Thats because vegetarians are actively putting more effort and concentration into deciding on what to eat and what they need to make their diet work. Being vegetarian is basically the same as putting yourself on any kind of diet, you'll be more healthy simply because you'll be consciously thinking about food more. It's just as easy to have an unhealthy veggy diet as it is to have an unhealthy omnivorous diet.

fukken saved in hopes of the unlikly return of anonymous picture comments and for 4chan maybe