I'm paraphrasing The Nuge because a scientific study shows that vegetarians can experience brain shrinkage linked to B-12 deficiency.
Unfortunately, the study also showed that excessive alcohol shrinks your brain. But that one's no big surprise. Also, being obese was linked to brain loss.
I wonder if Michael Moore drinks Key-Light by the case with the five or six vegan pizzas it looks like he eats daily.
Seriously though, meat is delicious. Human brain evolution is a testament to a steady, nutrient-dense diet. And while you can over-do anything, if a food is delicious there's a good chance it has something your body wants.