You do realize that lower amounts of one substance can be healthy while higher amounts of that same substance can have negative effects?
Also, think about it evolutionarily. Humans did not evolve eating meat 3 times a day like many modern Americans do, because we had to hunt each time and meat wasn't always accessible even when we could, plus storage technology was nonexistent for most of our evolutionary history. Our bodies weren't built to handle that.
1. We aren't talking about a drug or a herb, it's food. The negative effects is certain groups controlling what should or shouldn't be in our systems.
2. Humans evolved eating meat thousands of years ago. Maybe not as often as the modern people of today do but the canine teeth features in modern man show they have teeth that are designed for meat.
2a. Our bodies are built for anything but things that are extremely harmful like toxins in our food and water. Even vegetarians and vegans are getting sick and cancer from the pesticides in the fruits and veggies, maybe not as much as meat cause meat absorbs the chemicals longer and larger amounts of it.