Glycogen—stored glucose in liver and muscles—provides about 2,000-2,400 calories of energy, lasting roughly 24 hours of fasting. Once depleted, the body shifts to fat-burning: breaking down fat into fatty acids and ketones. Research by Cahill documented this metabolic switch.
This companion explores what glycogen is, the hour-by-hour timeline of a fast, why the first 24 hours feel different from later fasting, and the water weight connection (glycogen stores 3-4 grams of water per gram). (4 min read)