As far as I know it's a real thing. It's something like that where your body preserves all the fat (energy/sustenance) it can. I grabbed this from Wikipedia so it's questionable but it is a thing in animals and humans are an animal. So I don't see why it wouldn't happen in humans too. "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"