Glycogen

The body breaks down most carbohydrates from the foods we eat and converts them to a type of sugar called glucose. Glucose is the main source of fuel for our cells. Glycogen is the main way the body stores glucose for later use. Glycogen is synthesized and stored mainly in the liver, although it is also stored in the muscle and fat cells. After carbohydrate ingestion, more glycogen will be produced, and then released as blood glucose levels fall.

When glycogen is stored within muscle and liver cells, it retains water along with it. So, during the beginning of any weight loss diet, some "water weight" is lost. For example, in the first few days of starvation, glycogen is used by the liver to maintain blood sugar and by muscle metabolism, and the associated water is excreted from the body in the urine, accounting for a major part of 1-2 kg loss of weight.