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Fasting Blood Sugar: Normal, High, & Low + Ways to Improve

Written by Biljana Novkovic, PhD | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Biljana Novkovic, PhD | Last updated:

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A fasting blood sugar test is one way to check for and monitor diabetes. Both high and low blood sugar levels can be dangerous. Read on to learn more about the test and causes of high or low blood sugar levels. We will also go over the best strategies to lower your blood sugar levels to successfully prevent or manage diabetes.

What is a Fasting Blood Sugar Test?

Procedure

A fasting blood sugar test is also called a fasting blood glucose test. It measures your sugar levels after fasting for at least 8 hours. Glucose is the main sugar found in your bloodstream and high levels after fasting may point to diabetes.

While fasting before the test, you can still drink water (unless otherwise directed by your doctor), but anything like coffee, tea, juice, and sodas should be avoided. A healthcare professional will draw blood for a vein in your arm, which will be sent to the lab for analysis [1].

After a meal, your blood glucose levels will normally increase over a few hours as you break down and absorb dietary carbohydrates. In healthy people, the pancreas will produce insulin to move glucose into tissues and out of the bloodstream. Thus, glucose levels will gradually drop after eating and stay low during fasting [2, 3].

People with diabetes either don’t have enough insulin to reduce blood glucose (type 1 diabetes) or their body cannot effectively use insulin (type 2 diabetes). In a fasting blood glucose test, diabetics will have much higher blood sugar levels than non-diabetics, which can lead to many health problems in the long run [4, 1].

A fasting blood sugar test measures your blood glucose levels after fasting for at least 8 hours. High levels may point to diabetes.

Why Doctors Order It

A fasting blood glucose test is done to screen for prediabetes and diabetes. It also helps doctors monitor diabetes and determine if medications and dietary changes are having an effect [1].

Your fasting blood glucose level is the lowest your blood sugar can be because the influence of recent meals is minimized [5, 6].

Fasting blood sugar is often checked alongside HbA1c (also known as glycated hemoglobin), which is a measure of your blood sugar levels over the past three months. The more sugar there is in the blood, the more it will attach to hemoglobin, raising HbA1c [7].

Taken together, the two offer more information than each test alone. For example, even if your blood sugar levels are very high at one random point, they may have been mostly low over the past couple of months (low HbA1c). On the other hand, HBA1c could be higher but your glucose levels at the moment lower.

Apart from diabetes, abnormal levels can point out issues such as insulin resistance, hormonal imbalances, and pancreatic, liver, or kidney disease.

Doctors order a fasting blood sugar test to screen for and monitor diabetes and prediabetes, usually alongside HbA1c.

Who Should Do the Test?

According to the American Diabetes Association, screening for diabetes is recommended in people over 45 (every 3 years), or at any age if you have certain risk factors, including [8]:

  • Being overweight, obese, or physically inactive
  • Having a close (first or second degree) relative with diabetes
  • Belonging to a certain race/ethnic group (Native Americans, African-Americans, Hispanic Americans, Asians/South Pacific Islanders)
  • Having signs of insulin resistance or conditions associated with insulin resistance, such as high blood pressure (hypertension), low good cholesterol and/or high triglycerides (dyslipidemia), and polycystic ovary syndrome
  • Having had diabetes in pregnancy (gestational diabetes)

Blood Sugar Ranges

Normal Range

Normal fasting blood glucose levels in healthy people are 70 – 125 mg/dL or 3.9 – 6.9 mmol/L.

Low blood sugar or hypoglycemia is when blood levels drop below 70 mg/dL in diabetics or around 55 mg/dL (3 mmol/L) in healthy people.

High blood sugar or hyperglycemia is when your blood glucose levels are too high, above 126 mg/dL or 7 mmol/L.

The normal fasting blood sugar range in healthy people is 70 – 125 mg/dL.

Prediabetes Range

Prediabetes (also known as impaired fasting glucose) refers to blood glucose levels that are consistently on the higher end of the normal range – between 100 – 125 mg/dL (5.6 – 7 mmol/L). Prediabetes is dangerous because it can easily develop into diabetes [9].

Prediabetes means that you almost have diabetes, but you shouldn’t despair. Prediabetes is reversible. By making lifestyle changes early on, you can prevent diabetes. If you don’t make any changes, however, studies suggest that there’s about a 40% chance that you will get diabetes four years down the line [10].

If your fasting blood sugar is 100 – 125 mg/dL, you may have prediabetes.

Prediabetes Range

Levels above >126 mg/dL or >7 mmol/L are in the range of diabetes. At least two values in this range on two separate tests are needed for your doctor to diagnose diabetes.

Glucose Meter

Being able to self-monitor your blood glucose levels is very important if you have diabetes. With a blood glucose meter, you can regularly check your sugar levels and modify your lifestyle and meals accordingly. Meters let you track fluctuations caused by exercise, food, medications, stress, and other factors [11].

