When we eat, blood glucose levels are increased (by the food we eat) and this causes the pancreas to start producing insulin. Thanks to insulin that serves as a kind of "key" to open the cells to the glucose that enters them. All of this helps cells to produce energy, and this is fine! but also thanks to insulin, we store excess glucose in the form of fat in adipose tissue (that's how the happy love handles, cellulite, orange peel skin begin to form!) And therefore when we overeat, insulin transforms excess glucose into fat and this fat is stored by our dear wise body. And he keeps it, in case "his food" (that is, glucose) runs out! It is when our body will pull these food stores (this happens when we go on a diet, we put less food in the body and it has to pull from its stores).