Glycogen storage is one of the most important concepts for understanding why the scale moves the way it does in the first 1-2 weeks of any diet. It also explains carb loading, the post-workout 'pump', and the rapid early scale movement on low-carb diets.
Storage capacity:
- Liver glycogen: typically 80-120 g in adults. Gets used for blood glucose maintenance between meals.
- Muscle glycogen: typically 200-400 g in adults, higher in trained athletes (up to 600+ g). Gets used by working muscle during exercise.
- Combined: roughly 300-500 g for an average adult, more for athletes.
The water-binding is the key fact: each gram of glycogen carries roughly 3 grams of water. A fully glycogen-loaded adult is therefore carrying about 1,200-2,000 g (2.6-4.4 lb) of water in addition to the glycogen mass itself. When intake drops or carb intake is restricted, glycogen depletes partially, that bound water is released, and the scale drops 3-5+ lb in the first week.
This explains several common dieting patterns:
- Week 1 of any deficit produces 2-4 lb of weight loss even when the calorie math predicts 1 lb of fat loss. The extra is glycogen depletion plus water.
- Week 1 of a low-carb diet produces 4-8 lb of weight loss for the same reason, scaled up because carb restriction drives more aggressive glycogen drop.
- Returning to higher carbs after a deficit produces 3-5 lb of "weight gain" in the first 2-3 days that is glycogen and water re-storing, not fat.
- Carb loading before endurance events stores extra glycogen and water. The water is desired during long events because it supports cooling and circulation.
For calorie counting purposes, the practical rule: ignore the first 7-14 days of weigh-in data when starting or ending a diet. The glycogen-water shifts dominate fat changes during that window.