Approximately 1.9 million Americans ages 20 and older were diagnosed with diabetes in 2010, according to the Centers for Disease Control and Prevention. Another 79 million were expected to have prediabetes in the same year. Keeping an eye on your blood glucose level is wise if you're at risk for either of these conditions, as is understanding what fasting blood glucose is and how it is affected by food.

Normal Fasting Glucose

Your blood glucose level is a key indicator of a problem with insulin production or blood sugar regulation in your body. The normal blood glucose level before meals, referred to as fasting blood glucose, should be between 70 and 99 mg/dL, according to registered dietitian Christine Wendt. Glucose in the blood comes from the food you consume each day and is used as an energy source. Some is stored in the liver for use during periods where you don't eat, such as during the night when you're sleeping.

Fasting Glucose Testing

Fasting glucose tests are typically performed when you've not consumed any calories -- from food or beverages -- for at least eight hours. This would normally be done first thing in the morning prior to breakfast. Knowing your fasting glucose rate before the first meal of the day is important because it provides knowledge about how your body regulates blood sugar during the night. Physicians typically order the fasting blood glucose test during the diagnostic process for both prediabetes and diabetes.

Effect of Food

What -- and how much -- you eat directly impacts your blood sugar. Consuming carbs will cause your glucose level to rise the fastest and the most significantly. This is why carbs get the most attention when it comes to diabetes management. In addition, the more food you eat, the more your blood glucose will rise. Dietary habits may seem like a concern only when doing random glucose checks or postprandial tests, which are done two hours after you begin eating a meal. Yet what you eat can impact fasting levels as well. Eating large portions or high-carb foods just before bed affects glucose regulation during the night, thereby impacting your fasting glucose results the next morning.

Abnormal Blood Glucose

A fasting blood glucose level of 126 mg/dL or above is indicative of diabetes, although other conditions can cause high blood sugar as well. Endocrine conditions, such as Cushing's syndrome, can cause hyperglycemia. Stress and cardiovascular events cause high blood sugar as well. In contrast, pituitary tumors, kidney failure, hypothyroidism, malnutrition and cirrhosis of the liver may contribute to low fasting blood sugar. Low readings are below 90 mg/dL. A level below 70 mg/dL indicates a dangerous situation requiring medical attention, warns Dr. Barak Gaster of the University of Washington Medical Center.