l*u*l*u* and Chiron are quite right.
I would add that a fast (if conducted properly) allows you to become more "in tune" with the needs and wants of the body: a self realization or awareness develops that could have been lacking previously. A sensitization of the physical/mental/emotional and spiritual.
Re-feeding is not about calorie-counting but responding to appetite, just as you know when there is a need for rest/sleep or a need for fluids.