Glucose is a monosaccharide which represents an essential biological energy source, enabling the generation of ATP following glycolysis. Although many tissues can also use fats and protein as an energy source, the brain and red blood cells can only use glucose. Glucose is stored in the body, importantly in the liver, as glycogen. Circulating levels of glucose are controlled by two enzymes, insulin and glucagon. In response to high glucose levels, pro-insulin is released from pancreatic beta cells in the islets of Langerhans and is converted to the active form in the blood. Insulin stimulates the uptake of glucose and storage in the tissues as glycogen (glycogenesis). The uptake of glucose into mammalian cells is facilitated by glucose transporters (Glut). The individual Glut subtypes differ in their tissue distribution, substrate specificity, kinetic properties, and intracellular localization, allowing members of the Glut family to finely regulate whole-body glucose homeostasis.
In contrast, low glucose levels cause secretion of pancreatic peptide hormone glucagon from alpha cells. Glucagon promotes the conversion of liver glycogen to glucose (glycogenolysis) and release of glucose back into the blood. During starvation and intense exercise, glucose can also be generated from non-carbohydrate precursors (i.e. pyruvate, amino acids and glycerol), in a process called gluconeogenesis. Disruption of glucose homeostasis is most commonly studied in the field of diabetes mellitus, a metabolic syndrome in which patients do not produce sufficient levels of, or correctly respond to, insulin.