There are several different types of Glycogen Storage Diseases, but all of them are considered inherited metabolic disorders. A metabolic disorder is a disease that disrupts metabolism. Metabolism is the process by which our body breaks down the food we eat and converts it to energy. A person who has a metabolic disorder has a difficult time breaking down certain foods and creating energy. A metabolic disease is most frequently caused by an absence or deficiency in an enzyme (or protein). An enzyme can act to help the body break down food into energy. There are many enzymes in the body and each act like a machine on an assembly line. When one of the enzymes is not working properly, the process of breaking down of specific foods can go more slowly or shut down completely. (http://www.agsdus.org/html/whatisglycogenstoragedisease.html)
The underlying problem in all of the glycogen storage diseases is the use and storage of glycogen. Glycogen is the storage form of glucose (sugar). To briefly review metabolism, a simple form of sugar (glucose) is our bodies’ main source of energy. After we eat, we have too much glucose in our blood, so our bodies store the extra glucose in the form of glycogen (much like we deposit our extra money in a bank). When our bodies need more energy, certain enzymes convert the glycogen back to glucose and withdraw it from the liver and the muscles (just like we withdraw spending money from the bank). Glycogen is a complex material made of individual glucoses linked together in long chains with many branches off the chains (just like a tree). Sometimes GSDs are also referred to as glycogenoses because they are caused by difficulty in glycogen metabolism.
Glycogen is mainly stored in the liver and muscle cells, but the kidneys and intestines also store some limited amounts of glycogen.
A person with a Glycogen Storage Disease (GSD) has an absence or deficiency of one of the enzymes responsible for making or breaking down glycogen in the body. This is called an enzyme deficiency.