Glycogen is a polysaccharide molecule stored in animal cells along with water and used as a source of energy. When broken down in the body, it is transformed into glucose, an important source of energy for animals. In animals, it plays a role similar to that played by starch in plants. A great deal of research has been done on this molecule and its role in the body ever since it was recognized as a critical part of the body's energy storage system.
Animal derive this molecule from carbohydrates, producing it in the liver, muscles, and digestive tract during the digestion process. Glycogen is stored in muscle tissue and in the liver, with levels tending to peak immediately after a meal. In humans, the body can store around 2,000 kilocalories of it at any given time. When people eat, levels are refreshed, with the body working to keep the amount as stable as possible so that there is a steady supply of energy.
Storage of this molecule is less efficient than that for fatty acids, which might lead some to wonder why the body doesn't store all energy in that form. There are several reasons for animals to engage in glycogen storage despite the questionable efficiency. The first is that the brain needs glucose, so requires energy reserves that will supply its needs. The second is that this molecule is used to regulate glucose levels in the blood between meals.
Athletes can experience a situation in which their reserves are depleted. This occurs in endurance activities, in which the body slowly uses up its supplies over the course of an event like a marathon. When this point is reached, it is sometimes referred to as “hitting the wall,” thanks to the strain it puts on the body. The athlete's size and condition have an effect on when she or he will hit the wall. Athletes attempt to avoid this by carb loading before events, and they also eat quickly after events to rebuild their reserves.
Some people have conditions known as glycogen storage diseases. Such conditions are usually genetic in nature, caused by problems with the genes which regulate the process of its creation and storage. People may also have trouble breaking down the molecule into glucose. Individuals with these conditions can experience a wide variety of health problems, depending on the type of disease they have, and how early it is identified.