Continuous Glucose Monitors: Q&A
What can they do for you?
In “Why Some Habits Feel Impossible To Break”, I discussed how some technologies are helping people make significant changes in their habits. That article sparked a lot of interest in the main technology I discussed: continuous glucose monitoring. I’ve received some follow-up questions over on LinkedIn, including:
What can a CGM tell me? There’s got to be some things it can’t tell me, right? What are the limitations? What insights can I get?
How best to employ it?
What’s the best length of time to use it? Intensively for a few weeks, a few months, forever, what?
How does continuous glucose monitoring work?
How do I get one? Are there any companies you would recommend?
How much do they cost?
Will it really help me change my behaviors around diet, sleep, exercise, and stress?
First things first:
I’m not a doctor, and I don’t play one on the internet. I’ve just been around the health & fitness world for a long time (growing up with a physical therapist for a mom will do that to you). I’ve seen a lot of technologies come and go over the last 30+ years, and I’ve personally experimented with many of them. My observations are largely informed from those experiences.
Additionally, Dr. Peter Attia has an excellent post that strongly influenced my thinking on the requirements of good health-tracking tech. Here are his “Attia Postulates”:
What you’re measuring matters (e.g., HRV—matters vs. # steps taken—not so much)
What you’re measuring differs from day to day in ways that are not intuitive or linear (see above)
What you’re proposing to measure you can actually measure accurately (e.g., HR—can be measured accurately vs. subtle changes in body position—probably can’t be measured well enough yet)
You can report the data in a timeframe that is relevant to learn or act on (i.e., real-time info versus delayed info)
You can provide corrective, actionable feedback, if necessary (this is huge and largely missing in most applications)
CGMs are one of the few things that meet those criteria, and it’s why I’m such a big fan of their use.
One last point
This article is specifically for people like myself: the non-diabetic/non-insulin-dependent population.
I mentioned this in my other article, and it bears repeating: CGMs are not toys. For people whose survival depends on the precise application of exogenous insulin, CGMs are literally life-saving.
A non-insulin-dependent person can try out a CGM and take it off if they don’t like it.
An insulin-dependent person doesn’t have that luxury.
I know people who would not be alive today if not for their CGM, and they feel profound gratitude for the technology. But there are also moments where they feel deep resentment about being tied to a machine for their whole lives.
So…let’s be respectful while discussing the topic, okay?
Alright, on with the questions!
What can a CGM really tell me? What insights can I get from using one?
This was the most common question I received, so let’s start here.
First, a quick recap:
A CGM is measuring the amount of glucose in your blood. (Technically, it’s the amount of glucose in the interstitial fluid between the cells of your subcutaneous tissue)
There is a lot of evidence that high glucose levels correlate with adverse health outcomes, such as autoimmune conditions, cancer, and Alzheimer’s.
If your fasting blood glucose levels are above 90-100 mg/dL, you are likely pre-diabetic. A CGM can catch this early.
The mainstream recommendation for fasting blood glucose is less than 100 mg/dL. Many doctors focused on healthspan & longevity, however, suggest keeping average blood glucose below 100 and fasting blood glucose below 90, with the standard deviation less than 15 (mg/dL).
Diabetes is not your friend, so knowing your blood glucose levels is extremely valuable.
However, you can get fasting blood glucose from a regular blood draw, so why bother with a CGM?
HbA1c levels can fool you. Average blood glucose levels calculated by a CGM can’t.
You might be familiar with the term HbA1c, or glycated hemoglobin. HbA1c is a rough approximation of blood glucose levels over the prior 90 days.
In simple terms, an HbA1c test looks at how many sugar molecules are attached to a protein called hemoglobin in your red blood cells. Since red blood cells live for about 90 days, this gives us an approximate idea of how much sugar is floating around in your blood.
The problem, however, is that the HbA1C test can be easily confounded. Certain genetic conditions like beta thalassemia can significantly affect the amount of hemoglobin in a person’s red blood cells. Additionally, it is known that race/ethnicity is a confounding factor since genetics differ across racial genealogies. Lastly, diet can significantly affect the lifespan of a red blood cell by changing the probability of oxidation/glycation, thereby affecting the HbA1c calculations.
