Glycogen stores are depleted within approximately how long during fasting?

• A. 6-12 hours
• B. 24-48 hours
• C. 72 hours
• D. 96 hours

Answer: (B)

When fasting, the body first relies on liver glycogen to keep blood glucose steady through glycogenolysis. This hepatic glycogen reserve typically lasts about a day. Once those stores are depleted, which occurs around 24 hours into the fast, the body switches to gluconeogenesis from amino acids, lactate, and glycerol to maintain glucose levels. Therefore, glycogen stores are essentially exhausted within roughly 24–48 hours, with gluconeogenesis taking over as the primary source of glucose. Muscle glycogen isn’t released into the bloodstream, so its depletion doesn’t directly affect blood glucose, and its usage becomes more relevant with ongoing activity rather than fasting duration alone.