Humans are omnivores. We need both meat and veg in our lives to be considered "healthy". Some vegans claim humans aren't omnivores at all, but at one time used to be herbivores. Any time they present "evidence" on this, it usually isn't accurate or it's hearsay over the web by some self-appointed "expert". They also completely forget the canines in our teeth.
The majority of vegans I've met look like zombies or borderline anorexic. Whether it's caused by their diet or not is unknown to me, but it's weird how it seems to be a common trait. Vegetarians however have the ability to become fat, which also boggles my mind. A friend of mine is a strict vegetarian and she's getting quite chubby as of late. Probably the dairy products in work, along with excess carbs.
The reason why I can't really criticise people's eating habits is because I'm not exactly healthy myself. My diet mostly consists of meat and occasional junk food. That along with no exercise helped me become the fat bastard I am today.