What Is a Continuous Glucose Monitor (CGM) and Should You Try One?
Sneak Peek into Chapter 2 of Hack Your Blood Sugar.
📖👀Here’s a peek into one of the chapters I sent to my editor last month. I’d love your feedback! Did it answer your questions?
What Is a Continuous Glucose Monitor (CGM)?
If you grew up watching Star Trek, you might remember the hand-held device Bones (aka Dr. McCoy) used to scan the body for a quick assessment. Bones could make a very thorough diagnosis with that handy little gadget.
Wouldn’t it be amazing to have a device that told you, minute-by-minute, how your body was reacting to the food you eat? Gone would be the days of guessing how food affects you. You could also silence your nosy Aunt Susan, who’s always telling you how to eat, and stop wondering if your brother’s keto diet would work for you.
Welcome to the future. We now have just such a gadget. Continuous glucose monitors, or CGMs, have been around for years, and they are now so widely available that you can order one online and have it shipped to your doorstep in a matter of days. A CGM is one type of wearable that tests your glucose levels in real-time and sends your data to an app on your phone. They are in a class of gadgets we call wearables. Wearables are biosensors that can tell us about our sleep, blood sugar, heart rate, body temperature, and more. Their software puts the data together to tell us about our metabolic health, how well-rested we are, our ovulation status, and more. They send the data to an app on our mobile phone for easy access.
You might think it sounds creepy. But don’t you think, just a little, that it’s exciting to have such immediate feedback about how food affects your blood sugar?
Have you heard of people with diabetes pricking their fingers to test their blood sugar? Many diabetics gave up routine finger sticks years ago thanks to CGMs. CGM technology is a big jump up from sticking your finger multiple times per day, and not just from a pain standpoint. It’s inconvenient to have to stop and test your blood sugar 4-10 times per day. Many people with type 1 diabetes now wear a CGM that automatically tells an insulin pump how much insulin to inject. That’s how well CGMs can work.
Fingersticks vs. CGMs: What’s the Difference?
A finger stick gives you just a snapshot of one moment in time, while a CGM catches everything. The idea of sticking your finger after a meal is to see how high your blood sugar goes after said meal. With a finger stick, you have to guess on timing. What you're trying to do is guess when your blood sugar will be at its highest point after a meal. If you test an hour after a meal and it’s at 150 mg/dl (8.3 mmol/L), there’s a chance your blood sugar peaked earlier at 165 mg/dl (9.2 mmol/L) or is still rising. With a CGM, you can see the entire picture.
Also, when a person tests their morning fasting blood sugar, that data can be skewed. Many people with prediabetes have something called a dawn phenomenon. Their blood sugar might be at a healthy level all night long (say, 85 mg/dl or 4.7 mmol/L), but just as they begin to wake up, the dawn phenomenon causes their blood sugar to shoot up to 120 mg/dl (6.7 mmol/L)!! If they test at that point, they won’t know if their blood sugar was high all night long or if it just shot up in the morning. Knowing the difference gives you real insight into what’s going on. As you’ll learn later on, overnight CGM data can give you essential insight into your sleep and how to time your evening meal.
How Does a CGM Work?
A CGM is a small disk that ranges in size from a nickel to two stacked quarters. It has a small filament about 5mm long and is as thin as an acupuncture needle. It inserts just under the skin. The CGM doesn’t go deep enough to hit your bloodstream, and most people report it doesn’t hurt to install.
There are 3 parts to a CGM: The Sensor, the transmitter, and the receiver.
The CGM itself contains the sensor and a transmitter. The sensor measures your glucose levels and stores 8 hours of data, in case you are away from your reader. The transmitter will wirelessly send your data to your receiver, which for most people is their smartphone.
Your reader is typically a smartphone. All CGMs come with a free app that you install on your phone to show you your data. The prescription CGMs can also work with a separate reader for people without smartphones.
Where and How to Wear a CGM
The CGM typically goes on the back of your arm and stays put for about 2 weeks or until you remove it. You can exercise with it, shower, swim, and travel with it. Most of the time, you forget it’s even there. The CGM will work on just about any fleshy part of the body, such as the chest, back, or abdomen. Anywhere you’ve got a little fat will work.
Interstitial Glucose vs. Blood Glucose
A CGM works by testing your interstitial glucose levels. Your interstitial fluid is the fluid between cells in your body. It’s not the same as your blood glucose but it responds in a consistent manner to your blood glucose.
Imagine a nickel taped to the back of your arm for 2 weeks that you can just forget about while it collects glucose data for you to view on your phone anytime you need it.
The History of CGMs: From Finger Pricks to Smart Sensors
CGM use started back in 1999 with an implantable sensor that lasted 3 days. Researchers had long been looking for a way to improve compliance for their patients with diabetes in monitoring their blood glucose. As you can imagine, it’s a big ask to tell patients they have to stop and stick their finger multiple times per day. Frequent testing dramatically improves the health of those living with diabetes, so it was important to find a way to make it easier and less painful.
