What is fasting blood sugar?


As the name suggests, fasting means not eating or drinking any fluids (other than water) for several hours. Fasting blood sugar level is measured as a test for diabetes. After fasting of 8 hours, a carbohydrate metabolism test is done to measure blood glucose levels. In the early morning hours, hormonal changes in your body cause blood glucose to rise naturally. For people free from diabetes, the increase in blood glucose is reduced to no by increased insulin production. For the diabetics, glucose metabolism is a problem. The improper functioning of insulin is troublesome.

Insulin and Glucagon- The Partners in Carbohydrate Metabolism

Insulin and glucagon work together, rather antagonistically, to maintain the blood glucose level. When you eat something the carbohydrates are metabolized and glucose is carried to the various organs through the bloodstream. The excess of glucose obtained from carbohydrate metabolism is converted into glycogen and stored in the liver to be used when required.  When the blood glucose level falls glucagon converts the glycogen stored in the liver into glucose, which is carried to the body organs via the bloodstream. Thus, the carbohydrates consumed by us are properly utilized by the body and the normal blood glucose level is maintained. Insulin insufficiency or failure of the body to respond to insulin causes the diabetic condition.

Role of Glucagon During Fasting

During fasting, the hormone glucagon is stimulated which increases plasma glucose levels in the body. Glucagon allows the body to regulate the utilization of glucose and fats. Glucagon is released in response to low blood glucose levels and when the body needs additional glucose, such as during vigorous exercise and physical activities.

fasting blood sugar

If a person does not have diabetes, his body will produce insulin to rebalance the increased glucose levels. Thus, the level of blood glucose when measured is found within the recommended level.

Diagnosis of Diabetes

However people with diabetes either don’t produce enough insulin to rebalance their blood sugar (typically in type 1 diabetes) or their body is not able to use the insulin effectively enough (typical of type 2 diabetes). Thus, when blood glucose levels are tested, people with diabetes will have significantly higher blood sugar levels than people who do not have diabetes.

A blood sample is taken after an overnight fast (8 hours minimum). A fasting blood sugar level less than 100 mg/dL i.e., 70 to 99 mg/dl (3.9 to 5.4 mmol/L) is normal. A fasting blood sugar level from 100 to 125 mg/dl (5.5 to 6.9 mmol/l) is considered prediabetes. Fasting blood sugar of 126 mg/dL (7 mmol/L) or higher indicates that the person has diabetes.

The Use of Fasting Blood Sugar Test

The primary role of fasting sugar test is to detect the condition of carbohydrate metabolism in the body. The test is recommended by the healthcare experts to diagnose whether a person is having diabetes or not. The fasting blood sugar test is also used to check the effectiveness of different medication or dietary changes on people already diagnosed with diabetes.

Fasting Tests

The fasting test should be conducted on two separate occasions to ensure consistency in results and avoid a false diagnosis. The increased blood glucose levels may be as a result of Cushing’s syndrome liver or kidney disease, eclampsia and pancreatitis. However, many of these conditions are often picked up in lab diagnostic tests.

Fasting Test Results

The results of a fasting blood glucose test are as follows:

  • Normal: 3.9 to 5.4 mmol/l (70 to 99 mg/dl)

  • Prediabetes or Impaired Glucose Tolerance: 5.5 to 6.9 mmol/l (100 to 125 mg/dl)

  • Diagnosis of diabetes: 7.0 mmol/l (126 mg/dl) or above


Initially, the fasting blood sugar level was defined at 140 mg/dl. In 1997, the American Diabetes Association reduced the level of diagnosis from 140 to 126 mg/dl.

Fasting and Glucose Imbalance

Some physiological things that go on in your body can make your blood glucose rise in the morning. One of these is insulin resistance. This is a condition when your body’s muscle and fat cells are unable to utilize insulin effectively to reduce blood glucose. Not only this. Insulin resistance also affects the functioning of the liver – how it processes, stores, and releases sugar during the night.

The liver is supposed to release small amounts of glucose when we are not eating. In type 2 diabetes, the liver releases more glucose into the bloodstream than what is required, especially at night. So, while the hormones are causing the rise in blood glucose naturally, the liver adds up to the sugar level in your system. And because your insulin resistance prevents your muscle and fat cells from using the sugar, your blood glucose level rises.

fasting blood sugar

The mealtime blood glucose can be somewhat controlled by a strict diet and exercise. But, if fasting blood glucose is high it usually needs to be treated with medicines. Consultation with your healthcare expert about medications for good control of blood glucose is recommended.


Fasting blood sugar helps us to find the functioning of our carbohydrate metabolism. The ADA guidelines recommend regular screening for type 2 diabetes. In all adults aged 45 and older (who do not have prediabetes) and/or other risk factors for type 2 diabetes, screening should be done after every 3 years.

For individuals who are obese (body mass index of 25 kg/m2 or higher) and have any additional risk factors for developing type 2 diabetes, testing should be done more frequently; blood sugar testing is done yearly for individuals with prediabetes. You must consult your healthcare expert for the best advice.

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