Insulin molecules, illustration. Insulin is a protein hormone produced in the pancreas that regulates the metabolism of carbohydrates, proteins and fats. It does this by promoting the uptake of glucose from the blood into liver cells, skeletal muscle cells and fat cells. Disruption of insulin function can lead to diabetes. The insulin molecule is depicted in four styles: ball and stick (top left), ribbon (top right), line (bottom left) and molecular surface (centre).