Glycogen is a branched polysaccharide of glucose that serves as a form of energy storage in animals,[2] fungi, and bacteria. [3] It is the main storage form of glucose in the body.
Glycogen functions as one of two forms of energy storage, glycogen for short-term and the other form triglycerides stored in adipose tissue (i.e. body fat) for long-term storage. In humans, glycogen is manufactured and stored primarily in the cells of the liver and skeletal muscle. [4][5] In the liver, glycogen accounts for 5-6% of the fresh weight of the organ: the liver of an adult human, weighing 1.5 kg, can store approximately 100-120 g of glycogen. [4][6] In skeletal muscle, the concentration of glycogen is low (1-2% of muscle mass): approximately 400 g of glycogen are stored in skeletal muscle of a 70 kg adult. [4] Small amounts of glycogen are also found in other tissues and cells, including the kidney, red blood cells,[7][8][9] white blood cells,[10] and glial cells in the brain. [11] The uterus also stores glycogen to nourish the embryo during pregnancy. [12]
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]
