What Is a Fasting Blood Glucose Test?
After a meal, blood glucose levels will normally increase over a few hours. The major source of this is carbohydrates, which are eventually broken down into glucose in the body. Glucose is extremely important to monitor because it has many health implications [1, 2].
A fasting blood glucose test measures blood glucose after not eating for at least 8 hours. You can still drink water (unless otherwise directed by your doctor), but anything like coffee, tea, juice, and sodas should be avoided. Blood is drawn out of the veins and then analyzed at a lab .
During fasting, the hormone glucagon is produced, which promotes glucose production (gluconeogenesis) and glycogen breakdown (glucose reservoirs). In normal individuals, their body will produce insulin in order to rebalance and reduce elevated glucose levels .
Those with diabetes either don’t have enough insulin to rebalance blood glucose (type 1 diabetes) or their body cannot effectively use insulin (type 2 diabetes). In a fasting blood glucose test, diabetics will have blood sugar levels significantly higher than non-diabetics. This leads to a severe blood vessel (angiopathy) and nerve damage (neuropathy) if left untreated [4, 3].
Why Is a Fasting Glucose Test Done?
A fasting blood glucose test is done to check for prediabetes and diabetes. It is very useful in screening out diabetics versus non-diabetics, and also helps in monitoring. It also evaluates the effectiveness of different medications and dietary changes in diabetic individuals .
Also known as glycated hemoglobin, HbA1c levels give an indication of a person’s average blood glucose levels in the last two to three months. The more sugar in the blood, the more it will attach to hemoglobin (“glycate”), raising HbA1c. It is used for testing and monitoring diabetes and points to whether therapy has been working in the last few months .
Additionally, HbA1c gives a good indication of the fat (lipid) profile in the body. This reduces the risk of cardiovascular complications, one of the most common problems faced by diabetics .
This test is straightforward and safe, but may be painful for some users. Blood is drawn out of the veins in the arm in order to get analyzed .
Your fasting blood glucose level is the lowest your blood glucose levels reach because the influence of recent meals is minimized. As such, it determines whether your blood glucose levels are too low, too high, or normal relative to the rest of the population. Relative is a very important point here because having relatively normal fasting blood glucose levels does not equate to having optimal levels [7, 8].
Levels and Ranges
Normal fasting blood glucose levels of healthy individuals before meals (8-hour fast) are 80 – 130 mg/dL. Although these are just normal ranges, having blood glucose levels at certain points in this range can be detrimental over time 
Hypoglycemia is caused by too little blood glucose when levels reach below 70 mg/dL. This only occurs in diabetics and is even more dangerous than high glucose .
Hyperglycemia is caused by too much blood glucose. This is when fasting blood glucose levels go above 200 mg/dL for diabetics. In non-diabetics, glucose levels in this range over a longer period of time will cause diabetes .
You can request that your doctor test your fasting glucose levels. Conventional doctors will look at high or low fasting glucose levels and not mention anything. Sometimes, a lab result may be in the reference range, but not actually be in the optimal range. Reference ranges are not the same as optimal ranges. This is why even fasting glucose levels in the ‘normal’ range can be unhealthy and indicate that certain processes in the body aren’t optimal. Lab Test Analyzer will let you know if your fasting glucose levels are optimal and what you can do to get them there if they aren’t.
HbA1c is an indicator of diabetes and hyperglycemia. The table below summarizes HbA1c levels in non-diabetics, prediabetes, and diabetes, with the accompanying blood glucose levels :
*Cut-offs for prediabetes and diabetes with blood glucose levels slightly vary between studies
Both HbA1C levels as long-term markers and glucose blood levels, as markers of the glucose levels at the moment, need to be taken into account when considering what category a person would fall into.
For example, even if blood sugar levels are very high at one point, they may have been overall low over the past couple of months so HbA1c may be low. This points to the overall good management of sugar levels. But it may also happen that HBA1c is higher, but the glucose levels at the moment lower (due to insulin or other factors), pointing to the opposite.
