Living with Diabetes

Fasting Blood Sugar Levels

Fasting, as the name suggests, means refraining from eating of drinking any liquids other than water for eight hours. It is used as a test for diabetes.

After fasting, a carbohydrate metabolism test is conducted which measures blood glucose levels.

Glucagon during fasting

When fasting the hormone glucagon is stimulated and this increases plasma glucose levels in the body.

If a patient doesn’t have diabetes, their body will produce insulin to rebalance the increased glucose levels.

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).

Consequently when blood glucose levels are tested, people with diabetes will have blood sugar levels significantly higher than people who do not have diabetes.

What is the fasting blood sugar test used for?

The fasting blood sugar test is also used to test the effectiveness of different medication or dietary changes on people already diagnosed as diabetic


Fasting blood sugar levels are measured by taking a blood test after a period of fasting, usually of 8 hours without food. Typically, fasting blood glucose levels are taken in the morning before any breakfast is eaten. When having your fasting blood glucose levels take, you should not have any drinks apart from water during the period of fasting.

A fasting blood glucose test can be useful to see how well the body is able to manage blood sugar levels in the absence of food. When we do not eat for several hours, the body will release glucose into the blood via the liver and, following this, the body’s insulin should help to stabilise blood glucose levels.

A fasting blood glucose test therefore shows:

  • Whether the body is able to bring down the raise in blood glucose levels from the previous meal
  • How effectively the body copes with its own release of glucose

The target for fasting blood glucose levels are the same as the targets for before meal readings.

  • For adults with diabetes, the target level is between 4 and 7 mmol/l
  • For children with diabetes, the target level is between 4 and 8 mmol/l

However,  some people may be set individual targets, by their doctor, that differ to the above levels.

Fasting blood glucose tests for people with type 2 diabetes are useful for showing how well your body’s insulin responds to periods without food, such as overnight.

In type 1 diabetes, fasting blood glucose levels help to show whether your long term insulin, also known as background insulin, is set at the right dose. If you are in any doubt, Your healthcare team can help you to understand the levels you are getting and suggest any dosage changes.

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Fasting tests

The fasting test should be conducted on two separate occasions to ensure consistent results and in order to avoid a false diagnosis.

This is the case as 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 test with respect to glucose levels in the body are as follows:

  • Normal: 3.9 to 5.4 mmols/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 [361]

The American Diabetes Association reduced the level of diagnosis in this test from 140 to 126 mg/dl in 1997.

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