Background
If there is one “abnormal” blood test that I get the most questions on, it has to be the fasting blood glucose. As a result of implementing stringent lifestyle changes, often patients see their waistlines shrink, their lipids improve, but that stubborn fasting blood glucose often doesn’t seem to budge. Even more paradoxically I have patients who dramatically cut back carb intake but still notice a persistent elevation in their fasting blood sugar. Is this something to be concerned about if all other numbers look good? Let’s dig deeper so you understand why the interpretation of this single lab test has layers of complexity and must be part of a broader story.
Single Pixel vs Full Picture
Your fasting blood glucose is a singular data point that provides no background, longitudinal, or contextual information. Before I illustrate this with a personal example, I want to briefly explain the dynamic nature of blood glucose shifts. As you consume a serving of carbohydrate that is digested into glucose molecules, the glucose “cars” as I like to call them, move in various directions depending on your body’s metabolic needs. Below is an image from my book that illustrates the carb car at the intersection of 3 major lots…muscle (top left), liver (bottom left), and fat (right side).
In a resting state these glucose cars are stored in a form called “glycogen,” primarily in the muscle and liver lots. When we incur physical demands on the body, glucose appropriately flows to muscle while mental demands pull glucose molecules to the brain.
Commonly overlooked factors that can also increase glucose flux are emotional stress and sleep deprivation which can increase hormones that signal the liver to release glucose into the blood. Look at the impact of the stress hormone cortisol on glucose flux.
Chronic persistent cortisol elevation from chronic stress contributes to insulin resistance at the muscle (notice muscle lot not accepting glucose) while stimulating the liver to produce extra glucose, while also promoting extra belly fat storage. In other words you get the imperfect storm of reduced glucose disposal by the muscle combined by increase glucose production of glucose by the liver.
These are factors independent of exercise and diet, which is why many of my patients who are on a low carb diet and are exercising intensely are puzzled by their persistent elevations in glucose.
They fail to understand that their thoughts, their emotions, their late bedtimes, and their erratic schedules are significant contributors to blood glucose elevations.
This point is often driven home when they wear a CGM (continuous glucose monitor). To learn more about using CGMs, read my post here.
My Lab Results
I’m going to review a few data points below to clearly illustrate the fallacy of fasting blood glucose. Since we agree by now that glucose is a singular limited data point in time, I’ll now pair it with my blood ketones to provide more context.
What are ketones? Briefly, ketones are an alternative fuel source that are produced when your glucose levels are low and your fat cells respond by releasing fat into the blood in a form called fatty acids, which the liver converts into ketones. The keto diet is a low carbohydrate diet that keeps glucose (and thus insulin) levels low so your fat cells can release these fatty acids to fuel ketone production. These ketones can be used by key organs like your muscles, brain, and heart for fuel.
Let’s get back to my personal data below which is split into 3 different days with simultaneous measurements of fasting glucose and ketones. When ketones are present, it basically indicates my metabolism has switched to fat-burning mode. When it’s low (ketone screen shows “Lo”), I’m no longer in the optimal fat-burning zone. Look at day one first. This was a social event where I ended up carb binging on something sweet and also ate a high carb meal.
You might look at the corresponding glucose result and say that I’m lucky because it’s below 100 mg/dL despite my carb binge. However I can tell you that in the days leading up to this binge I was pretty low carb and physically active. That was deliberate. If there are planned social events, I often prep my body days in advance with fasting and regular physical activity to help deplete my overall glucose stores.
The social event literally becomes a carb restocking event where the carbs I occasionally binge on help replenish my depleted stores. My muscle and liver parking lots had extra room to park the glucose cars from my carb binge and I recall sleeping really well that night. All those factors kept my glucose in range, but I essentially had no ketones. That’s because my carb binge spiked insulin and that kept fat cells “locked,” meaning they would not release fatty acids into the blood to be converted by the liver into ketones. To put it simply, my metabolism was not in a fat-burning state.
I want to emphasize that this is not a frequent occurrence for me and I don’t recommend this becoming a weekly pattern of fasting during the week and binging on weekends.
Most of us don’t have the metabolic flexibility and reserves to keep switching back and forth between high carb binges and fasting to induce ketosis.
Even elite athletes that used to do this habitually (aka “carb loading”) to train for races paid a long-term price in the form of chronic inflammation. Listen to my podcast with elite endurance trainer Phil Maffetone here where we discuss the hazards of layering an unhealthy diet onto an excessively strenuous workout routine.
Now refer to day two in the above table where I did not binge on carbs the night before and did a fasted workout before checking my blood. The fast combined with my workout helped generate ketones, a sign that my fat cells are liberating stored fat. That’s a good thing. However, why is my glucose level elevated despite these two good deeds (fasting and exercise)?
My workout is a metabolic stress that caused my liver to produce glucose to fuel my exercising muscles. This is not what I consider an “abnormal” glucose, but rather represents the intrinsic dynamic nature of metabolism where my body is taking glucose out of my liver to fuel my muscles. This is not abnormal or pathological. It’s a survival mechanism, and since the excess glucose is being utilized by muscle for energy, it’s unlikely to cause damage.
Now what if I were a high carb eating sedentary office worker that rolled out of bed and checked his ketones and fasting glucose before sitting down at the computer for another marathon 14+ hour sitting session? If my ketones are undetectable (“Lo” on the reading) and my fasting glucose is let’s say 106 mg/dL, then that is a sign of an overloaded liver that is dumping extra glucose into blood despite there not being any muscular demand from exercise.
If the muscles are not pulling in this extra glucose to fuel exercise, then what will those extra glucose molecules do? Over the long term, they can deposit in small blood vessels causing microvascular damage to your eyes, kidneys, nerves, etc.
You want excess glucose in the blood to work for you by fueling exercising muscle, not against you by damaging vital structures.
