So much of the bad advice that was eventually proven wrong was exactly that: advice that seemed to make sense based on our understanding of biological processes

So much of the bad advice that was eventually proven wrong was exactly that: advice that seemed to make sense based on our understanding of biological processes, but was never actually tested. Stuff like the bad advice about avoiding eggs because they are high in cholesterol (turns out for the vast majority of people can eat eggs without negatively impacting blood cholesterol), stress causes ulcers (it was just believed that the stomach was too acidic to allow bacterial growth, until someone really looked), etc. The interesting thing is how strongly the existing "medical establishment" dug in when they had no actual studies to back them up.