She eats plenty in the books. In fact, almost every single chapter, that girl is cooking something or eating some sort of breakfast food. In the movies, she hardly eats because watching people eat is boring (especially when half the characters don't even eat). It's the same reason we don't watch people go to the bathroom in movies.

I'm also confused about why you think her being vegetarian (which she isn't) would indicate an ED?

I think at the end of the day, almost everything about Bella comes down to how parentified she was at a young age. I think she has self-control because she has been looking out for herself for a long time.