The main fuel use by the cells is called glucose, a simple sugar. Glucose enters your cells through receptors. Receptors are sites on cells that accept insulin and allow glucose to enter. Once inside, glucose can be used as fuel. But glucose has difficulty entering your cells without insulin.
Excess glucose is stored in the liver and muscles in a form called glycogen.
Between meals, when your blood sugar is low and your cells need fuel, the liver glycogen is released to form glucose. The pancreas is located in the abdomen, behind the stomach. It is attached to the small intestine and the spleen. Inside the pancreas are small clusters of cells called Islets of Langerhans. Within the islets are beta cells, which produce insulin.