> the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in liver cells and after significant protein loss.[2] After prolonged periods of starvation, the body uses the proteins within muscle tissue as a fuel source.
https://en.wikipedia.org/wiki/Starvation_response
> the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in liver cells and after significant protein loss.[2] After prolonged periods of starvation, the body uses the proteins within muscle tissue as a fuel source.