When I was a teenager I went on an extreme fast, down to one meal a day, for a 6 week period. Problem is, I struggled to eat a normal amount again after the time I set for myself. I had to go to a food therapist after becoming a twig to try and get my calories up again. Even now, years later, I can easily slip back into eating a bag of crisps and then forgetting to eat the rest of the day.