Conversely, a CGM bypasses all of these issues because it directly measures real-time glucose values. (Technically, there is a 5-20 minute delay since the glucose must transfer from the blood to the interstitial fluid - just clarifying for my fellow pedants.) Today’s CGM technology is getting so accurate that the FDA allows them to be used as primary lines of treatment.
Your blood glucose response to a meal says a lot about your underlying metabolic health
Imagine that two people both have fasting blood glucose values of 80 mg/dL. However, after eating a meal with a moderate amount of carbohydrates, Person A experiences a blood glucose spike to 160 mg/dL that takes 3 hours to return to baseline. On the other hand, Person B experiences a rise in blood glucose to 120 mg/dL and is back to baseline within 90 minutes.
Obviously, something very different is happening inside the bodies of these two people, even though they both have fasting blood glucose levels of 80 mg/dL.
This is where a CGM becomes invaluable. I have seen many instances of people thinking, “I work out, I’m not overweight, and my lab values are normal, so…I must be healthy”, only to find out that their metabolic response to food is a train wreck in the making.
This isn’t a small thing. There appears to be evidence that a person’s response to COVID, for example, is dependent on their metabolic health. This may be why some seemingly “healthy” people are having such negative reactions to COVID infection.
Kara Collier, the Director of Nutrition at NutriSense, recommends keeping peak glucose values below 140 mg/dL. If glucose levels consistently spike above 160-180 mg/dL, they can cause damage to the beta cells in the pancreas and oxidative damage to cells throughout the body. This is reflected in a rise in inflammatory markers like hs-CRP (high sensitivity C-reactive protein), AGE levels (advanced glycation end-products), high mitochondrial ROS production, and more.
Glucose levels aren’t just affected by the foods you eat. Sleep, exercise, stress, nutrient timing, hormonal fluctuations, biological sex, and dozens of other factors impact glucose levels.
Let’s say you want to eat a giant bowl of pasta. Consider the following:
Eating fat or protein before the meal will slow the release of glucose, thereby lowering insulin response
Eating the meal in the morning will result in better glucose management than at night (for most people)
Eating the meal after intense exercise will result in better glucose management
Eating fiber with the meal will slow transit time, thereby lowering the glucose load and the subsequent insulin response
A bad night of sleep the previous night will worsen your ability to manage the glucose, resulting in higher insulin levels
High levels of stress - which increases hormones like cortisol - will raise glucose levels and will impair your ability to partition the meal effectively
(Women) Eating the same meal at different times throughout your menstrual cycle will change your glucose/insulin response
Check out this tweet thread from a guy named Rishabh Srivastava who recently ran a 14-day CGM experiment:
The difference between these two graphs is stark. In the first one, Rishabh’s glucose levels spike above 160 mg/dL, which we know produces some amount of oxidative damage to the body. Conversely, adding a long walk after eating the exact same meal only spiked his glucose to just over 110 mg/dL.
Your body is not like a mechanical watch that operates in the exact same manner, day after day. Rather, your body is constantly adjusting itself based on the inputs from the environment.
This isn’t that surprising when you stop and think about it. Your body is composed of ~30 trillion semi-autonomous agents (cells) that are working together for the mutual survival of the organism (i.e. you).
On top of that, there are another ~30 trillion mostly autonomous agents (bacteria) living inside of you that exist in a sometimes-symbiotic-sometimes-parasitic-sometimes-we’re-not-really-sure state.
The point is: human physiology is messy.
Once again, this is where a CGM shines. A CGM samples your glucose levels every five minutes, generating 288 snapshots of your physiologic health per day. This won’t tell you everything about your health (we’ll talk about those limitations soon), but it’s a hell of a lot better than getting a single annual check-up at the doctor’s office.
Humans are more similar to one another than they are dissimilar, but the fact remains that your biochemistry is unique. A CGM can help you customize your nutrition, sleep, stress management practices, and exercise to deliver a tailor-made experience that optimizes YOU.
