If it’s new in the last 10,000 years (the beginning of agriculture) we have at least some hope that the worst problems have been dealt with, although we do have lactose and gluten intolerance as counter examples.
If it’s new to the human diet in the last hundred years, maybe it’s fine, and maybe it isn’t. It has to pretty obviously kill you for people to notice.
Animals, fruit, and root vegetables have been in the human diet since before humans left Africa. That’s my rule of thumb.
The entire point of diet advice and nutritional science is trying to find out what "in moderation" means for different foods. The amount of donuts that is moderate and the amount of broccoli that is moderate are not equal.