April 15, 2020
Blood glucose levels are frequently checked during standard yearly check-ups as a screening test for diabetes. Additionally, many people track their glucose at home with an over-the-counter finger prick method, as a way to track their health and make sure they are staying out of danger. When blood glucose (or blood “sugar”) is checked, either by your healthcare provider or with a home finger stick glucose monitor, the results are measured in milligrams (mg) of glucose per deciliter (dl) of blood.
One of the most common glucose measurements you will hear discussed is the fasting plasma glucose (FPG) level, also referred to has fasting blood glucose (FBG) levels. Fasting plasma glucose (FPG) is determined by checking your blood glucose levels after not having anything calories at least 8 hours before the test. According to the American Diabetes Association (ADA), people can be classified into one of three categories depending on their fasting plasma glucose levels: normal, prediabetes, and diabetes. To be considered “normal,” fasting glucose must be under 100 mg/dl.
Post-meal glucose levels are also meaningful with regards to your health, and high post-meal glucose levels are linked to worsened glucose control over time, development of obesity, diabetes, and cardiovascular disease, and impaired exercise and cognitive performance. While it is not unexpected for glucose levels to increase after a meal as the glucose from the meal is released into the blood, if this level is too high it is not good for health and can predispose one to disease over time. According to the International Diabetes Federation (IDF) guidelines for the management of post-meal glucose levels, nondiabetic people should have a glucose level of no higher than 140 mg/dl after meals, and glucose should return to pre-meal levels within 2-3 hours. Post-meal hyperglycemia (elevated glucose) is defined as a glucose level >140 mg/dl 1-2 hours after ingestion of food or drinks.
These glucose measurement methods mentioned so far rely on a single point-in-time measurement to determine if your levels are normal. Recent advances in continuous glucose monitoring (CGM) technology allow you to track your glucose levels over a 24-hour period and gain insight into deeper trends that are also associated with health, such as glycemic variability, a measure of the up-and-down swings in glucose throughout the day. However, there are no standardized, universally accepted criteria for what “normal” 24-hour glucose values are using CGM technology. Scientists are continuing to gather information about glucose levels in healthy people using CGM technology.
Of note, CGM devices measure interstitial glucose levels (glucose from the fluid in between cells) in contrast to blood/plasma glucose levels (glucose in the blood) measured in the FPG tests. While interstitial glucose and blood/plasma glucose levels correlate highly, they are not precisely the same thing and diagnoses are not made from interstitial measurements.
Below is a summary overview of data about 24-hour glucose trends in nondiabetic individuals wearing CGM to gain a better understanding of “what’s normal.”
One study from 2009 entitled “Reference Values for Continuous Glucose Monitoring in Chinese Subjects” looked at the glucose levels of 434 healthy (nondiabetic, non-obese) adults using CGM and found the following:
A 2010 study, “Variation of Interstitial Glucose Measurements Assessed by Continuous Glucose Monitors in Healthy, Nondiabetic Individuals,” looked at a healthy population of 74 individuals that included children, adolescents, and adults during daily living using CGM. This research showed that:
Compared to the first study mentioned, these healthy, nondiabetic individuals appeared to have a tighter range of glucose, spending the vast majority of the 24-hour period between 71-120 mg/dl.
