Fasting blood sugar levels give vital clues about how a person is managing their blood sugar. Blood sugar tends to peak about an hour after eating and declines after that.

High fasting blood sugar levels point to insulin resistance or diabetes, while abnormally low fasting blood sugar can be due to diabetes medications.

Knowing when to test and what to look for can help people stay healthy, especially if they have diabetes or are at risk of developing the condition.

Keep reading to learn more about fasting blood sugar levels, including information on testing and tips for maintaining normal levels.

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The body needs glucose for energy, and glucose comes from the food a person eats. However, the body does not use all of this energy at once. Insulin makes it possible to store and release glucose as necessary.

Following a meal, blood sugar levels rise and usually peak about an hour after eating.

How high blood sugar rises and the precise timing of the peak depends on a person’s diet.

Food-related factors that can trigger significant rises include:

  • eating large meals
  • consuming sugary foods and drinks
  • eating foods with simple carbohydrates, or carbs, such as bread and sweet snacks

As blood sugar rises, the pancreas releases insulin. Insulin lowers blood sugar, breaking it down so the body can use it for energy or store it for later.

However, people who have diabetes have difficulties with insulin in one of two ways.

Those with type 1 diabetes do not produce enough insulin because their body attacks its insulin-producing cells. Those with type 2 diabetes do not respond well to insulin in their body and, later, may not make enough insulin.

In both cases, people experience high blood sugar levels and have difficulty using glucose, or blood sugar.

Fasting blood sugar depends on three factors:

  • the contents of a person’s last meal
  • the size of their previous meal
  • their body’s ability to produce and respond to insulin

Blood sugar levels between meals offer insight into how the body manages sugar. High levels of fasting blood sugar suggest the body was not able to lower blood sugar levels.

This points to insulin resistance, inadequate insulin production, or in some cases, both.

Very low blood sugar levels may indicate that diabetes medications are lowering the blood sugar too much.

To assess fasting blood sugar levels, people or healthcare professionals can use a glycosylated hemoglobin (HbA1c) test, a conventional blood sugar test, or continuous glucose monitoring.

The HbA1c test

HbA1c is a test doctors use to assess overall blood sugar control. The HbA1c test measures how the body manages blood sugar over time, usually during the past 3 months.

A person will undergo this test at a doctor’s office or in a lab. If levels are very high, the individual may need a second test. The results appear as a percentage.

Blood sugar testing at home

A person can test their blood sugar levels at home.

In most cases, doctors ask people to measure fasting blood sugar immediately upon waking and before they have anything to eat or drink. It may also be appropriate to test blood sugar before eating or 2 hours after a meal, which is when blood sugar returns to normal levels.

The right time to test depends on treatment goals and other factors. For example, most people with diabetes do not need to test between meals unless they are using a diabetes drug that can lower blood sugar. Other people may test between meals if they feel their sugar levels may be low.

Because people with type 1 diabetes do not make enough insulin, they need to test their blood sugar levels several times a day so they can adjust their insulin doses.

A person will test blood sugar levels by:

  • preparing the testing strip and glucose monitor to be ready for the blood sample
  • using an alcohol swab to clean the testing area, which is usually the side of a fingertip
  • lancing the testing area and bracing against a firm surface to help resist the impulse to pull away
  • squeezing the testing area around the wound to maximize blood flow
  • squeezing a drop of blood onto the test strip
  • putting the strip into the monitor
  • recording the time, blood sugar reading, and recent food intake in a log

Find out more about home blood sugar testing here.

Continuous glucose monitoring

Another option for daily use is continuous glucose monitoring (CGM).

For CGM, a person wears a monitor 24 hours a day. The monitor records their blood glucose levels on an ongoing basis.

CGM can give a more accurate picture of a person’s levels and fluctuations throughout the day. However, this type of kit can be more expensive in some cases, depending on a person’s insurance coverage.

There are also nonfasting blood tests.

These include the random plasma glucose (RPG) test and oral glucose tolerance test (OGTT).

For an RPG, a doctor does a conventional blood sugar test when a person is not fasting.

