Insulin is a hormone produced by beta cells in a part of the pancreas known as the islets of Langerhans. Glucose is the fuel that provides energy for cells throughout your body. Insulin controls how much glucose the liver produces and also helps to move glucose from the bloodstream into your cells, where it is needed as a source of energy. Here's a simple explanation of this complex process.

Insulin performs many crucial functions, none more important than its role in allowing your body to turn food into energy. Your intestine breaks down dietary carbohydrates into glucose, a simple sugar that serves as your body's main source of fuel. When glucose is absorbed through the intestine, blood glucose levels rise. This causes beta cells in the pancreas -- an organ that lies behind the stomach -- to produce and secrete insulin.

Insulin circulates in the blood, eventually attaching to receptor sites on cells. This signals the cells to increase the number and activity of proteins called "glucose transporters," which remove glucose from the blood by carrying it across the cell membrane to the inside of the cell.

What happens to the glucose once it's inside the cell? That depends on the type of cell. For instance, muscle cells burn glucose to make energy. Fat cells, on the other hand, convert glucose to triglycerides and store the triglycerides for later use.

As blood glucose levels return to normal, the pancreas produces less insulin. However, beta cells continue to make small amounts of the hormone to prevent blood glucose from rising.

Diabetes occurs when there are insulin problems. The insulin may be missing entirely (type 1 diabetes) or just not working properly (usually type 2).

In people with type 1 diabetes, the immune system destroys the pancreas' beta cells, resulting in a near or total loss of insulin. Type 2 diabetes occurs when the body's tissues become more resistant to insulin, so that the pancreas must produce a greater volume of the hormone in order to usher glucose into cells. Over time, this phenomenon, known as insulin resistance, may become so severe that the pancreas can no longer keep up with demand.

Left untreated, either form of the disease causes glucose to build up in the blood, damaging organs and other tissues. All people with type 1 diabetes, as well as some individuals with type 2 diabetes, must inject themselves with insulin to prevent hyperglycemia (excessive blood glucose levels). Many people with type 2 diabetes take oral medications to increase insulin production in the pancreas.