Insulin
Insulin is a hormone that is primarily responsible for carbohydrate metabolism. It is produced in the pancreas, and is secreted into the bloodstream to deal with homeostatic control of blood glucose. Insulin shuttles the nutrients to the target cells, so is responsible for ensuring we actually receive the nutritional benefit from our foods. Insulin is also the hormone responsible for fat storage, so is ideally kept under control with suitable food choices.
Sufferers of Type 1 Diabetes Mellitus usually have to inject insulin as they don not produce their own. Patients with Type 2 Diabetes Mellitus have decreased insulin sensitivity, often (but not always) down to poor nutritional choices and little exercise.