If you have diabetes, or if your doctor suspects that you may have diabetes, he or she may have you do a fasting blood test.  Why do you have to fast before a diabetes test, when other blood tests may be run at any time?  To understand this, it is important to understand diabetes as an illness and its relationship to blood sugar, or blood glucose.

Diabetes, Food, and Sugar

diabetes-testWhen a person has diabetes, their body does not respond in the correct manner to insulin, which is a hormone produced by the pancreas.  This hormone regulates the processing of glucose in the cells.  A person with diabetes either does not produce enough insulin, or their cells resist the effects of insulin.  Therefore, the levels of unabsorbed glucose in the blood rise, which has adverse effects on the body.  For example, it can cause increased need to urinate–the body needs to rid itself of the glucose somehow, and so it stimulates the bladder to remove the excess glucose via urination.  This is one of the tell-tale signs that a doctor will look for when diagnosing diabetes.

Since diabetes and the hormone imbalance is directly tied to sugar, this, in turn, is affected by diet.  Glucose is the fuel for the body–converted into ATP to power cells.  All foods must be broken down into glucose eventually; however, foods that are more complex take longer to be broken down.  Foods that are simple carbohydrates: such as bread, rice, and anything sugary, are broken down by the body into their sugar components very quickly.  If a person with diabetes eats too many of these carbohydrate-heavy foods, it is like a thousand extra cars entering an already full parking lot.  The body cannot process the sugars properly, and they build up very quickly.

Fasting Blood Sugar Testing

Therefore, to get the most accurate reading of a person’s blood sugar baseline, a blood test to determine glucose levels must be run on blood that has not been “fed” with glucose recently.  By fasting for about 12 hours before the blood test, the patient ensures that the level being measured in the laboratory is their base blood glucose level, unaffected by any extra glucose.  If the blood glucose level comes back as above a certain level, this indicates that the glucose is not being properly absorbed by the body and diabetes will be diagnosed by the doctor.

To consider it another way, pretend that you have to measure the clarity of water in a bowl.  In order to judge how clear it is, you would want it to be as free of contaminant as possible.  Before the test, would you squirt some food dye in, and then attempt to judge the clarity?  Of course not, because the food dye taints the water and skews the results of the test.  Similarly, any food taken in and broken down into sugar by the body will skew the result of a blood test.  This is why fasting before a diabetes test is so important.

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