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The 4-pound overnight swing is fuel and water, not fat.
Your muscles and liver store carbohydrate as glycogen — roughly 12 g per kg of muscle if untrained, up to 20 g/kg when endurance-trained, plus about 100 g in the liver. Every gram binds ~3 g of water, which is why keto dieters "lose 5 pounds" in week one and carb-ups put it back. This calculator estimates your tank size, its energy content, and its real scale impact.
Keto dieters use it to decode the week-one scale crash; athletes use it to size the fuel tank before race week. Coaches point athletes at it when the post-refeed scale spike causes a panic — 3 g of water per gram of glycogen explains everything.
A typical 180 lb trained athlete stores roughly 700–800 g total — about 600–700 g in muscle and ~100 g in the liver — worth around 3,000 kcal. Storage capacity scales with muscle mass and rises with endurance training and carb-rich diets; supercompensation protocols can push muscle stores above 20 g/kg.
Each gram of stored glycogen binds about 3 g of water. Refilling 400 g of glycogen puts roughly 3.5 lb of glycogen-plus-water on the scale — fuel in the tank, not fat gained. This is also why low-carb diets show dramatic first-week losses: mostly the glycogen tank and its water draining.
A hard lifting session burns roughly 25–40% of the glycogen in the muscles worked. High-intensity endurance work at 70–80% VO₂max empties most of the tank in 90–120 minutes — the marathon "wall" around mile 20 is glycogen running out. Liver glycogen falls steadily between meals and overnight.