You don't eat enough !

[anthrax]

After a prolonged starving period, when you go back to regular diet with much more calories, your body stock those calories as fat in order to have enough reserve for the next starving period

 

[JoBu]

The problem isn't so much with total calorie intake but how it is distributed over the day. People think that by skipping breakfast and not eating anything till the afternoon when they just can't stand it anymore (and probably tend to eat more than they can process at one time) they are doing something useful for loosing weight. But they are really conditioning their body to conserve as much energy as possible when there is food available because it "expects" to be starved. They would be much better off by spreading total food intake over the entire day but still keeping below daily requirements to maintain current weight.

After a prolonged starving period, when you go back to regular diet with much more calories, your body stock those calories as fat in order to have enough reserve for the next starving period