Day 3 is a similar picture where again my fasting glucose may look a bit high at 102 mg/dL, but my ketones are also elevated. Again, the ketones represent my fat-burning state, while the glucose represents the shuttling of fuel to supply exercising muscle.
There are days where these same habits and patterns lead to even lower fasting glucose levels in the 80s or low 90s, but I deliberately selected these higher numbers to make the point clear. Let’s review some of the reasons for elevated fasting blood glucose below:
1. Exercise Effect: We discussed this already. Just to clarify, the more intense your exercise effort, the more likely the glucose will be elevated above normal. When I exercise in a lower heart rate zone my glucose levels often stay within a more normal range. This is one of the reasons I try to keep 80 percent of my workouts below my higher intensity, anaerobic threshold. To learn more, read this post and/or listen to my Phil Maffetone interview referenced earlier.
2. Physiological Insulin Resistance: When your muscles become adept at burning fat for fuel, they require less glucose. This is what you want. You want your muscles to draw fat from your fat tissue to use for energy. To be more graphic, you want your metabolism to be cannibalistic, so your own muscle greedily feeds on your internal fat stores for energy, instead of depending on external food sources.
This is why pairing fasting with exercise can be magical. You turn to your internal pantry for energy, instead of the one in your kitchen.
Since muscles have less of a need for pulling glucose from your blood, the glucose remains in the blood and can cause a slight elevation in your fasting glucose.
3. Emotional stress: Refer back to my image above regarding the impact of chronic stress on glucose levels. Refer to my stress page here for advice and tools to help with stress management.
4. Sleep disturbance: Late bedtimes, poor quality sleep, and conditions like sleep apnea or other sleep-related breathing disorders can cause an elevated morning blood sugar. Refer to my free e-book on sleep here if you struggle with sleep.
5. Nutritional stress: This can be individuals who are too low carb which is a signal to the liver to pulse out glucose to prevent a hypoglycemic event. This can also be a carb overloaded liver from the day/night before where the liver needs to unload some of this extra glucose burden.
Out of all the above triggers, stress and sleep are the ones my patients have the hardest time with. They often refuse to acknowledge these key factors and would much rather fast longer and exercise harder than manage stress and sleep issues.
The liver is a glucose-making life support system and just trying to manage it with diet and exercise is important, but often not sufficient if you want to completely reverse persistent glucose elevations.
If you’re still skeptical, consider wearing a continuous glucose monitor (CGM) and simultaneously track sleep, carb intake, and activity/exercise. You can read more about CGMs here. For exercise you need to pay attention to intensity, since excess anaerobic exercise (HIIT, sprints, or even casual runs at a higher heart rate) will spike glucose while optimal aerobic intensity usually reduces it. If you don’t know the difference between these exercise doses, read this post.
Beware of the Cortisol-Adrenaline Trap
I want to make a really important point here. In my personal experience (and those of many of my patients), it’s the combination of emotional stress, high-intensity exercise, and excessive fasting/low carb eating that together can cause a more significant elevation in fasting glucose. I’ve had to acknowledge my natural tendency to fall into this trap because often I medicate my own stress with higher intensity exercise and prolonged fasting.
Although my energy levels feel great from this powerful combination and my elevated ketones tell me I’m burning fat, I often see my fasting glucose rise up into the 110 mg/dL or higher range despite me doing all of these things that you would think should cumulatively drive down my glucose levels.
Rather than chalk it up to physiological insulin resistance and tell myself it’s a normal response, I now look at these higher glucose levels as a direct signal from my liver that I’m overdosing on cortisol and adrenaline.
It’s often a sign that I’m engaging in some combination of overtraining, overthinking, and undereating. Mental rest, physical rest, and nourishing my body properly help get my glucose levels back on track again by driving down cortisol and adrenaline, two chemicals that push the liver to produce more sugar. I guarantee many of you reading this post who are checking off all the boxes on healthy lifestyle habits, but still seeing little movement on your glucose might also be a victim of this trap.
What you define as a low or moderate intensity workout is likely too high. What you define as an adequate subjective sense of “feeling energetic” can be an addictive adrenaline high that you’ve just gotten used to. What you perceive as a sense of “feeling calm” might be a habitual tendency to internalize multiple thoughts and complex emotions that internally are causing persistent elevations in cortisol.
I’ve lacked this type of self awareness for years, so it has been a revelation and something I need to work on every single day. My wife Shally knows it and reminds me often….”don’t you need to take a break today from work, working out, etc.” Objective measures like heart rate (during exercise and at rest) and blood glucose are additional self-restraint mechanisms to help me rest, reset, and recover.
Summary
Hopefully by now you realize that fasting glucose alone is a highly problematic marker for disease risk. A metabolically healthy individual who is physically active might have the exact same result as a metabolically unhealthy sedentary person, yet the former person will likely live far longer and better. I also have many patients who consistently have fasting glucose levels in the 70-80 mg/dL range which is better than me, but they carry extra belly fat and often have cholesterol abnormalities and fatty liver. Does their “normal” fasting blood glucose mean they are metabolically healthy with minimal future risk of chronic disease? Absolutely not.
As mentioned above, a CGM can provide a broader picture of glucose metabolism that goes far beyond an individual data point. Your hemoglobin A1C (aka glycohemoglobin) is another longer term marker of glucose control, but this can also be an imperfect measurement in certain individuals. Context matters and that’s something our current way of reporting lab results doesn’t represent. Remember, when it comes to your health data (like financial and other pieces of data), don’t over fixate on single results. Think of the broader landscape of health which includes a combination of diagnostic tests and lifestyle markers. Also, pay attention to persistent and unexpected elevations in fasting blood glucose which may be an indicator that you are falling into the cortisol-adrenaline trap.