You might start to realize that your lack of energy, food cravings, and feelings of anxiety aren’t “normal”
It is common in Western societies to disassociate the mind from the body. Many people are under the illusion that things like personality, empathy, patience, or emotional stability are unchanging aspects of their character.
That’s simply not true.
Your mind is not a disembodied entity riding around in a physical body. Instead, your mind is a direct output of your brain. The brain is an organ, and like the rest of your organs, its function is intimately connected to your overall physical status.
If you drank every night, would you expect your liver to function well? Of course not.
If you smoked every day, would you expect your lungs to function well? Of course not.
So if you sleep poorly, eat nutrient-deficient foods, don’t exercise, and live under constant stress, then what do you think will happen to brain function?
Okay, guilty as charged.
But I think you see the point. Your brain function, like every other part of your body, depends on the right environment for optimal performance.
If you find that you regularly feel lethargic, anxious, depressed, irritable, or compulsively need to eat every couple of hours, then that’s a really good sign that something is going awry metabolically.
Managing glucose levels won’t necessarily solve all of those issues, but you might be surprised how many of them can be resolved once glucose levels are stabilized. Remember:
Tracking glucose is about tracking energy management in the body.
There is essentially nothing more important than energy management. So tracking glucose disposal & management (via a CGM) is a way to get a front-row seat into what’s happening in your body at a metabolic level.
How will using a CGM help me change my behavior?
In the first article, I discussed one of the fundamental principles of behavioral change: Feedback Loops. As a reminder, the principle states:
“That which is immediately rewarded is repeated more often – that which is immediately punished is avoided more often.”
One of the biggest challenges in changing behaviors is figuring out how to get the feedback loops tight enough.
In other words, how do you create scenarios where the consequences of your actions occur close enough to the original action such that it influences future behavior?
In some cases, it’s fairly easy to find these loops. For example:
If eating certain foods cause you to feel bloated and gassy, then you stop eating those foods
If drinking alcohol before bed causes you to toss & turn all night, then you drink less alcohol before bed
If performing certain movements cause discomfort in your joints, then you find different movements or you correct your technique
But what feedback does your body give you that the foods you are consuming are causing plaque accumulation in your arteries? Or that the TV you watched the night before raised your cortisol levels which led to your snack cravings the next day?
In situations like these, the feedback from your actions is delayed by days, months, or even years. We simply did not evolve to effectively respond to feedback loops of that duration.
I think this is why wearing a CGM can be such a powerful behavioral modification tool.
As an example, consider the following exchange between Dr. Peter Attia and Kevin Sayer, the president of Dexcom (one of the “Big 3” CGM manufacturers in the US). In their conversation, Peter is describing a recent experience with his CGM (he uses a Dexcom G6):
Peter: “Just yesterday I was in an airport..and there are certain snacks that I love and are kind of my comfort food when my flights are delayed. Like, I feel like I deserve a treat cause I’m stuck in the airport (laughs). And I bought one, I bought my treat, my treat is trail mix.
So I get on the plane…and I’m looking at the bag, I’m looking at my glucose and my glucose level is great…and I just decide I’m not eating it. So I didn’t eat it. But I promise you if I didn’t have that CGM, I would have eaten it. But I just didn’t feel like looking at that glucose, cause I know, I’ve done it in the past, I eat that whole bag of trail mix and my blood glucose will be 130, easily.
I love this exchange because it so closely matches what we see in the real world.
Speaking for myself, tracking something immediately causes me to become more competitive with myself. It doesn’t matter what it is that I’m tracking: weight lifted in a deadlift, 100m time on a Concept2 Rower, number of minutes of cardio per day, going to bed at the same time each night, whatever. As soon as I start tracking something, I become accountable for the results.
Maybe you aren’t wired like I am. But even if you don’t get personally competitive, then I bet you respond well when you’re accountable to others.
