During fasting, which storage carbohydrate is used by the liver to supply glucose?

• A. Triglycerides
• B. Cholesterol
• C. Protein
• D. Glycogen

Answer: (D)

During fasting, the liver maintains blood glucose by breaking down a stored carbohydrate called glycogen. This process, glycogenolysis, uses the enzyme glycogen phosphorylase to remove glucose units as glucose-1-phosphate, which is converted to glucose-6-phosphate and then released as free glucose into the bloodstream by glucose-6-phosphatase. This rapid source of glucose comes from glycogen, the storage form of carbohydrate in the liver.

The other options are not storage carbohydrates: triglycerides are fats stored in adipose tissue and yield fatty acids for energy rather than glucose; cholesterol is a lipid used for membranes and steroid synthesis; protein can supply amino acids for gluconeogenesis, but that’s not a carbohydrate store and occurs later in fasting when glycogen is depleted.