A fasting blood sugar test is one way to check for and monitor diabetes. You only need to fast for about 8 hours before the test to get accurate results. Read on to learn more and check if your levels are normal.
What is a Fasting Blood Sugar Test?
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 point to diabetes.
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 bring glucose into tissues and out of the bloodstream. Thus, glucose levels will gradually drop after eating and stay low during fasting [2, 3, 2].
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].
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 .
Fasting blood sugar is often checked alongside HbA1c (also known as glycated hemoglobin), which reveals your sugar levels over the past couple of 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). This points to good sugar control. On the other hand, HBA1c could be higher but your glucose levels at the moment lower, revealing poor sugar control.
Normal Fasting Blood Sugar Range
Normal fasting blood glucose levels in healthy people are 80 – 130 mg/dL 
Low blood sugar or hypoglycemia is when your blood glucose is too low—below 70 mg/dL. It usually only happens in diabetics who took too much insulin. That’s why you need to check your sugar levels regularly, as low blood sugar can be even more dangerous than high blood sugar .
High blood sugar or hyperglycemia is when your blood glucose is too high–above 200 mg/dL for diabetics. People with glucose levels in this range over a longer period of time have diabetes .
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. The World Health Organization set the prediabetes cut-off at 110 mg/dL. 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 often stall diabetes in its tracks. If you don’t make any changes, however, there’s about a 40% chance that you will get diabetes four years down the line .
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 [10, 11].
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.
Health Risks of High Fasting Blood Sugar
Diabetes is characterized by consistently elevated blood glucose levels (> 140 mg/dL). There are two types of diabetes, caused by either a lack of insulin release (type 1) or insulin resistance (type 2). More details on diabetes can be found in our post here. Additionally, fasting blood glucose levels in the upper end of the “normal range” (80 – 130 mg/dL) also point to diabetes .
A prospective study suggests that fasting blood glucose ranges between 91-99 mg/dL increases the risk of type 2 diabetes. An additional studied evaluated 46,578 healthy individuals (fasting blood glucose < 100 mg/dL). Individuals in the 90-94 mg/dL range were 49% more likely to get diabetes than those below 85 mg/dL [12, 13].
2) Damages Blood Vessels
The high glucose levels in the body damage the inner lining of blood vessels. This is because high glucose increases damaging (reactive oxidative species, CRP) and inflammatory compounds (cytokines), and makes it harder for the vessels to relax (via reducing NO) [14, 15].
The resulting oxidative stress in the blood vessels causes inflammation. Proteins and leukocytes accumulate, harden blood vessels, and eventually lead to the formation of plaques that block blood flow (atherosclerosis). This is the root cause of heart disease in people with diabetes [14, 15].
If left untreated, high blood sugar leads to stroke, heart disease, and other blood circulation disorders – the most common complication of type 2 diabetes. A recent analysis of 20 studies concluded that even in the nondiabetic range, higher blood sugar increases the risk for cardiovascular disease [14, 16].
In one study, 1973 non-diabetic men with fasting glucose levels below 110 mg/dL were evaluated once, and then again after 22 years. Over half of these men died from heart disease, and men with fasting blood glucose levels above 85 mg/dL had a significantly higher death rate than at the lower ranges. This is after adjusting for age, smoking habits, fat, fitness and blood pressure .
In another study of over 68,000 people without diabetes, higher blood sugar levels increased the risk of heart attacks over 4 years .
3) Damages Nerves
Diabetic neuropathy happens when high glucose causes nerve damage in patients with diabetes. High glucose increases inflammation in the nerves, resulting in mild numbness and pain in the legs and feet or problems with digestion, urination, and the heart [19, 19].
A common complication of diabetic neuropathy is called the diabetic foot, with ulcers, infections, and minimal to no feeling in feet or legs. Loss of feeling in the foot may lead to more injuries and open wounds, while a lower immune response in people with diabetes makes it harder to fight the infection off .
High blood sugar can also cause brain damage in the long term. In a 10-year follow-up study of 87 people, high HbA1c damaged brain cells and slowed down signals between them .
4) May Cause Cancer
17,860 men were categorized into three groups: non-diabetic, prediabetes and diabetes, of which 1663 got prostate cancer (after a follow-up). Diabetic individuals had a higher risk of prostate cancer, which may be due to hormonal changes caused by elevated glucose levels. The exact causes are unknown, though .
A case-control study evaluated fasting blood glucose levels of around 1100 participants. Individuals with higher fasting blood glucose levels over the last 11 years had a higher risk of pancreatic cancer. Long-term hyperglycemia may damage the liver and cause cancer, but elevated glucose could also be a side effect of pancreatic cancer .
5) May Cause Vision Loss & Dry Eye Syndrome
A study comprised of 700 diabetic men and women evaluated the effectiveness of fasting blood glucose levels in predicting vision loss (retinopathy). After a 10-year follow-up, vision loss was more frequent in the participants with higher fasting blood glucose levels (strongest predictor for vision loss is > 108 mg/dL). Hyperglycemia is a significant risk factor for retinopathy .
In another study, 199 diabetic individuals (type 2) were selected for evaluation. Of these, 108 patients (54.3%) suffered from dry eye syndrome. This is a condition that causes irritation and blurred vision .
6) May Cause Osteoporosis
33 women with osteoporosis tested for milk intolerance had higher fasting blood glucose levels than the controls (did not have osteoporosis), although they had lower body weight. The study concluded that higher fasting blood glucose levels may trigger osteoporosis in women .
7) Weakens the Immune System
High blood sugar weakens the immune system in the long run, which can be seen from studies that looked at HbA1c. In a study of over 1000 healthy people, elevated HbA1c impaired the immune response (T-cell) .
In a study of 38 patients with Fournier’s Gangrene (a mixed infection causing tissue death), those with elevated HbA1c had more serious symptoms and were more likely to die .
A genome-wide association study on 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. Having the overexpressed version of this gene is a risk factor for diabetes and high blood sugar .
An additional study found nine SNPs in the DNA that affect glucose. Expression of all these increases fasting blood glucose levels, but this differs based on ethnicity. More genes that impact fasting blood glucose levels are G6PC2 and GCK [32, 33, 34].
Many different positions in the DNA (loci) have been associated with HbA1c levels. One variant (rs1050828) in the G6PD gene artificially lowers HbA1c levels, which will cause a missed diagnosis of diabetes .
Irregular Fasting Glucose Levels?
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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 point to diabetes. The normal range in healthy people is 80 – 130 mg/dL.
Levels on the higher end of the normal range (above 110 mg/dL) signal prediabetes, which often leads to diabetes if you ignore it and delay making healthy lifestyle changes.
Doctors typically order the fasting blood sugar test alongside HbA1c, which can reveal your sugar levels over the past couple of months. Together, these two markers give a more complete picture of your blood sugar control.
In the long run, high blood sugar can damage your nerves, blood vessels, heart, and immune system. If you have diabetes, monitor your sugar levels regularly (with both lab tests and glucose monitors) to make sure they’re stable.