Think about a time you were on a team or had a group of people that you were accountable to for something (sports, job, hobby, etc.). I bet you got more serious about your results because of the external accountability.
Wearing a CGM has a similar psychological effect:
For those of you like Peter Attia or myself, it may cause you to become more self-accountable (“Do I REALLY want to eat that snack?”)
For others, you can share the data with people you care about to produce the same effect (“Do I REALLY want to let my friends/family down?”)
When it comes to health - and most of life, for that matter - adherence is the Holy Grail. This pretty much boils down to a central point: consistency matters.
I’ve written about the power of adherence in my articles on Behavioral Design. If I had to throw percentages on it, I would say that success in life is 20-30% about Knowledge and 70-80% about Adherence. If you can just do the right thing for long enough, then results are nearly guaranteed to follow.
How long do I need to wear a CGM for it to be useful?
If you are going to use a CGM, then I would recommend wearing it for at least 4 weeks.
If you’ve ever tracked any of your health markers, then you know how much they can fluctuate on both a daily and weekly basis. As we discussed in a previous Q&A, the human body is a biological organism and not a mechanical machine. As the old joke goes, “The only constant is change”.
This is why most experienced personal trainers will have their clients track metrics like bodyweight based on rolling averages rather than day-to-day numbers.
For example, let’s say you weigh yourself every day and you get the following numbers (in pounds):
178.2, 177.6, 179.4, 177.4, 176.8, 177.8, 177.2
If we add those up and divide them by 7 days, then we get an average weight of 177.8 lbs. But the real question is: did you lose weight?
The answer is: who knows!
However, this is easy to solve. Next week, you’ll do the same thing, and then you’ll compare your average from Week 1 to your average from Week 2.
This is a FAR more accurate method for determining changes to your weight over time since it (mostly) removes the random fluctuations that occur day-to-day.
This exact same process is true for your glucose levels. If you only wear a CGM for 1-2 weeks, then you aren’t giving yourself enough time to determine average values. You need a sufficiently large data set to help increase the signal-to-noise ratio. Otherwise, you run the risk of seeing trends that aren’t actually there. In the machine learning/artificial intelligence world, this phenomenon is called “over-fit”.
This is especially true if you are female since your monthly cycle causes hormonal fluctuations which affect metrics like insulin sensitivity. It is well known in strength & conditioning circles, for example, that female athletes respond to carbohydrate intake differently throughout their cycle.
All that being said…
This doesn’t mean you can’t see changes in your health in as little as 7 days. As glucose levels come into normal ranges, it is common to see energy levels increase, satiety levels increase, and anxiety levels decrease.
What can’t a CGM tell me?
We talked about some of the insights a CGM can provide, so let’s talk about what it can’t tell you.
A CGM is not meant to track markers like fasting insulin, ALT/AST (liver function), inflammation, lipid levels (triglycerides, HDL, LDL, etc.) and others. It’s still possible for other negative things to be “hiding in plain sight”, even if the CGM looks okay (though it’s less common).
It’s also not giving us the full picture in terms of fat metabolism, ketone metabolism, lactate throughput, etc. There are other energetic substrates in the body besides glucose, and all of them play a critical role in your health & performance.
Lastly, a CGM won’t (necessarily) catch the deleterious impact of things like vegetable oils, smoking, or other oxidative insults to the body. As I mentioned elsewhere, these types of markers are better captured through tests like hs-CRP (high sensitivity C-reactive protein), homocysteine levels, 8-hydroxy-2-deoxyguanosine, ferritin, fibrinogen, and others.
Every tool has a job for which it is best suited. A CGM’s primary benefit is in the realm of nutrition, with secondary benefits in sleep, exercise, and stress management.
The upside, however, is that glucose is used in the functioning of every cell in the body. Therefore, while great glucose management doesn’t necessarily mean you are healthy, you can guarantee that poor glucose management indicates real problems.
How do I get a CGM? How much do they cost?
