You mean eating only plants is not in our nature and it might be harmful for us? That should be relatively easy to validate by observing a lot of long time vegans.

Eating soy beans is not in the nature of cows or fish but we smart cookies make them eat it in our animal factories anyway because it is convenient and cheap. To make that work, we have to give them lots of nasty stuff that we know is harmful to both them and us. But us pretending to not be herbivores on behalf of cows is fine somehow I guess.