit yourself to 2 runs a week!

Then find out some other factor (such as special interest funding, industry manipulation, someone trying to get published) was skewing the interpretation of the data, and change the advice again. This ping pong advice is why no one trusts it.

https://www.sciencealert.com/surprise-discovery-of-elevated-cancer-risk-in-elite-runners-can-be-explained