Glycogen storage disease type 0 is a disease characterized by a deficiency in the glycogen synthase enzyme. Although glycogen synthase deficiency does not result in storage of extra glycogen in the liver, it is often classified with the Glycogen storage diseases because it is another defect of glycogen storage and can cause similar problems. Mutations in the liver isoform, GYS2, causes fasting hypoglycemia, high blood ketones, increased free fatty acids and low levels of alanine and lactate. Conversely, feeding in these patients results in hyperglycemia and hyperlactatemia.
There are two versions: the muscle version involves GYS1 and the liver version involves GYS2.