When you schedule blood work for tests regarding your health condition, you may be instructed to fast ahead of time. This clears your system of dietary elements that might interfere with the outcome of tests to measure blood concentrations of glucose, hormones or other substances in question. Your doctor will provide guidelines for fasting before your particular test. Consider that anything you ingest -- even noncaloric beverages -- can affect your metabolism and your blood work results.
Definition of Fasting
Fasting is withholding food for a period of time to remove the effects of digestion or specific compounds in foods from your body chemistry. Water is allowed during a fast because it is essential to the balance of fluid and electrolyte minerals in cellular tissue and blood that is needed to sustain normal body function. Other than water, however, fasting is absolute. You can't eat or drink "just a little something" without changing the nature of your internal condition. Whatever you ingest may sway the test outcome and your doctor's accurate interpretation of your medical condition.
Tests That Require Fasting
Blood work may be prescribed by your doctor as a baseline reading of your normal blood composition or to diagnose or rule out a suspected health problem. The tests for which you might have to fast may or may not be related to your dietary habits. Levels of blood glucose, triglycerides and cholesterol may be checked individually or included in groups of tests such as lipoprotein or basic metabolic panels. You may be instructed to fast prior to a blood draw for a non-fasting assessment such as a complete blood count if the same sample will be examined for other purposes.
Normal Dietary Preparation
Your health care provider may want to measure the effects of your regular diet on your blood composition first and remove that variable for a second test. Follow any detailed instructions regarding when to eat normally and when to stop eating. You might be allowed to eat a full meal and drink regular or diet soda the night before your blood is drawn up until a certain time, depending on when your test is scheduled. This gives your body enough blood sugar to function during the eight to 12 hours that are typically required for fasting.
Fasting Dietary Preparation
Eat nothing and drink only water during the eight- to 12-hour fasting period. Beverages are considered food, even those such as diet soda, coffee and tea that contain few or zero calories. These still contain compounds that affect your body and your blood work. The artificial sweeteners in diet sodas, in particular, may have unknown effects on your blood glucose or digestive rate. While the artificial sweeteners in food products are approved by the Food and Drug Administration, they may or may not have been extensively studied in human clinical trials.
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