During extended periods of fasting (starvation), the main source of energy is from:

• A. Protein catabolism
• B. Gluconeogenesis
• C. Glycolysis
• D. Ketogenesis

Answer: (D)

When fasting lasts a long time, the body shifts its energy source from glucose to fatty acids and ketone bodies. After liver glycogen is depleted, adipose tissue releases fatty acids that the liver oxidizes to acetyl-CoA. With oxaloacetate later diverted to gluconeogenesis, acetyl-CoA is used to make ketone bodies, which then circulate and serve as the main energy supply for most tissues, including the brain. This switch preserves lean tissue by reducing the need to break down protein for glucose. Gluconeogenesis and a limited amount of glycolysis continue, but they are not the primary energy sources during extended fasting.