You can choose from a broad range of blood glucose meters with varying accuracy. Meters that have very high accuracy tend to be more expensive. Some meters can now be integrated with a smartphone, and other newer models may not require finger pricking (some can be inserted under the skin) or use tiny and almost painless needles [11, 12].

Fasting blood glucose and HbA1c tests are more accurate, but blood glucose meters are easy to use, accessible, and indispensable for checking your sugar levels daily.

Be sure to get an accurate glucose meter to monitor your sugar levels regularly if you have diabetes.

High Fasting Blood Sugar

For the purposes of this article, we will treat blood sugar in the range of prediabetes and above (>100 mg/dL or >5.6 mmol/L) as high.

Causes

Causes shown below are commonly associated with high glucose levels. Work with your doctor or another health care professional to get an accurate diagnosis. Your doctor will interpret this test, taking into account your medical history and other tests results.

Chronic conditions that can increase fasting glucose levels include:

  • Insulin resistance [13, 14]
  • Obesity [15, 16, 17, 18]
  • Type 1 and 2 diabetes [19, 4]
  • Pregnancy and gestational diabetes [19, 20]
  • Fatty liver and other liver disease [21, 22, 23]
  • Kidney disease [24]
  • Overactive thyroid (hyperthyroidism) [25]
  • Stress due to illness, injury, or surgery [26, 27, 19]
  • Endocrine disorders, such as Cushing’s syndrome (too much cortisol), pheochromocytoma (benign tumors of the adrenal gland), acromegaly (excess growth hormone) [19, 28]
  • Pancreatic inflammation (pancreatitis) or cancer [19]

The following factors also increase blood sugar levels:

  • Overeating [29]
  • Acute and chronic stress [30, 31, 32]
  • Poor sleep quality or not enough sleep [33, 34, 35]
  • Smoking [36, 37, 38]
  • Air pollution [39]
  • Chronic exposure to toxins such as polychlorinated biphenyls (PCB) and organochlorine pesticides (OCP) [40]

Many drugs can increase glucose levels, including:

  • Antidepressants [41]
  • Drugs that treat high blood pressure, such as thiazide diuretics and beta blockers [42, 42]
  • Epinephrine/adrenalin [43]
  • Estrogen and oral contraceptives [19, 44]
  • Lithium [45]
  • Anti-seizure medication Phenytoin (Dilantin) [19]
  • Glucocorticoids [19, 46]
  • Niacin (vitamin B3) [47, 48]

Symptoms

When your blood sugar is high, you may experience the following symptoms [19]:

  • Feeling very thirsty
  • Urinating often
  • Fatigue
  • Blurred vision
  • Slow-healing infections

Ways to Decrease Blood Sugar

The most important thing is to work with your doctor to find out what’s causing your high glucose levels and to treat any underlying conditions!

Discuss the additional lifestyle changes listed below with your doctor. None of these strategies should ever be done in place of what your doctor recommends or prescribes!

Lifestyle Changes

If you are overweight, losing some weight will improve the ability of your body to use and respond to glucose more efficiently and reduce your risk of diabetes. Being obese is the number one risk factor for developing diabetes [15, 16, 18]! Losing even a small amount of your body weight can be very beneficial.

Physical activity is a great way to manage your blood glucose levels. Muscle activity burns glucose for energy and makes cells more sensitive to insulin. Find something fun that you will enjoy doing regularly (over 3 times a week for over 30 minutes) [49, 50, 51, 52, 53].

Get adequate rest. Sleep deprivation and poor sleep quality decrease the ability of cells to react to insulin, and over time causes increases in blood sugar levels [33, 35].

Manage stress. Stress can increase blood sugar levels, by increasing hormones such as cortisol and inflammatory molecules in your body [30, 54, 32, 55].

Diet

A healthy diet will help control your blood sugar levels!

Beneficial foods include vegetables, fruits, nuts, whole grains, fish, and olive oil [56, 57, 58]

Foods to avoid include red and processed meat, refined carbs and sweets, sugar-sweetened beverages, and high-fat foods. Most importantly, avoid overeating in general [59, 29].

Eat regularly, and especially take care not to skip breakfast. Starting your day without breakfast can lead to increased blood glucose [60].

On the other hand, you may want to refrain from nighttime snacks. A study shows that these are associated with obesity and higher blood sugar levels [61].

Drink plenty of water [62, 63]. It’s especially beneficial if you replace sugary drinks with water.

Studies show that moderate alcohol consumption (1 drink per day) can lower blood glucose levels and prevent diabetes and heart disease [64, 65, 66, 67]. However, heavier consumption has a negative effect and increases the risk of type 2 diabetes. Discuss your alcohol intake with your doctor.

