Fasting sugar levels are typically measured by what is described as the fasting blood glucose, otherwise known as FBG. The normal glucose range for fasting is from 70-99. If you have type 1 or type 2 diabetes, you likely need to maintain a lower level, usually 35-80.
The two major causes of high fasting blood sugars are insulin resistance due to obesity and increased production of insulin in response to a meal (excess postprandial hyperglycemia).
Excess postprandial hyperglycemia is often referred to as "rebound" because it can cause blood sugar levels to rapidly rise after an overnight fast and during exercise.
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