The body reacts in this way—by reducing energy expenditure—because the body is smart and doesn’t want to die. What would happen if the body continued to expend 3000 calories daily while taking in only 1500? Soon fat stores would be burned, then protein stores would be burned, and then you would die. Nice. The smart course of action for the body is to immediately reduce caloric expenditure to 1500 calories per day to restore balance. Caloric expenditure may even be adjusted a little lower (say, to 1400 calories per day), to create a margin of safety. This is exactly what the body does.