Taken together, the two offer much more information than each alone.
Impaired Blood Glucose
Prediabetes, also known as impaired fasting glucose, refers to consistently elevated blood glucose levels, but not high enough to diagnose diabetes (below hyperglycemic level). These individuals are at higher risk for diabetes, and their fasting glucose levels are consistently between 100 – 125 mg/dL. The World Health Organization define the prediabetes cut-off at 110 mg/dL .
Being able to self-monitor your blood glucose levels is very important in the management of diabetes. Blood glucose meters allow patients to monitor their blood glucose levels and modify their treatment and lifestyle accordingly. They allow you to track fluctuations caused by exercise, food, medications, stress and other factors .
There is a broad range of blood glucose meters to choose from with varying accuracy. Some meters have very high accuracy, but will also be much more costly. Some can now be integrated with a smartphone, and other newer ones may not require finger pricking, or the needles will be very small and almost painless [10, 11].
Fasting blood glucose and HbA1c tests are more accurate, but blood glucose meters are easy-to-use, accessible, and important for checking your sugar levels daily.
Health Implications of Fasting Glucose Tests
Fasting blood glucose tests check for and monitor diabetes. It provides caretakers with current fasting blood glucose values, which helps to change and adapt treatments .
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 (endothelium) 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. This accumulates proteins and leukocytes that harden the blood vessels and eventually lead to the formation of plaques, which block blood vessels (atherosclerosis). This is the root of all diabetes-caused cardiovascular diseases [14, 15].
If left untreated, high blood sugar leads to a number of cardiovascular diseases such as stroke, heart disease, and other blood circulation disorders – the most common complication of type 2 diabetes. A recent meta-analysis (of 20 studies on nondiabetic subjects concluded that even in the nondiabetic range, elevated glucose increases the risk for cardiovascular disease [14, 16].
1973 non-diabetic men with fasting glucose levels below 110 mg/dL were evaluated once, and then again in a 22-year follow-up. 53% of these men died from cardiovascular diseases, 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 .
A prospective study included 68,297 participants without diabetes and free of stroke, heart attacks, and cancer prior to the follow-up. There was a significantly increased risk of heart attacks with increasing fasting blood glucose levels in the non-diabetic individuals (4-year follow-up) .
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 .
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
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 .
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 .
Health Implications of HbA1c Tests
This is a very useful test for monitoring diabetes. It gives insight into an individual’s blood glucose values over the past few months, providing more long-term information. This is the recommended test for diabetes, and other tests (i.e. fasting blood glucose) should only be used if this is unavailable. It is the best choice for chronic management of diabetes [5, 5].
A person’s long-term blood glucose levels are directly proportional to their HbA1c levels. Because of this, the health implications below are caused by damage from high glucose levels over an extended period of time (the mechanisms are the same as mentioned above) .
1) High HbA1c and Diabetes
While a fasting blood glucose test is excellent for “in the moment” glucose levels, HbA1c tests give much more detailed information over a longer course of time. High HbA1c levels increase the risk of diabetes and the risk of health complications in diabetes. The best diagnosis is done by evaluating HbA1c and fasting blood glucose levels at the same time .
Fasting blood glucose levels may not always reflect HbA1c levels. Sixty cases were found in which their fasting blood glucose levels were in the prediabetes range (< 130 mg/dL) but their HbA1c levels were in the diabetic range (> 53 mmol/mol). The study concluded that fasting blood glucose should not be the only measure of optimal glucose control .
2) High HbA1c Damages Blood Vessels
Blood samples were taken from 562 elderly individuals to evaluate HbA1c, blood glucose, and C-reactive protein levels. Elevated HbA1c levels increased the amount of C-reactive protein in the body, which caused an inflammatory response in the blood vessels, leading to more complications .
When HbA1c levels increase, blood vessel generation (angiogenesis) and function reduce. This is seen in individuals with type 1 diabetes. Additionally, elevation in HbA1c is also related to increases in thrombomodulin, an enzyme involved in blood clotting. High HbA1c increases the risk of blood clotting (thrombosis) [27, 26].
