why is it necessary to fast before blood work
Fasting blood glucose test. Do not eat or drink anything except water for 8 to 10 hours before a fasting blood glucose test. These are used to diagnose diabetes, a condition caused by too much glucose (sugar) in the blood.
The two most common tests that require you to not eat are the blood sugar test and the blood cholesterol test. The blood sugar test requires that you don't eat because eating prior to the test will raise your blood sugar. If you have a large carbohydrate meal prior to the test your sugar will be high. This is normal.