The body needs protein to function (the body is partially built out of protein, and also proteins perform many important functions in the body), and it cannot b

The body needs protein to function (the body is partially built out of protein, and also proteins perform many important functions in the body), and it cannot be synthesized from fat or carbs. The body also has no store of protein. This means that outside of dietary protein, breaking down muscles is the only option. Secondary to that, the body needs some amount of glucose. The best option for that is stored glycogen. When those stores run out the body can enter ketosis which reduces but does not eliminate the need for glucose. Fat consists of three fatty acids joined by a glycerol part. Most of the energy is in the fatty acids, but the glycerol can serve as a limited supply of glucose. However that is not really sufficient, so to get more glucose, and the other option is breaking down protein.