Is eating meat REALLY bad for the environment?

Basically every vegetarian I have ever known has had mental or physical problems brought on by their restricted diet. My mother had 3 sisters and only one of them was a vegetarian. They all lived to between 88 and 91 years of age but the one who was a vegetarian most of her life had severe Alzheimer's her last 15 years while the other 3 showed no signs of such a problem. So I would have to guess it was the vegetarian diet that brought on the severe Alzheimer's in the one sister.