During a 24-hour fast with no carbohydrate intake, the liver maintains blood glucose by

• A. Producing glucose directly from carbon dioxide
• B. Converting lipids into glucose
• C. Converting amino acids into glucose
• D. Converting glycogen into glucose

Answer: (D)

When fasting, the liver first relies on stored glycogen to keep blood glucose steady. This happens through glycogenolysis, where glycogen is broken down into glucose-6-phosphate and then free glucose that can be released into the bloodstream. This provides a quick, direct source of glucose to meet ongoing needs between meals. After glycogen stores diminish, the liver shifts more toward gluconeogenesis, making glucose from noncarbohydrate sources like amino acids and glycerol. The idea of directly turning lipids into glucose isn’t accurate, since fatty acids can’t be converted into glucose (though glycerol from fats can feed gluconeogenesis). And producing glucose from carbon dioxide isn’t a process that occurs in human metabolism.