Along with Exersice, you need to supplement your diet with vitamins and minerials. Everyone knows when you sweat you need to replace the water that you loss but not everyone knows that that sweat contains more than just water and salt. It also contains all the vitamins and minerals your cells need to function. That's the main reason you get hungry after a good workout because you just sweated out a bunch of stuff your body needs so it tells the brain it's time to eat and replenish those vitamins and minerals.