A fasting blood sugar level is usually ordered by a physician either to check for a new diagnosis of diabetes or to monitor a person who is known to have diabetes. Ideally fasting blood sugar is tested shortly after you get up in the morning, 8 to 12 hours after eating or drinking anything other than water. The normal range is from 70 to 99 mg/dL. Levels above 100 mg/dL may indicate impaired glucose metabolism. Various factors can affect fasting blood sugar levels.
Any foods eaten within 8 hours of the test may cause glucose levels to be elevated. After food is digested, higher levels of glucose remain in the blood for some time. Alcoholic beverages consumed even the night before the test may cause a drop in blood sugar.
Medications such as corticosteroids, estrogen -- present in birth control pills, some diuretics, certain antidepressants, anti-seizure medication and even plain aspirin can increase glucose levels. Glucose levels can be decreased by medications that include insulin, oral hypoglycemic agents, anabolic steroids and even acetaminophen.
Exercise can cause an increase or a decrease in blood sugar levels. During exercise, insulin becomes more efficient. This effect can persist, lowering blood sugar levels for hours afterward. An hour of afternoon exercise may lower glucose levels until the next morning, affecting the fasting blood sugar test. Exercise can also affect glucose levels by releasing adrenaline. This raises blood sugar temporarily. Physical exertion or other activities that cause excitement may increase fasting sugar levels if performed shortly before the test.
Many medical conditions can affect blood sugar levels, such as liver disease, disorders of the pancreas and disorders of the thyroid gland. Acute and severe trauma -- such as major surgery, a heart attack or an automobile accident with injury can negatively affect glucose metabolism and result in blood levels far above 100 mg/dL, even during fasting.
Impaired Glucose Tolerance and Diabetes
Fasting blood glucose levels are elevated in disorders of glucose metabolism such as diabetes. Gestational diabetes is abnormal glucose metabolism that occurs in pregnant women. It almost always resolves shortly after the child is born. Fasting glucose levels between 100 mg/dL and 125 mg/dL are often indicative of pre-diabetes or impaired glucose tolerance (IGT), according to the criteria of the American Diabetes Association. Fasting sugar levels above 126 mg/dL are indicative of diabetes. Weight loss, proper diet and exercise can bring fasting levels down over time, even in a person who has been diagnosed with IGT or diabetes.
- American Diabetes Association: Diagnosing Diabetes and Learning About Prediabetes
- Journal of Diabetes Science and Technology: Factors Affecting Blood Glucose Monitoring -- Sources of Errors in Measurement
- Journal of Diabetes Science and Technology: Blood Glucose Regulation during Prolonged, Submaximal, Continuous Exercise -- A Guide for Clinicians