A third study, from 2008, entitled “Characterizing Glucose Exposure for Individuals with Normal Glucose Tolerance Using Continuous Glucose Monitoring and Ambulatory Glucose Profile Analysis” looked at 32 individuals with normal glucose tolerance wearing CGM for approximately 29 days and showed the following findings:
A fourth study, “Continuous Glucose Monitoring Profiles in Healthy Nondiabetic Participants: A Multicenter Prospective Study," from 2019, examined 153 healthy, nondiabetic children and adults ages 7-80 with normal mean BMI of 24 ± 3.2 kg/m2 wearing CGM for up to 10 days. This study showed:
A 2007 study, “Continuous Glucose Profiles in Healthy Subjects under Everyday Life Conditions and after Different Meals” looked at 21 healthy young individuals using CGM. These participants were between age 18-35, had a healthy BMI of 22.6 ± 1.7 kg/m2, and were examined eating standardized meals as well as regular meals of their choosing. Mean fasting glucose for these participants was 80 mg/dl. This study found:
Under everyday life conditions:
Under standardized meal conditions with a moderately low percentage carbohydrate (50 grams, 26.8%), high fiber (12.8g), high fat meal (47 grams, 56.7% fat), and high protein (30.9 grams, 16.5%) participants displayed:
Finally, 2018 paper, “Continuous glucose monitoring is more sensitive than HbA1c and fasting glucose in detecting dysglycaemia in a Spanish population without diabetes,” assessed 254 people with normal glycemic function wearing CGM for 2-5 days. The mean BMI of these participants was overweight, at 27.3 ± 4.7 kg/m2. Their results found:
In summary, based on ADA criteria, the IDF guidelines, a person’s glucose values are “normal” if they have a fasting glucose <100 mg/dl and a post-meal glucose level <140 mg/dl. Taking into account additional research performed specifically using continuous glucose monitors, we can gain some more clarity on normal trends, and can suggest that a nondiabetic, healthy individual can expect:
These are not standardized criteria or ranges but can serve as a simple guide for what has been observed as normal in nondiabetic individuals.
Exact numbers for what is considered “optimal” glucose levels to strive for while using CGM to achieve your best health are not definitively established; this is a question that is individual-specific and should be discussed with your healthcare provider. With that said, research shows that there is an increased risk of health problems as fasting glucose increases, even if it stays within the “normal” range, making finding your “optimal” glucose levels all the more important.
While the International Diabetes Federation and other research studies have shown that a post-meal glucose spike should be less than 140 mg/dL in a nondiabetic individual, this does not determine what value for a post-meal glucose elevation is truly optimal for your health. All that number tells us is that in nondiabetics doing an oral glucose tolerance test, researchers found that these individuals rarely get above a glucose value of 140 mg/dL after meals.
So, while this number may represent a proposed upper limit of what’s “normal,” it may not indicate what is going to serve you best from a health perspective. It is likely that many people may do better at lower post-meal glucose levels. Similarly, while the ADA states that a fasting glucose less than 100 mg/dL is normal, it does not indicate what value is optimal for health.
Lastly, there are no specific recommendations regarding what average glucose levels should be over a 24-hour period using CGMs. This lack of standardization is likely because CGMs are relatively new and not widely used in a nondiabetic population.
Why? Previously we discussed that the ADA considers normal fasting glucose as anything <100 mg/dl. However, multiple research studies show that as fasting glucose increases, there is an increased risk of health problems like diabetes and heart disease -- even if it stays within the normal range. The highlights of some of the study results include:
Why? In a study looking at healthy, young, nondiabetic adults who had normal BMI (mean of 22.6 ± 1.7 kg/m2), the average pre-meal glucose levels were in the range of 72-90 mg/dl.
Why? In a study looking at healthy nondiabetic adults, researchers found that the average post-meal glucose peak was 99 ± 10.5 mg/dL after a standardized balanced meal. In contrast, meals with less fiber and more refined sugars caused a higher post-meal glucose spike (up to an average of 133 ± 14 mg/dl) in the same population. Another study also looking at healthy, nondiabetic adults found an average post-meal spike of approximately 122 ± 23 mg/dl. Taking the standard deviation of these averages into consideration, aiming for a post-meal glucose level of less than 110 mg/dl with no more than a 30 mg/dl increase from pre-meal levels is a reasonable goal to strive for.
Why? These numbers represent the mean 24-hour glucose range in a young, very healthy population. We looked at several different studies of nondiabetic populations wearing CGMs, and this was one of the overall healthiest populations under normal living conditions, and therefore we think that 79-100 mg/dl is a safe and healthy range to orient towards.
