Insulin's function is to help regulate glucose levels in the blood by transporting glucose into the cells of the human body. If a person doesn't produce enough insulin or doesn't respond well to insulin, he may have diabetes.
Insulin greatly controls the concentration of glucose in the body. When there is too much glucose, circulating insulin signals to liver, muscle and adipose cells to absorb the excess glucose. These cells use the glucose for energy. If a body has low blood glucose levels, insulin can communicate with the liver and signal it to release glycogen, which is the stored form of glucose.