The ultimate goal was a “closed-loop system.” Researchers and advocates wanted to create a sort of artificial pancreas where the CGM would sense glucose levels and talk to an insulin pump, which would automatically deliver the correct dose of insulin to the person. All this would happen without the user having to think about their meal, prick their finger, count carbs, or consider their activity levels. The first 3-day CGM in 1999 was still a long way off from delivering on this dream but it was a major step forward.
It wasn’t until 2004 that health technology company Medtronic came out with a wireless CGM that would automatically transmit glucose levels to a reader, which was followed by the appearance of the first Dexcom CGM in 2006. Unfortunately, these still had a measly 3-day lifespan. Dexcom stretched their lifespan to seven days in 2007. But in 2014, Abbott joined the field with their CGM and, in 2018, launched their FreeStyle Libre in the US, lasting a whopping 14 days.
The story of the CGM parallels the evolution of the smartphone. While CGMs could (and still can) transmit to a separate reader, modern CGMs transmit glucose data to a smartphone. The first iPhone came out in 2007, followed by Android in 2008. Because CGMs and smartphones make such a great pairing, the emergence of modern smartphones helped CGM technology take a giant leap forward.
In 2016, Eversense came out with a 90-day sensor, but this is an implantable device. While it’s an excellent concept for individuals dependent on insulin, it’s not a practical model for a more casual user.
CGMs and the Rise of Biohacking
In 2017 and 2018, with both Dexcom’s G6 CGM and Abbott’s FreeStyle Libre CGM available for installation on top of the skin for 10 to 14 days of use, it became of interest to early adopters. Today, we call these early adopters biohackers.
Yes, these devices are for people with diabetes. But those of us working in the field of nutrition and metabolic health know how important blood sugar is to overall health, even in the absence of diabetes. Dave Asprey started the official biohacking movement in 2011, and he coined the term biohacking, which the Merriam-Webster dictionary added (and his name with it) to their lexicon in 2018. Biohacking is described as “biological experimentation done to improve the qualities or capabilities of living organisms.” Using a CGM to optimize blood sugar was an obvious tool for them.
Another well-known biohacker or self-experimenter, as he refers to it, Tim Ferriss, was known for wearing the earliest CGMs back in 2007. I want you to understand just how hardcore this was. These days, installing a CGM is painless and easy and it sends your information directly to your mobile phone. Back in 2007, it had to be implanted under your skin with the equivalent of a small pitchfork, and you had to carry a tablet around to read your data. Like other biohackers, he did not have diabetes or anything close to it. It came from his interest in self-experimentation and as research for one of his books.
More recently, Dr. Peter Attia has brought attention to using CGMs in his medical practice. Dr. Attia, through his popular podcast The Drive, focuses on Longevity and Health Span and has spoken at length about using CGMs with his patients to optimize their blood sugar and reduce their risk of chronic diseases. The use of CGM in non-diabetic individuals in his medical practice started to make other healthcare practitioners sit up and think of new ways to prevent diabetes rather than waiting to treat it. This led practitioners like me to seek CGMs out for our patients and use them in a preventative fashion, not just to manage insulin use.
Prescription vs. Over-the-Counter CGMs in 2025
For those living in the United States, you could still only purchase a CGM with a prescription from your doctor until August 2024. This, along with the higher price point, put them out of reach for most people. Multiple tech companies have popped up over the years to bridge this gap. Companies like Signos, Nutrisense, Levels, Veri, and Zoe have prescribers on staff who could write prescriptions for members. Consumers paid a premium for the CGM to be shipped to them and for a custom app subscription that helped interpret their glucose data. It wasn’t cheap, but it was within reach of many interested in optimizing their health.
2020 IN MY PRACTICE
You might remember this year for making masks a fashion statement, but it was an exciting year for my work. It was in 2020 that I started using CGMs with my patients. I found it fairly simple to request prescriptions from my patients’ doctors. Most doctors saw the obvious benefits of using CGMs preventively to improve blood sugar control well in advance of diabetes. The prescription made it possible to get CGMs for a good price at local pharmacies. Creating this team of patients, doctors, and nutritionists with this CGM data proved to be a winning strategy. My patients quickly learned how to improve blood sugar levels with less work, and that gave them a greater sense of control. When I say “quickly,” I mean some transformed type 2 diabetes glucose levels into healthier levels in as little as 4 weeks with simple diet and exercise changes. This allowed doctors to make better and more timely decisions about adding or dropping medications. I soon had nearly all my patients using them for at least 4 weeks.
2024: THE YEAR OF THE OTC CGM
2024 was a pivotal year in the US, as two new CGMs that were available without a prescription hit the market. We refer to this as “over-the-counter” (OTC) when you don’t need a prescription for a device or medication. In August of 2024, anyone could order a CGM online and have it delivered 2 days later.
I, for one, am firmly in the camp that believes we should all have access to these sensors, given the prevalence of blood sugar disorders in this country. Type 2 diabetes is often decades in the making. By having earlier access, we can course-correct early enough to avoid diabetes and the related chronic health conditions for many years.
However, I’ve seen over the months that many people using a CGM on their own feel even more lost. They aren’t sure how to interpret their data. Many don’t know what their blood sugar should look like. It can result in more confusion.