It is not uncommon for your glucose levels to increase after a meal: you just ate food that may contain glucose, and now your body is working to get it out of the bloodstream and into the cells. With that said, we know that we want to prevent excessive spiking of glucose levels, because studies show that high post-meal glucose spikes over 160 mg/dl are associated with higher cancer rates, and spikes are also associated with heart disease. Repeated high glucose spikes after meals contributes to inflammation, blood vessel damage, increased risk of diabetes, and weight gain. Additionally, the data shows that the big spikes and dips in glucose are more damaging to tissues than elevated but stable glucose levels. Therefore, you should strive to keep your glucose levels as steady as possible, at a low and healthy baseline level, with minimal variability after meals.
Keeping your glucose levels constant is more complicated than just following a list of “eat this, avoid that” foods. Each person has an individual response to food when it comes to their glucose levels; studies have shown that two people can have different changes in their glucose levels after eating identical foods. The difference can be quite dramatic. One study found that some people had equal and opposite post-meal glucose spikes in response to the same food.
So how do you keep your glucose levels stable? How do you know when you have a sugar spike and which foods caused it? That’s where CGM comes into play. Continuous glucose monitoring allows you to see your blood glucose levels in real-time and store that data for future reference; this makes CGMs uniquely positioned to help you optimize your diet and lifestyle. Foods affect each person differently, and it is hard to know what your blood glucose is doing at any one time without measuring it. CGMs can give you the data you need to optimize your health. Choosing foods and lifestyle habits that consistently keep average glucose lower and post-meal spikes lower will improve glucose patterns over time.
Studies have shown that the information gathered from CGMs can provide more detail and more potential areas for modification than the single glucose level that you get with a glucometer or laboratory blood test. One study looked at sub-elite athletes and found that 4 out 10 study participants spent more than 70% of the total monitoring time above healthy glucose levels, and 3 of 10 participants had fasting glucose in prediabetic range.
Similar results have been found in other studies: one reported that 73% of the “healthy” nondiabetic participants had glucose levels that were above normal in the range of 140-200 mg/dl at some point during the day.
CGMs can not only give you data on your blood glucose, but they can help you use the data to make changes to your diet and exercise routines. Studies have shown that continuous glucose monitoring can be used to characterize an individual's glucose response to specific foods, and, in turn, predict their responses to other foods. This technology has the potential to allow individuals to create personalized meal plans that suit their unique metabolic needs and improve glucose control.
Why is it unhealthy for glucose levels to be too high (hyperglycemia) or too low (hypoglycemia)?
Hyperglycemia refers to elevated blood glucose levels. This usually occurs because the body is not appropriately removing glucose from the blood; this can happen due to many complex reasons. Elevated glucose levels can damage blood vessels and nerves over time; this can then lead to problems in the eyes, kidneys, and heart, as well as numbness in the hands and feet. Very high levels can lead to coma and even death in some cases. People with fasting glucose levels that are higher than 100 mg/dl have impaired glucose tolerance and should speak with their healthcare provider.
Some people may think that, to avoid all these issues, they should just keep their blood glucose levels as low as possible. If too high is bad, then low must be good, right? Not exactly. When glucose gets too low, it’s called hypoglycemia. The threshold for hypoglycemia is typically thought to be when glucose falls below 70 mg/dl. When this happens, the body may release epinephrine (adrenaline), the “fight or flight” hormone, which this can lead to a fast heart rate, sweating, anxiety, blurry vision, and confusion, but also helps the body mobilize glucose into the blood. If blood glucose levels stay too low for too long, it can cause seizures, coma, and in very rare instances, death.
