Glucagon is a hormone that opposes the action of insulin. It is secreted when blood glucose levels fall. Glucagon increases blood glucose concentration partly by breaking down glycogen in the liver. Following a meal, glucose is absorbed into the blood. In response to increased blood glucose levels, insulin is secreted causing rapid uptake, storage, or use of glucose by the tissues of the body.
Unused glucose is stored as glycogen in the liver.
Between meals, when blood glucose is at minimal levels, tissues continue to require an energy source to function properly. Stored glycogen, via glucagon, is converted to glucose by a pathway known as glycogenolysis. Gluconeogenesis is the production of glucose in the liver from noncarbohydrate precursors such as glycogenic amino acids. Since sorbitol cannot readily diffuse through cell membranes, cell edema and changes in function can ensue. An additional complication of hyperglycemia is nonenzymatic glycosylation.