The sugar in the blood must enter the body cells before it can be used to make energy. When there is enough insulin in the blood and on the outside of the cells, it acts like a key to open the cells for the sugar to enter. The body cells use the sugar for fuel to make energy and the level of sugar in the blood drops. The blood sugar level stays in a safe range because the insulin keeps it there. In people who have diabetes, this system doesn't work.
Sugar builds up in the blood stream instead of going into the cells of the body.
Your blood sugar level can even rise when you haven't eaten, if too little insulin is available. Without enough insulin to keep it in check, the body can make extra sugar and release it into the bloodstream. This can happen any time insulin is in short supply or is not doing its job. It's more likely to happen when the body is under stress, like sickness, an injury, or severe emotional stress. Insulin is the key to keeping your blood sugar at the right level, but it doesn't work alone.