Now look at the right side of the above diagram, the sources. To keep you alive, your body needs two things, plain energy to burn in the form of carbs or fat and then amino acids in the form of protein. Your body has an amazing series of emergency backup system to keep you alive should food be scarce. Your stomach is the gas tank for ordinary use. Your bodyfat is the backup up generator to be turned on when food is scarce. And your muscles are the emergency backup, your body wont turn on this nuclear power plant to cannibalize muscle unless its a DIRE emergency. Energy can come from any of the three systems but amino acids can only come from the stomach or the muscles.