For an OGTT, a healthcare professional tests a person’s blood sugar levels over 2 hours, starting with a fasting blood test. Next, the person drinks a sugary beverage, and the doctor takes a blood sample every 30–60 minutes over a 2-hour period.

Blood sugar target levels are the ideal range that people should try to maintain as often as possible.

Because blood sugar levels vary throughout the day due to factors such as medications, food intake, and physical activity, no single blood sugar reading can reveal how well someone processes sugar.

HbA1c results

According to the American Diabetes Association, the results of an HbA1c test will be one of the following:

  • normal: less than 5.7%
  • prediabetes: between 5.7% and 6.4%
  • diabetes: 6.5% and over

Prediabetes is when blood sugar levels are higher than normal levels but not high enough to indicate diabetes. People can take measures to reverse prediabetes and stop diabetes from developing. Find out more here.

Home testing

Doctors express target blood sugar numbers for people with diabetes in milligrams per deciliter (mg/dL) as follows:

  • fasting (testing in the morning before food or water): 80–130 mg/dL
  • 2 hours after eating a meal: under 180 mg/dL

Meanwhile, for people without diabetes, target blood sugar levels are:

  • fasting (testing in the morning before food or water): 72–99 mg/dL
  • 2 hours after eating a meal: under 140 mg/dL

For people with prediabetes, the goal is to reach the same target blood sugar levels as people without diabetes.

However, the target numbers will vary between people. A healthcare professional will help a person identify their target levels.

Fasting blood sugar test

Doctors consider a fasting blood sugar level of less than 99 mg/dL as normal.

On the other hand, fasting blood sugar levels ranging from 100–125 mg/dL indicate prediabetes, and levels of 126 mg/dL or above indicate diabetes.

It is vital to follow a healthy diet to keep fasting blood sugar from rising too high. Strategies include:

People who take diabetes medications and are at risk of dangerous blood sugar dips should follow a similar diet. They also must take proactive steps to prevent blood sugar from dropping. This includes:

  • eating regular meals throughout the day
  • increasing food intake and snack frequency during intense physical activity
  • avoiding or limiting alcoholic beverages
  • consulting a doctor if vomiting or diarrhea make it difficult to manage blood sugar

People are likely to experience symptoms if their blood sugar levels are too low or too high.

Low blood sugar levels

Blood sugar that is too low can cause symptoms such as:

  • shaking and sweating
  • feeling jittery
  • difficulty concentrating
  • lack of energy
  • skin that looks paler than usual
  • fatigue or tiredness
  • headaches or muscle aches
  • fast or irregular heartbeat
  • weakness
  • lack of coordination

In extreme cases, low blood sugar can trigger seizures, loss of consciousness, confusion, and an inability to drink or eat.

Find out more about hypoglycemia or low blood sugar here.

High blood sugar levels

High blood sugar levels, or hyperglycemia, can cause the following symptoms:

As with low blood sugar, high blood sugar may cause loss of consciousness or seizures if people do not get treatment. Persistent high blood sugar can increase the risk of serious complications that doctors relate to diabetes, such as cardiovascular disease.

Any significant change in blood sugar patterns warrants a visit to a doctor. People with diabetes and those at risk of diabetes should also speak with a doctor if:

  • blood sugar levels become unusually high or low
  • well-managed blood sugar levels suddenly start fluctuating
  • they have new or worsening symptoms of diabetes
  • they change their medication or stop using it
  • they experience abnormally high blood pressure
  • they develop an infection or sore that will not heal

Diabetes needs ongoing monitoring, and the treatment can change over time. Information about diet and exercise is vital for a doctor to outline a proper treatment plan for each individual.

People with diabetes can assist their doctor by keeping detailed logs of their blood sugar levels and any relevant information, such as changes in activity, medications, or sleep. They should talk with a doctor about dietary and lifestyle habits that affect their blood sugar levels.

To manage diabetes, it is important to monitor and maintain healthy fasting blood sugar levels. For people with prediabetes, managing blood sugar levels can help reverse the condition and prevent the development of diabetes.

People who have diabetes or prediabetes need to test their blood sugar levels regularly and record their results. They should talk with a doctor if they notice any changes or unusual symptoms.