3) High HbA1c Damages Nerves
In a 10-year follow-up study, 87 individuals (36 non-diabetic, 9 prediabetes, and 42 type 2 diabetic) were tested at the beginning and end of the time period for nerve and neuron health. Elevated HbA1c caused nerve and neuron degeneration, while also minimally slowing down signals between them .
The most common consequence of nerve damage in individuals with high HbA1c is diabetic neuropathy. Similarly to the blood vessels, nerve damage occurs due to elevated reactive oxidative species and inflammation. In a study on 563 patients with diabetes (type 2), those with diabetic neuropathy (18.1%) had higher HbA1c levels, making it a potent predictor for this condition .
4) High HbA1c May Cause Cancer
Four major studies (systematic review) concluded that HbA1c increases the frequency and death rate of all types of cancer. An additional review analyzed data from other studies linking HbA1c to several types of cancer. High HbA1c increases risk of liver, pancreatic and uterus (of the endometrium, inner lining of the uterus) [31, 32].
5) High HbA1c May Cause Vision Loss
700 diabetic individuals got their HbA1c levels tested in order to see its use as a predictor of vision loss (retinopathy). The higher the HbA1c levels, the greater the frequency of retinopathy. HbA1c levels greater than 42 mmol/mol has the biggest risk for retinopathy [5, 23].
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) High HbA1c Weakens the Immune System
HbA1c levels and the immune system of 1103 healthy individuals was assessed in a cohort study. Elevated HbA1c caused changes in immune cell responses (T-cell), which ultimately leads to an impaired immune system. The exact mechanism for this is still under speculation, but the damaging effect on the immune system is well-known .
In a retrospective study, 38 patients with Fournier’s Gangrene, a mixed infection causing tissue death, were investigated for the effects of HbA1c. 18 patients had diabetes and 20 were non-diabetic. Those with elevated HbA1c (> 53 mmol/mol) were in significantly worse condition and had increased mortality .
7) High HbA1c May Cause Osteoporosis
An investigation of 58 diabetic children measured their bone mineral density levels and compared it to their HbA1c levels. 3 children had osteoporosis, while 14 had osteopenia (pre-osteoporosis: reduced bone mineral density). The greater the amount of HbA1c, the greater the reduction in bone mineral density in diabetic children .
A cross-sectional study on 93 diabetic individuals (type 1) compared the relationship between HbA1c and osteocalcin, a bone-specific protein that promotes bone health. HbA1c reduces osteocalcin levels, which increases the risk of fractures, breaks, and osteoporosis .
Additional to food and lifestyle, genetics also plays a role in your fasting blood glucose and HbA1c levels. It’s important to consider both of these in diabetes because your genetic makeup can influence your HbA1c and fasting blood glucose [38, 39]
A genome-wide association study on seven Mexican cohorts found a relationship between fasting blood glucose levels and the MTNR1B gene. The activation of this gene may block insulin release caused by glucose elevation .
There are two associated versions of the MTNR1B gene, one of which is expressed more. The expression of this gene is higher in individuals with type 2 diabetes, which infers that they also have elevated fasting blood glucose levels. Having the overexpressed version of this gene is a risk factor for diabetes and hyperglycemia .
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 [41, 42, 43].
Many different positions in the DNA (loci) have been associated with influencing HbA1c levels. This is through genes that influence glucose and red blood cell pathways. One variant (rs1050828) in the G6PD gene artificially lowers HbA1c levels, which will cause missed diagnosis of diabetes based on the threshold .
Genetic factors that influence red blood cell pathways do not predict future type 2 diabetes. Interestingly, people of European and Asian ancestry have a few loci that modestly impact HbA1c levels. Whereas in people of African American ancestry, the genetics of HbA1c is dominated by a variant of the G6PD gene [45, 46].
Irregular Fasting Glucose Levels?
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