General principles of glucose homeostasis
Glucose is derived from dietary carbohydrates, glycogenolysis (breakdown of glycogen, which is a storage form of glucose), and gluconeogenesis (formation of glucose by the liver). Insulin, glucagon, and other hormones maintain normal plasma glucose levels.
High glucose stimulates insulin production. Insulin enhances glucose uptake, use, and storage. As a result, blood glucose returns to normal.
Fasting lowers blood glucose and insulin. It also results in the release of counter-regulatory hormones (i.e., glucagon, epinephrine, cortisol, and growth hormone). These hormones limit glucose use and stimulate hepatic glucose production. In addition, lack of insulin causes lipolysis in adipose tissue, proteolysis in muscle, and ketosis in the liver. Ketones are an alternative energy source and its formation is glucagon dependent