Glycogen is a polymer (homopolymer) of glucose. It is the analogue of starch, another highly branched polymer of glucose. Glycogen is the storage form of glucose in humans and other vertebrates and is comprised of monomers of glucose.
Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen breaks down to release glucose in a process scientists call glycogenolysis. In humans, glycogen acts as a glucose energy reserve. It is primarily deposited in the liver and skeletal muscle, however, it is also present in other tissues, including heart, kidney, and adipose tissue.
Two-thirds of the body’s glycogen (approximately 200-300 grams) is stored in muscles in the form of shorter, lighter chains. Muscle glycogen is a readily available source of energy for the working muscle. Unlike liver glycogen, the glycogen contained in skeletal muscle cannot be released into the bloodstream to be utilized by other tissues, but only by the muscle itself, primarily during activity. This is because the muscle is deficient in the enzyme glucose-6-phosphatase (the enzyme that removes the phosphate group from glucose-6-phosphate). After glycogen is phosphorylated in muscle cells by the enzyme hexokinase (the enzyme that attaches the phosphate group to glucose), it cannot be dephosphorylated. Because phosphorylated glucose cannot be transported outside the cell, glucose-6-phosphate remains trapped inside. The glycogen contained in skeletal muscle is approximately 12-16 g/kg, totaling approximately 300-400 g.
The rate at which muscle glycogen is oxidized depends largely on the intensity of exercise and the energy system activated. At low and moderate intensities of aerobic exercise, much of the energy can be obtained from oxidative phosphorylation of lipid-derived acetyl-CoA, with minimal consumption of carbohydrates. As intensity increases, lipid oxidation fails to meet energy demands, so muscle glycogen begins to increasingly dominate, becoming at high intensities the primary substrate. At even higher intensities in endurance exercise, the anaerobic threshold is reached, in which carbohydrates, and therefore glycogen, become the only substrate used, this is because anaerobic energy is given mainly by its catabolism.
In lactacid anaerobic exercise, carbohydrates represent practically the only energy substrate, and muscle glycogen in this case assumes a decisive role. Glycogen binds to itself a considerable amount of water, equal to 2.7 g for each gram, so after an intense and prolonged physical effort, the weight loss can be significant: the depletion of glycogen reserves involves a weight loss of about 400 g to which must be added 1080 g of water. In the muscle, unlike in the liver, the uptake of glucose is slow, and it is the limiting factor of carbohydrate metabolism.
In the liver is stored one third of the glycogen of the whole body (about 80-100 g) in the form of chains longer and heavier than in the muscles. The main role of liver glycogen is to maintain stable and constant blood glucose levels, so it has a highly variable content. Unlike skeletal muscle, liver glycogen must be released into the bloodstream, as it represents the main endogenous supply of carbohydrates that can be utilized by the various tissues of the body.
Liver cells are capable of defosphorylating glucose. In this way, thanks to the dephosphorylation of hepatic glycogen, the liver can release glucose into the blood stream, in order to regulate blood glucose, contrary to muscle glycogen that cannot be used for the same purpose. Glucose is the main, and under normal conditions the only, substrate used by the brain and other glucose-dependent tissues. The average weight of the liver is 1.5 kg, with an approximate storage of 75-110 g of hepatic glycogen in the adult human in a post-absorption state. The liver has a proportionally higher concentration of glycogen than skeletal muscle.
Glycogen is broken down in the liver to glucose and then released into the bloodstream. This reserve lasts about 12 hours (variable period depending on the saturation of the reserve), after which the concentration of glucose in the blood lowers causing weakness, listlessness, and deconcentration. During night fasting, the liver releases glucose into the bloodstream due to the lack of external carbohydrate intake for several hours, in synergy with the release of fatty acids from adipose tissue, to provide a source of energy to tissues. At the end of the overnight fast, hepatic glycogen levels are greatly reduced to approximately around 20 g.
The tissues that have priority in glucose consumption are the so-called glucose-dependent tissues, which, contrary to other body tissues, are not able to exploit lipids for energy purposes. Nervous tissue is cited as one of the main tissues requiring glucose, although under critical conditions it is able to exploit ketone bodies, unlike other systems. It is estimated that the brain under resting conditions consumes about 0.1 g of glucose per minute. During exercise, glucose utilization by extramuscular tissues does not change much.
The kidneys are also capable of accumulating glycogen and releasing it into the bloodstream, but quantitatively this is less important than hepatic glycogen.