Carbohydrates account for a major portion of the human diet. These carbohydrates are composed of three principal monosaccharides: glucose, fructose and galactose; in addition glycogen is the storage form of carbohydrates in humans. The failure to effectively use these molecules accounts for the majority of the inborn errors of human carbohydrates metabolism.Glycogen storage diseases are enzyme deficiencies that impair glycogen synthesis, glycogen degradation, or glycolysis. The two organs most commonly affected are the liver and the skeletal muscle. Glycogen storage diseases that affect the liver typically cause hepatomegaly and hypoglycemia; those that affect skeletal muscle cause exercise intolerance, progressive weakness and cramping.Inborn errors of carbohydrate metabolism are the inborn error of metabolism that affect the catabolism and anabolism of carbohydrates. An example is lactose intolerance.