Several of the human congenital defects of glycogen metabolism are listed in Table 16.2. These conditions, which are called glycogen storage diseases, usually result from storage of abnormal quantities of glycogen or storage of glycogen with abnormal properties.

Deficiencies of enzymes, such as glucose-6-phosphatase or glucose-6-phosphatase translocase, which are only peripherally related to glycogen metabolism, affect the relative levels of glucose and glycogen because their deficiency can drastically alter the normal metabolism of glucose. Thus, liver cells lacking glucose-6-phosphatase are unable to make glucose for release into the bloodstream and instead convert the accumulating amounts of G6P to G1P and metabolize that to glycogen.

Similarly, glycogen storage diseases may result from deficiencies of or defects in the debranching or branching enzymes. Individuals deficient in debranching enzyme can degrade glycogen only until branch points are reached and no farther. Individuals with defective debranching enzyme accumulate glycogen with very short outer branches in their livers. Finally, individuals with defective branching enzyme accumulate glycogen with very long outer branches.