EDIT: Ignore ... I didn't realise this was all about the environment ... though that being said ... it's basic common sense ... each stage of processing adds inefficiencies.

It's fairly universal common sense that plenty of fruit and vegetables and core to a healthy diet. From there it really isn't a stretch to at least wonder whether going vegan is pretty healthy. The only things that would preclude such an idea from seeming feasible are probably propaganda around dairy and meta products, unfortunately. But even then, you can start thinking about the health problems of red meat and high fat diets.