Less energy expenditure makes you more lethargic, less cholecystokinin means more hunger, less leptin signalling means more hunger and more ghrelin also means you're hungrier.
Research on leptin, a hormone produced in fatty tissue for controlling fat deposits, has so far failed to produce the expected slimming drug breakthrough.
Eating sugar causes an excess release of insulin, and when there is too much insulin present in the body, your hypothalamus cannot pick up on its leptin signal.