Which hormone triggers the liver to convert glycogen into glucose during fasting?

• A. Insulin
• B. Epinephrine
• C. Cortisol
• D. Glucagon

Answer: (D)

When blood glucose drops during fasting, the pancreas releases a hormone called glucagon. This signal tells the liver to start breaking down stored glycogen into glucose, a process called glycogenolysis. Glucagon binds to receptors on liver cells and activates a cascade (increasing cAMP and activating protein kinase A) that turns on glycogen phosphorylase to cleave glycogen into glucose-1-phosphate, which is then converted to glucose and released into the bloodstream. At the same time, glycogen synthase is inhibited, preventing storage of new glucose during this fasted state.

Other hormones can influence blood sugar, but they don’t directly trigger this rapid glycogen-to-glucose switch during fasting in the same primary way. Insulin promotes storage and lowers blood glucose, epinephrine can promote glycogen breakdown during stress, and cortisol supports glucose production mainly through gluconeogenesis rather than immediate glycogenolysis.