Glucose is a major source of energy for the body. It is stored in the form of glycogen in both the liver and muscles and later released with the help of enzymes. Persons affected by GSD have an inherited defect in one of the enzymes responsible for forming or releasing glycogen as it is needed by the body during exercise and/or between meals. GSD can affect the liver, the muscles or both.
There are about eleven known types of GSD. All forms of GSD, except some forms of the liver phosphorylase kinase deficiency (GSD IX), occur when a child inherits the affected gene from both parents (autosomal recessive inheritance) each of whom is a carrier but not affected them selves. This means that for each pregnancy there is a 1 in 4 chance that the child will inherit both defective genes and thereby be affected.