My sister was like that (fad diets, only water and lettuce, etc) when we were both in boarding school, and she was only 13!
Anyway 6 months later my parents took her to the GP and she was diagnosed with anorexia nervosa. She's 24 now and she is very short and overweight. No one else in our family is or ever has been as short as she is, and none of us have ever been overweight either.
I think that she probably had stunting to her growth because of not eating properly, and her overweight is partly an overcompensation for the lack of eating she did when she was younger (many studies show that people who were malnourished as children are more likely to be overweight as adults).