To slim you need to exercise. Eating less can be an option if you have a serious case of obesity. But if you are only slightly over weight, eating less is not always the answer.
Some people become anorexic which makes them see their body as really fat and they stop eating. Sometimes they even become Bulimic from when they do eat. Bulimic is when they make themselves sick which rots your teeth because of the acid in sick.
If you become too thin, the body has no fat left to feed on and starts on the muscle.
About one in 100 of teenage girls suffer from eating problems. But some of them have them in a very small way.
But the older you get, the more people tend to get eating problems.
One in 50 young women and one in 500 young men suffer from anorexia and bulimia.