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The main source of energy for your body is a kind of sugar known as glucose.
Your blood carries glucose through your body. Glucose is necessary to nourish your cells. When you have diabetes, however, your body has trouble using glucose for energy.
Your blood glucose level is the measurement of how much glucose there is in your bloodstream. Usually this test is done when you are fasting—in other words when you have not eaten for at least 8 hours. Normal fasting blood glucose levels are less than 100 mg/dl. If you’ve eaten, your glucose level will be higher but it still should be less than 140 mg/dl.
A related condition is hypoglycemia. In this case, a person's blood sugar drops too low.