glycogenolysis

Glycogenolysis is the body raiding its pantry. Glycogen is a large, branched storage form of glucose kept mainly in the liver and muscles; glycogenolysis is the process of breaking it down to release glucose when blood sugar drops or energy is suddenly needed. It is the fastest way to restore blood glucose, kicking in within minutes.

Glucagon and epinephrine trigger it by raising cyclic AMP, which activates protein kinase A, which switches on the enzyme glycogen phosphorylase that chops glucose units off the glycogen chains. Crucially, only the liver can release this glucose into the blood, because liver cells have the enzyme glucose-6-phosphatase; muscle keeps its glucose for its own use.

Liver glycogen stores are limited and can sustain blood sugar for only roughly a day of fasting. After that the body must rely increasingly on gluconeogenesis. This sequencing — glycogenolysis first, then gluconeogenesis — is the orderly way the fasting body keeps its brain supplied with fuel.