"In prolonged starvation, gluconeogenesis provides the glucose oxidised by the brain. The major substrates for gluconeogenesis are amino acids derived from skeletal muscle protein breakdown. Circulating ketone body concentrations rise during prolonged starvation."
>What’s the long term prognosis if the muscle tissue happens to be your heart?
The body rebuilds the muscle tissue from protein every day and your body has a lot of muscle mass.
Glycogen reserves are only about 1,500 to 2,000 calories. Which means if you ever do any sort of heavy physical activity for a prolonged period then you’re body will also use glyconeogenesis to produce Glycogen. It’s not starvation mode, it’s normal mode.