Glycogen is a sugar based molecule that acts as a secondary store of energy in addition to fat. It consists of a base of a protein, glycogenin, and thousands of branches made up of individual sugar molecules. When glycogen breaks down, it releases glucose. About 1-2% of the weight of muscle consists of glycogen. However, most glycogen is stored in the liver where it can be released to serve the needs of the rest of the body's organs. The amount of glycogen in the liver varies, but after a meal, it can make up almost eight percent of the weight of the liver. A smaller amount of glycogen is found in blood cells and the kidneys. During pregnancy, large amounts can be found in the uterus to nourish the developing fetus.

The body requires energy to make glycogen, but some of that energy is released back into the body's tissues when glycogen is broken down.

The formation of glycogen is an important part of the metabolism. Glycogen will only be formed if both glucose and insulin are plentiful in the bloodstream. As such, both diabetes mellitus and hypoglycemia result in abnormal formation and breakdown of glycogen. In addition, several genetic diseases result in deficiencies in the body's ability to store glycogen.

Glycogen can also be exhausted through intense exercise and is a particular threat to marathon runners and long-distance cyclists. When glycogen is depleted, extreme fatigue results.

Glycogen at Wikipedia

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