Not very hard actually—those who eat lots of meat and dairy have plenty of health issues. A change brought to you in part by natural selection.
EDIT: For the downvoters: https://nutritionstudies.org/the-china-study/
Just one example...
EDIT: For the downvoters: https://nutritionstudies.org/the-china-study/
Just one example...
https://deniseminger.com/the-china-study/
Natural selection seems to have selected us from ancestors that mostly followed herds of ruminants around and ate them in place of vegetation that we can't digest.
Now imagine there was no hog farms and it was all regular plant crops and/or vegetation.
Bad diets generally do not make people die during the period where they are able to have children. Natural selection has very little effect. Even if it did, the time period it would take for these effects to matter is significantly longer than most people care about when confronting the climate crisis that requires immediate action.