In a recent study, researchers reviewed the published literature to see if low fasting glucose levels affected healthy people’s long term risks of health problems, like strokes and heart attacks. They found that healthy nondiabetic people who had baseline fasting glucose levels of less than 72 mg/dl had a 56% increase in all-cause mortality compared to people with normal fasting blood glucose levels. In addition, the risks for heart attacks and strokes were higher in people with baseline fasting glucose levels less than 72 mg/dl. This result is likely due to the body releasing more epinephrine to counteract the low glucose levels; too much epinephrine for too long leads to heart problems. Interestingly, people with low fasting glucose levels of less than 83 mg/dl but higher than 72 mg/dl did not have an increased risk of future heart attacks and strokes.
While there has been an association between low fasting plasma glucose levels and worse health outcomes, it is not clear whether transient dips in glucose levels (less than 70 mg/dL) during a continuous 24-hour period are unhealthy for nondiabetic individuals. Part of the reason that this is unknown is that continuous glucose monitoring is a relatively new technology and has been studied more extensively in diabetic individuals than in healthy individuals.
Long-term health outcomes relating to 24-hour glucose profile metrics are still being evaluated. In one study looking at nondiabetic, healthy individuals wearing CGMs over a 24-hour period, data showed that glucose dips below 70 mg/dL actually occur quite frequently. In fact, 41% of these healthy individuals experienced glucose levels less than 70 mg/dL in a 24-hour period and the men’s levels were below 70 mg/dl for 2.7 +/- 6.1% of the 24-hour period (2.1 +/- 4.4 % in women). Based on this data, healthy individuals can reasonably be expected to spend an average of 39 minutes with glucose values less than 70 mg/dL (in men). Furthermore, taking into account one standard deviation higher than the average, it could reasonably be considered “normal” to spend up to 126 minutes (8.8% of a 24-hour period) with CGM-measured glucose values less than 70 mg/dl. The clinical significance of these low glucose levels is unknown at this time but research suggests that many healthy individuals wearing CGMs do spend some amount of time with glucose levels less than 70 mg/dL.
Research also shows that glucose levels decrease by an average of 5% during REM sleep, as compared to non-REM sleep stages, which may contribute to periodic dips seen at night in nondiabetic people. In fact, healthy people who have glucose dips below 70 mg/dl have twice as many dips as night as compared to during the day. Additional, pressure on the CGM sensor from laying on it can cause aberrant low values.
Lastly, glucose dips below 70 mg/dL that occur just after a post-meal glucose spike may indicate reactive hypoglycemia; which is an exaggerated insulin response to a high carbohydrate meal, causing an overshoot in the amount of glucose that is absorbed out of the blood stream and into cells, and is not good for health. Again, we don’t want high highs and low lows; stable glucose appears to be better for the body. These glucose dips are typically characterized by symptoms including fatigue and lack of energy, and can be avoided by a low-carbohydrate/low-glycemic eating pattern with reduced post-meal glucose spikes.
Even though there is no defined low point for nondiabetic fasting blood glucose levels, keeping your blood glucose levels above a minimum threshold of 72 mg/dl may be beneficial for healthy, nondiabetic people.
What does all this mean? It means that while there are well-established “normal” ranges of fasting and post-meal glucose levels, these don’t give clarity into what glucose trends should be throughout at 24-hour period. They also don’t specify what ranges are optimal for best health.
Even people with “normal” glucose levels may be at higher risk of health problems than they realize because of frequent glucose spikes and dips, or elevated fasting glucose even if in the normal range. Your optimal glucose levels depend on many individual factors, and setting those ranges should include a discussion with your healthcare provider.
The studies show that keeping your blood glucose in the normal range is important, but also that preventing too many spikes and dips is key to maintaining your health. A personalized dietary and lifestyle plan that promotes metabolic health should also accomplish three main goals:
Figuring out which diet and lifestyle choices will allow you to achieve these goals is an iterative process; there is no one-size-fits-all plan that works for everyone to keep blood glucose in their optimal range. Continuous glucose monitoring can help you establish your optimal diet and lifestyle choices by serving as a continuous feedback mechanism, closing the loop between specific actions and the body’s reaction, and paving the way for improved current and future health.
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