It is real but not in the way fatties use it. They think that if they dont eat or eat x ammount of kcal they will go into "starvation mode" where they will then gain a lot of weight. In shot they think that you can magically gain fat weight by eating actually less that you use. There are a lot of "Why are you fat?> Well I started a diet of 1500kcal, went into starvation mode and gained 50 pounds" or the kind of facebook post "dont diet!!11! you will go onto starvation mode and get even fatter!!111uno!!1one". About the real "starvation mode", wikipedia says:   "Starvation mode is a state in which the body is responding to prolonged periods of low energy intake levels. During short periods of energy abstinence, the human body will burn primarily free fatty acids from body fat stores. After prolonged periods of starvation the body has depleted its body fat and begins to burn lean tissue and muscle as a fuel source."  So you actually need to burn your fat to get into a real starvation mode