Glucose is a large energy source for the body. It is stored by the body in the form of glycogen and later released into the body with the help of special proteins called enzymes.
There are different types of GSD. People who have GSD are born with the disease. In GSD, an abnormal amount of glycogen is stored in the liver. When a person has GSD:
- The liver cannot control the use of glycogen and glucose.
- Certain enzymes are missing that control the change of sugar (glucose) into its storage form (glycogen) or release of glucose from glycogen.
Many sugars (including glucose) are found in foods and are used by the body as a source of energy. After a meal, blood glucose levels rise. The body stores the extra glucose that is not needed right away as glycogen in the liver and muscles. Later, as the blood glucose levels in the body begin to drop, the body uses this stored energy.
These sugars, stored in the form of glycogen, need to be processed by enzymes in the body before they can carry out their functions. If the enzymes needed to process them are missing, the glycogen or one of its related starches can build up in the liver, causing problems.
There are at least 10 different types of GSDs. The types are put into groups based on the enzyme that is missing. The most common forms of GSD are types I (one), III (three) and IV (four). About one in 20,000 people can have a type of GSD.
- GSD I, also known as von Gierke disease: Results from a lack of the enzyme Glucose-6-Phosphatase.
- GSD III, also known as Cori disease: Results from a lack of the debrancher enzyme. This causes the body to form glycogen molecules that have an abnormal structure. This abnormal structure also prevents the glycogen from being broken down into free glucose.
- GSD IV, also known as amylopectinosis: There is not an increased amount of glycogen in the tissues. Instead, the glycogen that does build up in the tissues has very long outer branches. With this type of GSD, there is lack of the branching enzyme. This abnormal glycogen is thought to stimulate the immune system. The result is a great deal of scarring (cirrhosis) of the liver as well as other organs, such as muscle and heart.