During what situations does the body start to burn muscle (protein) for fuel?

If you eat poorly: Body needs fuel to run the body machine 24 hours a day. This comes from foods you eat. But if you do not eat adequately like upto about 1500-1800 calories per day, the body will get its energy from normal tissues of the body like stored fats or from the proteins in the muscles.

Starvation.: The body starts to use protein for energy in a starvation state.