Collateral fattening is a phenomenon where people gain more weight than they lost after a period of starvation or dieting. This happens because starvation or dieting causes muscle loss, which lowers the basal metabolic rate and increases the food intake. However, restoring muscle mass is not easy and depends on the initial body fat percentage. The leaner the person was before starvation or dieting, the more fat they will regain and the longer it will take to recover their muscle mass. This results in fat overshooting, which is when the person ends up with more fat than they had before.