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, according to the Mayo Clinic.
Blood sugar, also known as glucose, is the body's main source of energy. A fasting blood sugar level is a measurement of how much glucose is in a person's blood after 12 hours without eating or drinking anything other than water, according to the Cleveland Clinic. That's why this simple blood test is usually done in the morning, before breakfast. If your fasting blood sugar is high, it is likely a sign of diabetes.
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In order to understand what can affect a fasting blood sugar level, it's helpful to know a bit more about how the body processes glucose.
The body gets most of its glucose supply from metabolizing the carbohydrates in food, per Kaiser Permanente. Insulin, a hormone produced by the pancreas, helps move glucose out of the bloodstream and into the cells, lowering blood sugar levels in the process. But people with diabetes have systems that either don't make enough insulin or can't use the insulin that they do have effectively. This leads to high blood sugar, or hyperglycemia.
Fasting Blood Sugar Test Results
Here are the possible outcomes of a fasting blood sugar test, according to the Mayo Clinic:
- Normal fasting blood sugar: less than 100 milligrams per deciliter (mg/dL)
- Impaired fasting glucose (aka prediabetes): between 100 and 125 mg/dL
- Diabetes: greater than 126 mg/dL
It takes two test results above 126 mg/dL to receive a diabetes diagnosis. If your first fasting blood glucose test is high, repeat the test to confirm, or have a related check (such as the A1C test) done on the same day. If that second test is also high, then it confirms the diabetes diagnosis.
Before getting any blood work done, talk to your doctor about how things like medications you take or your favorite foods could alter your test results. Here are a few things that affect fasting blood glucose to know about.
Steroids and anti-psychotic medications can cause higher-than-normal blood sugar levels, according to the American Diabetes Association (ADA). Hormonal contraceptives can elevate blood sugar too, according to the University of Colorado.
Other meds may cause low blood sugar, or hypoglycemia. Most of these are diabetes medications such as insulin injections. But in rare cases, certain heart arrhythmia drugs, pain relievers and antibiotics can also cause low blood sugar, according to the U.S. National Library of Medicine.
Before taking a fasting blood sugar test, be sure to tell your doctor about any and all medications you may be taking — including over-the-counter medications and herbal supplements.
Foods and Drinks
Of course, any food eaten within 12 hours of your test can cause high blood sugar. But dehydration can also raise blood sugar levels, according to the Centers for Disease Control and Prevention (CDC). In short, less water in the body means a higher concentration of blood sugar.
On the other hand, drinking alcohol the night before your test can lower blood sugar the next morning, according to the National Library of Medicine. This is because alcohol prevents the liver from producing glucose, which the body uses for energy when it's not getting glucose from food.
Read more: 7 Foods That Won't Cause Blood Sugar Spikes
Exercising before a fasting blood sugar test can cause blood sugar levels to be either higher or lower than normal. According to the ADA, exercise increases the body's ability to use insulin effectively (known as insulin sensitivity), which helps lower blood sugar. Being active also means that the body needs more fuel, which prompts cells to take in more glucose, regardless of how much insulin is available. This also reduces blood sugar levels.
Discuss your exercise plan with your doctor before taking a fasting blood glucose test, as physical activity can lower blood glucose levels for up to 24 hours.
Exercise can also temporarily raise blood sugar, according the Joslin Diabetes Center. During exercise, the body releases extra glucose to keep up with the higher energy demand. In people with diabetes, that extra glucose can remain in the bloodstream due to impaired insulin function, causing hyperglycemia. So if you exercise right before your test (i.e., going for a morning jog) your numbers may be elevated.
When the body is stressed — by any number of circumstances, including serious illness, infection or any type of emotional stress, according to the University of California, San Francisco (UCSF) — it prepares itself for any situation by making sure it has plenty of available glucose. It does this by lowering insulin levels while raising glucagon and adrenaline levels (two hormones involved in raising blood sugar), per UCSF. Muscle and fat cells also become less sensitive to insulin. As a result of these changes, more glucose stays the blood, potentially causing hyperglycemia.
Lack of sleep (a form of stress) can also affect fasting blood sugars, according to Samar Hafida, MD, an endocrinologist at Harvard's Joslin Diabetes Center. Not getting enough sleep has "the potential to give a false fasting blood sugar reading," Dr. Hafida says. "People who don't sleep well or who work night shifts [and will therefore be on short sleep for their test] could be affected."
Underlying Health Conditions
A number of illnesses or physical conditions can elevate or lower blood sugar levels. These include hormonal deficiencies and infections, according to the Cleveland Clinic. In rare cases, conditions affecting the liver can also cause low blood sugar, per the Mayo Clinic.
- Mayo Clinic: "Diabetes - Diagnosis and Treatment"
- Cleveland Clinic: "Blood Sugar Tests"
- Kaiser Permanente: "How our bodies turn food into energy"
- American Diabetes Association: "Understanding Blood Sugar and Control"
- University of Colorado: "Diabetes & Birth Control"
- National Library of Medicine: "Drug-Induced Low Blood Sugar"
- American Diabetes Association: "Blood Sugar and Exercise"
- Joslin Diabetes Center: "Why Do Blood Glucose Levels Sometimes Go Up after Physical Activity?"
- University of California, San Francisco: "Blood Sugar and Stress"
- Cleveland Clinic: "Hyperglycemia (High Blood Sugar)
- Mayo Clinic: "Hypoglycemia"
- CDC: "10 Surprising Things That Can Spike Your Blood Sugar"
- National Library of Medicine: "Diabetes and Alcohol"
Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.