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.
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 .
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 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 .
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 .
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.
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 :
- 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)
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.
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 .
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 .
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.
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 .
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.
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 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 
- Overactive thyroid (hyperthyroidism) 
- 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 
The following factors also increase blood sugar levels:
- Overeating 
- Acute and chronic stress [30, 31, 32]
- Poor sleep quality or not enough sleep [33, 34, 35]
- Smoking [36, 37, 38]
- Air pollution 
- Chronic exposure to toxins such as polychlorinated biphenyls (PCB) and organochlorine pesticides (OCP) 
Many drugs can increase glucose levels, including:
- Antidepressants 
- Drugs that treat high blood pressure, such as thiazide diuretics and beta blockers [42, 42]
- Epinephrine/adrenalin 
- Estrogen and oral contraceptives [19, 44]
- Lithium 
- Anti-seizure medication Phenytoin (Dilantin) 
- Glucocorticoids [19, 46]
- Niacin (vitamin B3) [47, 48]
When your blood sugar is high, you may experience the following symptoms :
- Feeling very thirsty
- Urinating often
- Blurred vision
- Slow-healing infections
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!
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].
A healthy diet will help control your blood sugar levels!
Eat regularly, and especially take care not to skip breakfast. Starting your day without breakfast can lead to increased blood glucose .
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 .
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.
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:
- Aloe [68, 69, 70, 71]
- Alpha-lipoic acid [72, 73]
- Berberine [74, 75, 76, 77]
- Caffeine/Coffee [78, 79, 80, 81, 82, 83]
- Chromium [84, 85]
- Cinnamon [86, 87]
- Fenugreek [88, 89, 90]
- Fiber, such as glucomannan or beta-glucans [91, 92, 93, 94, 95, 96, 97, 98, 99]
- Flaxseed [100, 101, 102, 103]
- Garlic [104, 105]
- Green tea [106, 107, 108]
- Magnesium [109, 110]
- Milk thistle [111, 112]
Look out for deficiencies in the following nutrients and work with your doctor on correcting them if present:
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 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 
- Malnutrition/starvation 
- Too much alcohol [124, 125]
- Weight loss surgery (bariatric surgery) 
- Underactive thyroid (hypothyroidism) 
- 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) 
- 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 
- Drugs used to treat high blood pressure, such as beta-blockers 
- Blurred vision
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!
When we exercise, our muscles burn glucose fast. Make sure you are eating well beforehand, to prevent drops in blood sugar levels.
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 .
Refrain from drinking alcohol, especially on an empty stomach. It can decrease your blood sugar .
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.
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 .
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 .
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!
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.