The body's first choice for fuel is stored glycogen (blood sugar). After the glycogen has been depleted, the body must obtain more glucose to keep its nervous system operating. At this point, an inactive, under fed body will turn to protein, its own lean muscle mass, to feed its basal metabolic rate.
Many dieters believe the body will burn fat as its alternative source of energy. This is wrong. At this stage, fat stores are of no use to the nervous system.
Here's why. The nervous system and brain are the central controllers in the body. They can also use glucose (blood sugar) as fuel, and it is imperative that they find fuel. The muscle and organs may use fat as fuel, but the nervous system cannot. Also, the body possesses no enzymes that can convert fat to glucose. The body does, however, have enzymes that convert protein to glucose.
So, if fuel is not available, the body converts its own muscle mass into glucose to feed its nervous system and metabolism. In fact, if the body were to continue to consume its lean tissue unchecked, with no other fuel sources, death would ensure within a few weeks. After all, not only skeletal muscle, but also the liver, heart muscle, lung tissue, blood cells -- all vital tissues -are being burned as fuel.