Glycogen and glucose remain plentiful. In this postprandial or "fed" state, the liver takes in more glucose from the blood than it releases. After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. When it is needed for energy, glycogen is broken down and converted again to glucose. Glycogen phosphorylase is the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen is the primary source of blood glucose used by the rest of the body for fuel. Glucagon, another hormone produced by the pancreas, in
Glucose The free energy of formation of α-D-glucose is 917.2 kilojoules per mole. In humans, gluconeogenesis occurs in the liver and kidney, but also in other cell types. In the liver about 150 g of glycogen are stored, in skeletal muscle about 250 g.<ref name="Löffler/Petrides 389">Peter C. Heinrich: "Löffler/Petrides Biochemie und Pathobiochemie." Springer-Verlag, 2014, , p. 389. (german)</ref> However, the glucose released in muscle cells upon cleavage of the glycogen can not be delivered to the circulation because glucose is phosphorylated by the hexokinae, and a glucose-6-phosphatase is not expressed to remove the phosphate group. Unlike for glucose, there is no