"The process of using and storing food energy that occurs when we eat goes in reverse when we fast. Insulin levels drop, signaling the body to start burning stored energy. Glycogen (the glucose that’s stored in the liver) is the most easily accessible energy source, and the liver stores enough to provide energy for twenty-four hours or so. After that, the body starts to break down stored body fat for energy.
So you see, the body really only exists in two states — the fed (high-insulin) state and the fasted (low-insulin) state. Either we are storing food energy or we are burning food energy. If eating and fasting are balanced, then there is no net weight gain.