It's what happens when you are starving to death. It's debatable which phase can be called "starvation mode", as the body goes through several stages to survive actual starvation. But, it's generally agreed that a person can't enter anything like starvation mode until they've gone at least 3 days with absolutely no food, or many days with very little food.  In basic, it's the point where the body changes its metabolic strategy. It realizes that new food won't be coming in to the stomach, so instead it starts consuming itself. After a week with no food, your body will consume your own muscles to produce the energy that your vital organs need.  Very fat people can go much, much longer with no food. The record is 380 days - though that person was medically supervised and given supplements of vitamins and minerals. The body is very, very good at energy management. Evolution has been perfecting the strategy since life started. Your metabolism is so good that it will outlive you by several weeks. I kid you not; the internal decay of a dead body is conducted by that person's metabolism.