The amount of glycogen stored in the body mainly depends on physical exercise, basal metabolic rate and dietary habits [13] (especially oxidized type 1 fiber [14][15]). Different levels of resting muscle glycogen are achieved by altering the number of glycogen granules rather than increasing the size of existing granules [15], although most resting glycogen granules are smaller than their theoretical maximum. [16] Approximately 4 grams of glucose is always present in the human blood;[4] in fasting individuals, blood glucose remains constant at this level but depletes glycogen stores in the liver and skeletal muscle. [4] Glycogen storage in skeletal muscle is a form of energy storage in the muscle itself;[4] however, the breakdown of muscle glycogen blocks the muscle's uptake of glucose from the blood, thereby increasing the amount of blood glucose available to other tissues. [4] Hepatic glycogen stores serve as storage for glucose for use throughout the body, especially the central nervous system. [4] In fasted, sedentary people, the human brain consumes approximately 60% of blood sugar. [4]

Glycogen is an analog of starch, a glucose polymer that functions as an energy store in plants. It has a structure similar to amylopectin (a component of starch), but is more extensively branched and dense than starch. Both are white powders in dry state. Glycogen exists as granules in the cytosol/cytoplasm of many cell types and plays an important role in glucose cycling. Glycogen forms an energy store that can be quickly mobilized to meet sudden demands for glucose, but this energy store is not as compact as that of triglycerides (lipids). Therefore, it is also found as a storage reserve in many parasitic protozoa.

Glycogen is a branched biopolymer consisting of linear chains of glucose residues with an average chain length of approximately 8-12 glucose units, with 2,000-60,000 residues per glycogen molecule.

Like amylopectin, glucose units are linked together linearly from one glucose unit to another by α(1→4) glycosidic bonds. Branches are connected to the chain from which they branched by an α(1→6) glycosidic bond between the first glucose of the new branch and the glucose on the stem chain.