Insulin is a peptide hormone produced by β cells of the pancreatic islets, and by the Brockmann body in some teleost fish. It has important effects on the metabolism of carbohydrates, fats and protein by promoting the absorption of, especially, glucose from the blood into fat, liver and skeletal muscle cells.
When the pancreatic β cells are destroyed by an autoimmune process, insulin can no longer be synthesized or be secreted into the blood. This results in type 1 diabetes mellitus. Combined influence of genetic susceptibility and environmental factors result in insulin resistance in cells. This is characterized by type 2 diabetes.