This scientific weight control fact about the metabolism and how it works stems from our ancestry as cavemen, when there were famine periods between hunts. The body would store and hold onto fat for survival. The metabolism has not evolved from this self-defense tactic and doesn't realize we can just run to the grocery store and get whatever we want. On restrictive diets, weightloss can plateau when the metabolism thinks it is experiencing a famine period, and it will only burn what is taken in. Even on some of the popular low carbohydrate diets, it is advised to eat pizza for two days when your weight plateaus to get the metabolism going.