Supplements

Talk to your doctor about the following foods and supplements. Initial studies suggest they may help decrease blood sugar levels or may be otherwise beneficial in prediabetes and diabetes:

Look out for deficiencies in the following nutrients and work with your doctor on correcting them if present:

Low Fasting Blood Sugar

Low blood sugar is called hypoglycemia (hypo = low, glycemia = blood sugar). It’s blood sugar below 70 mg/dL (3.9 mmol/L) in diabetics or around 55 mg/dL (3 mmol/L) in healthy people.

Causes

Causes shown here are commonly associated with lower blood sugar. Work with your doctor or another health care professional to get an accurate diagnosis. Your doctor will interpret this test, taking into account your medical history, signs and symptoms, and other test results.

Hypoglycemia or low blood sugar can be caused by:

  • Type 1 or 2 diabetes [120, 121]
  • Intense exercise or overtraining [122]
  • Malnutrition/starvation [123]
  • Too much alcohol [124, 125]
  • Weight loss surgery (bariatric surgery) [125]
  • Underactive thyroid (hypothyroidism) [126]
  • Severe infections or illness, including liver, kidney, or heart failure [127, 125, 128, 24]
  • Adrenal insufficiency (low adrenaline and cortisol) [129, 123]
  • Pituitary insufficiency (hypopituitarism) [130]
  • Pancreatic tumors [123, 131]
  • Some rare genetic disorders or autoimmune conditions affecting insulin [132, 133, 125]

Hypoglycemia is often caused by medications. Many drugs can decrease blood sugar levels, including:

  • Insulin and antidiabetics, including metformin [134, 125]
  • Aspirin [125]
  • Drugs used to treat high blood pressure, such as beta-blockers [135]

Symptoms

Symptoms of low blood sugar include [136, 137]:

  • Sweating
  • Hunger
  • Trembling
  • Headaches
  • Anxiety
  • Blurred vision

Ways To Increase Blood Sugar

The most important thing is to work with your doctor to find out what’s causing your low blood sugar and to treat any underlying conditions!

Discuss the additional lifestyle changes with your doctor. None of these strategies should ever be done in place of what your doctor recommends or prescribes!

Lifestyle

When we exercise, our muscles burn glucose fast. Make sure you are eating well beforehand, to prevent drops in blood sugar levels.

Diet

Make sure you eat regularly, ideally frequent smaller meals throughout the day, and that your diet is well-balanced. Go for quality carbs and foods rich in dietary fiber, vitamins, and minerals and low in added sugars, fats, and sodium [138].

Refrain from drinking alcohol, especially on an empty stomach. It can decrease your blood sugar [124].

Supplements

If you are susceptible to low blood sugar due to diabetes, carry a glucose supplement with you and take it when you start to feel symptoms of low blood sugar.

Genetics

Additional to food and lifestyle, genetics also plays a role in fasting blood glucose and HbA1c levels [139, 140]

A genome-wide association study in seven Mexican cohorts found a relationship between fasting blood glucose levels and the MTNR1B gene. Interestingly, this gene codes for the melatonin receptor and helps you fall asleep, but its over-activation may block insulin release and increase the risk of diabetes [141].

There are two associated versions of the MTNR1B gene, one of which is expressed more. Studies suggest that having the overexpressed version of this gene may be a risk factor for diabetes and high blood sugar [141].

An additional study found nine SNPs that were associated with glucose levels. More genes that impact fasting blood glucose levels are G6PC2 and GCK [142, 143, 144].

One variant (rs1050828) in the G6PD gene artificially lowers HbA1c levels, which makes this test less sensitive to detecting diabetes [145].

Remember, although genes and gene variants may have some effect on your glucose levels, the biggest effect is still due to your lifestyle and your dietary choices. Choose wisely!

Takeaway

A fasting blood sugar test is also called a fasting glucose test. It measures your sugar levels after fasting for at least 8 hours. Glucose is the main sugar found in your bloodstream and high levels after fasting may point to diabetes. The normal range in healthy people is 70 – 125 mg/dL. Levels on the higher end of the normal range (above 100 mg/dL) signal prediabetes, which often leads to diabetes if you ignore it and delay making healthy lifestyle and dietary changes. Doctors typically order the fasting blood sugar test alongside HbA1c, which can reveal your sugar levels over the past couple of months. Both low and high levels of blood sugar can be dangerous. If you have diabetes, monitor your sugar levels regularly (with both lab tests and glucose monitors) to make sure they’re stable.

About the Author

Biljana Novkovic

Biljana Novkovic

PhD
Biljana received her PhD from Hokkaido University.
Before joining SelfHacked, she was a research scientist with extensive field and laboratory experience. She spent 4 years reviewing the scientific literature on supplements, lab tests and other areas of health sciences. She is passionate about releasing the most accurate science and health information available on topics, and she's meticulous when writing and reviewing articles to make sure the science is sound. She believes that SelfHacked has the best science that is also layperson-friendly on the web.

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