According to the FDA, a CGM is classified as a medical device. Due to that, the only way to get a CGM in the US is through a doctor’s prescription. I’ll leave aside my thoughts about how absurd that is and focus on the question at hand 😒
Broadly speaking, there are two ways you can go about getting a CGM as a non-diabetic:
Ask your doctor if they will prescribe one for you if you pay for it out of pocket
Apply for a CGM through a private company.
Let’s look at each option briefly.
#1 Ask Your Doctor
Unfortunately, the state of the US insurance industry means you won’t be getting your CGM compensated by your insurance company. You can try, but let me save you the hassle: they won’t cover it. Apparently, it’s much better to pay significantly more money for reactive medicine than it is to invest in proactive approaches. Yay, America. /end-sarcasm
The second challenge you’ll face going through your doctor is that many doctors won’t prescribe you a CGM if you aren’t pre-diabetic. Many people don’t realize that doctors generally only receive 24 hours (+/- 4 hours) of formal nutritional training during school. As such, a significant number of doctors don’t realize how useful technologies like CGMs can be for proactively addressing health issues. Sad, but true.
Despite the challenges of going through a doctor, it can be worth it from a cost perspective. Even though you’ll be paying for it out of pocket, it is typically cheaper to go through a hospital system because of the price discount they get.
For example, one of my family members recently got a CGM through his doctor for about $80/month. Note that you can typically use your HSA dollars towards the cost of a CGM as well.
Compare that to $250-400/mo that it typically costs through private companies, and you can see why going through a doctor might still be worth it.
When you get a CGM, it will typically come from one of the 3 major CGM providers:
Usually, you will get the default software that comes with the CGM. Since CGMs are typically only prescribed to diabetics, the user interface won’t be set up for someone who doesn’t need insulin. As such, the user experience may feel lacking.
#2 Get a CGM through a Private Company
Due to the public’s growing interest in preventative health, more and more private companies are getting into the CGM space.
Two examples of private companies offering CGMs are NutriSense and Levels. I don’t have an affiliation with either company, though I have referred several people to NutriSense over the past year. Due to that, I’ve also gotten to see their technology in action, and I can confirm it is fantastic.
I see two major benefits of going through a private company:
It’s easy to get a CGM. All you do is fill out an intake questionnaire, and apart from a few exclusion criteria, most people qualify to get a CGM. (Some of you might be wondering how these companies can prescribe a CGM if it requires a doctor. In fact, these companies do use doctors behind the scenes to write prescriptions for the CGMs. In essence, the company is acting as a middleman between you and the doctor.)
The software is designed with non-diabetics in mind, meaning the user experience is WAY better.
That second point is worth reiterating. Remember how I mentioned that when you are prescribed a CGM through a doctor it typically comes from one of the 3 major CGM companies (Dexcom, Abbott, or Medtronic)?
When you order a CGM through NutriSense, it’s the exact same process. In the case of NutriSense, the CGM you get is the Abbott Freestyle Libre. However, NutriSense has built a custom front-end mobile app and website for its users. This UI is built specifically with non-diabetics in mind and provides a beautiful interface for tracking things like sleep, exercise, nutrition, and stress inputs.
Additionally, NutriSense has a support team of trained dietitians that provide coaching and feedback to their users to ensure a smooth experience that maximizes results.
Lastly, the user experience is constantly getting better because NutriSense can use machine learning to extrapolate best practices from their user base. That’s a bigger deal than most people may realize. Large data sets don’t really exist for non-diabetics wearing CGMs (at least not in the US), so this is an area ripe for innovation.
Personally, I’m really damn excited to see how better data helps resolve many of the ridiculous debates in the nutrition world. As Nutrisense’s Director of Nutrition, Kara Collier, is fond of saying:
“Data over dogma.”
Does wearing a CGM hurt?
But if you want to watch a funny video where a lady psyches herself out in her first attempt at applying a CGM, then watch this (outtakes start at 7:02). Makes me smile every time:
So good 🤣
I truly hope you try out a CGM soon. There is a lot of silly BS in the world of “health & wellness”, but CGMs are a true bright spot.
If you have more questions, leave a comment down below and I’d be happy to update the article.