My sister-in-law is a vegetarian and has been for over 10 years now. While she was pregnant and nursing, she did crave tuna every once in awhile, so she ate it because she figured that's what the baby needed at the moment. She went back to being vegetarian before her daughter had her first birthday. The problem is that she eats crap. She doesn't really cook for herself and she's not terribly fond of salads. She mostly eats spaghetti and bean burritos. Her best friend is a vegan, meaning no animal products whatsoever. She has to take dietary supplements to get all of the nutrition that her body needs, despite eating a rich and diverse diet. She even had a vegan cake at her wedding, with no butter in either the cake or the frosting.
My friend's fiance grew up as a vegetarian because his parents didn't eat meat. When he was finally on his own in college, he ate like the rest of the omnivores and actually enjoyed meat. But he developed a rare arthritis condition, so he can't eat most meats anymore. He can still eat seafood, but there's something in meat, poultry and pork that makes his arthritis flare up.


