Glycogen is the storage form of glucose, the source of human energy derived from carbohydrates consumed through food. To assist in the storage process, molecules of glucose, a sugar composed of carbon, oxygen, and hydrogen atoms, are strung together to form glycogen, a complex molecule known as a polysaccharide.
The skeletal muscles and the liver are the two chief storage facilities for glycogen. Approximately 1% of muscle mass is glycogen; between 8% and 10% of the liver's weight is stored glycogen. The skeletal muscles store two times as much glycogen as does the liver.
The breakdown of stored glycogen, and the further utilization of glucose, is a process known as glycogenolysis. When carbohydrates are first consumed, the digestive process creates useful units of glucose, whose presence is a signal to the body that is registered at the pancreas, the organ responsible for the monitoring of glucose levels in the bloodstream. The recognition of the glucose presence triggers the production of insulin, a hormone created in the pancreas for the regulation of the amount of glucose sugar present in the bloodstream. Excess glucose is subsequently directed to the liver to be stored as glycogen.
The body has a complex regulatory mechanism in which the liver is prompted to release glycogen in its glucose form when required to balance blood sugar levels. Muscle-stored glycogen is not so flexible in terms of its deployment in the body; once stored in a muscle, the glycogen is not capable of being shared with or transported to other areas that might require fuel. Muscle glycogen must be used at the point of storage.
Once reconverted to glucose, a series of chemical reactions will take place; the single glucose molecule interacts with phosphate compounds to ultimately generate two molecules of adenosine triphosphate (ATP), the ultimate body fuel source. Very small amounts of ATP are present in the skeletal muscles at any given time, sufficient for the generation of power in circumstances in which the anaerobic alactic energy system will be required. Such events are almost exclusively immediate activities that last less than 10 seconds. In all other circumstances, ATP must be manufactured through glycogenolysis.
Seventy-five percent of the glycogen available to the body through carbohydrate consumption, in its energy-convertible glucose form, is used to service the energy requirements of the brain and the central nervous system. The balance of glucose stores is directed to the purposes of erythrocyte (red blood cell) formation, skeletal muscle development, and the function of the heart muscle.
The relationships that exist between glycogen and athletic performance are straightforward, and each may be summarized as follows:
- The greater the ability of the body to store glycogen, the greater the ability to carry out physical tasks.
- The lower the levels of glycogen present in the body, the less intensity with which the athlete can perform or train, and the lesser amount of work time will be available to the athlete.
- The average total storage of reserves of glycogen will last a typical adult person between 12 and 14 hours; when the adult person is engaging in exercise of a moderate level of intensity, such as marathon running, the glycogen supply will be exhausted in approximately two hours of activity. In a marathon, "hitting the wall," the feeling of a pronounced loss of energy and fatigue, is in part a function of glycogen depletion.
- When the body has sustained a complete or a near-total depletion of its glycogen stores, it will take approximately 24 hours for the body to both ingest sufficient food of the appropriate carbohydrate proportion, as well as convert the ingested carbohydrates into glycogen.
There are a number of mechanisms employed by elite athletes to increase the ability of their body to store greater amounts of glycogen. One such method is commonly known as "carbo loading," whereby the athlete begins to consume large carbohydrate-rich meals as endurance training is tapered in anticipation of a key competition. This process tends to effectively increase the amount of glycogen stored in the body, and thus aids performance, so long as the athlete does not sustain a weight gain due to too severe a tapering of training.
A reverse method, which is employed by some endurance athletes, is to reduce the intake of carbohydrates during training, with a corresponding reduction in glycogen, as a stimulation to the body to make a maximum use of available fat stores.