Forbes disease (GSD-III) is one of several glycogen storage disorders (GSD) that are inherited as autosomal recessive traits. Symptoms are caused by a lack of the enzyme amylo-1,6 glucosidase (debrancher enzyme). This enzyme deficiency causes excess amounts of an abnormal glycogen (the stored form of energy that comes from carbohydrates) to be deposited in the liver, muscles and, in some cases, the heart.
There are two forms of this disorder. GSD-IIIA affects about 85% of patients with Forbes disease and involves both the liver and the muscles. GSD-IIIB affects only the liver.