Insulin is a hormone that is produced in specialized cells in the islets of Langerhans, a part of the pancreas. The main role of insulin is to regulate the body's use of sugars and other nutrients.
The process is initiated during and immediately after the process of digestion breaks down carbohydrates into sugar molecules (including glucose) and proteins into amino acids.
Right after a meal, glucose and amino acids are absorbed directly into the bloodstream, and blood glucose levels rise sharply. The rise in the blood glucose levels signals important cells in the pancreas, called beta cells, to secrete insulin, which pours into the bloodstream.
Insulin enables glucose and amino acids to enter cells in the body, particularly muscle and liver cells. Most cells of the body have insulin receptors which bind the insulin to the cell. When a cell has insulin attached to it, the cell then is able to activate the other receptors. These receptors are designed to absorb glucose from the blood stream and direct whether these nutrients will be burned for energy or stored for future use. (The brain and nervous system are not dependent on insulin; they regulate their glucose needs through other mechanisms.) Without insulin, the cells in our bodies would not be able to process the glucose and therefore have no energy for movement, growth, repair, or other functions. Insulin is the access point to unlocking the door of the cell to allow the glucose to be transferred from the bloodstream into the cell. When insulin levels are high, the liver stops producing glucose and stores it in other forms until the body needs it again.
As blood glucose levels reach their peak, the pancreas reduces the production of insulin. About 2 to 4 hours after a meal, both blood glucose and insulin are at low levels, with insulin being slightly higher. The blood glucose levels are then referred to as fasting blood glucose concentrations. (1)