CGMs Aren’t Magic — When Data Isn’t Enough
In the summer of 2023, I was at an event and met a woman who saw me wearing a CGM. She introduced herself and told me that she used one too. She had been using one of the tech company’s CGMs for a year now. However, she confided in me that she was taking a break. “I’m just kind of frustrated,” she explained. “I get all these readings telling me my blood sugar is too high, but I don’t know what to do about it.”
She’s not alone. Since I started doing this work, I’ve been contacted by many people who have been using CGMs for months, if not a whole year. They have a mountain of data but no improvements. It gives them stress and anxiety, being unable to make sense of things.
This experience is not unusual. I want to be very clear that the “one-to-many” model that tech companies have created works well for many. They created apps that suit the majority of people with blood sugar issues like prediabetes and metabolic syndrome. Users who like data, enjoy a DIY approach, and feel confident turning that data into effective strategies will have success using these custom apps. However, there are two categories of people who don’t fare as well with this approach and may benefit from working with a nutrition professional.
The first type is someone with blood sugar data that isn’t as straightforward as others. It might be a woman in menopause who is otherwise metabolically healthy but finds her blood sugar labs are starting to creep up. It might be a person with insulin resistance and all the signs of metabolic syndrome (which we’ll cover in the next chapter), but because they are making so much insulin, their blood sugar still looks good on the surface. It could also be a person who is eating well but they have sleep issues, chronic stress, or inflammation. The stress hormones and inflammation can disrupt blood sugar without any contribution from food. These people need a practitioner who can do some sleuthing with them to help uncover the root causes of their blood sugar issues.
The second type is someone who simply prefers working with a human. They may not get on well with smartphone apps and might get frustrated with the data. They want another human to put it into perspective for them, coach them through the process, and smooth out the tech hurdles.
There is no one perfect way to use a CGM. There is only the approach that suits you best.
Real-Life Success Stories Using CGMs
Robert is a 64-year-old man who came to me with type 2 diabetes. We worked together for 3 months and got his blood sugar down well into the prediabetes range. He was already a fairly active person who was eating a relatively healthy diet.
We noticed right away that his blood sugar was very sensitive to even small amounts of carbohydrates. He seemed to have poor insulin production or delayed insulin production. I’ve seen this happen as we age, and it can be genetic.
Once we figured this out, Robert switched to a low-carbohydrate diet with more protein, non-starchy vegetables, and healthy fats. He paid attention to timing his meals by not eating at night and saw big improvements in his A1c and his health.
A year later, he mentioned to me that his adult son was having the same problem. He immediately shared what he had learned with his son and recommended his son to my practice. However, his son had a very busy life and didn’t need to add more appointments to his calendar. He decided to try one of the tech companies instead. For him, the app and the DIY approach did the trick, and he was able to lower his A1c on his own.
Even though Robert and his son chose different methods for using a CGM to hack their blood sugar, they both got great results. The common denominator was being able to see exactly what foods and habits were problematic so they could make immediate changes for rapid results. The two options accommodated different lifestyles and personal preferences, but both delivered the desired results.
JoAnne’s experience had little to do with food
JoAnne contacted me after using a CGM with a tech company’s paid app for 3 months. JoAnne is a 56-year-old woman who had an A1c in the prediabetic range. She’s been watching her CGM data over the months and getting tips from the app, but nothing was changing. She was nearly ready to throw in the towel on her CGM when I talked to her.
JoAnne did a full intake with me so I could see her health history and lab results from the previous year. After reviewing her history, I already had a good idea that she had a history of gastrointestinal issues, higher levels of inflammation, and wasn’t sleeping well. She appeared to be making good food choices but was starting to be more restrictive of her food because the CGM data made her question her choices.
It became clear to both of us that the combination of poor sleep and inflammation was keeping her baseline blood sugar levels elevated. You will learn more about why this happens in the coming chapters. For JoAnne, more food restrictions weren’t going to help. She needed help improving her digestion and gut health and strategies to improve her sleep. So that’s where we put our focus! She needed a home sleep study, and we did some functional gut testing to help us discover what approach to take with her gut health.
If a person has poor sleep, there is a limit to what you can achieve with food. If there are gastrointestinal issues causing increased inflammation in the body, this can also be a root cause of elevated baseline glucose levels. Unlike the quick wins that Robert and his son got using their CGM, some people will learn how to improve their diet but then realize that there is more work to be done elsewhere. We’ll go into more detail on the roles of gut health and inflammation a little bit later.
Which CGM Brands Are Available in 2025?
I don’t want to get into the weeds too much on this topic because this technology is changing all the time. The current CGM brands will continue to upgrade, and new devices will emerge. That said, I feel that it is helpful to describe the current field of CGM technology to give you a better understanding of what it is and how it works.
In reality, no matter what Google tells you, there are only 2 two manufacturers that you’ll have access to for home use of a CGM. They are Abbott and Dexcom.
Prescription-only: (Can be used with a reader or smartphone)
Abbott FreeStyle Libre
Dexcom G6 and G7
Over-the-counter and available to purchase online (Must be used with a smartphone)
Abbott Lingo
Dexcom Stelo
All of these come with their own app at no extra charge.
Users in the United States require a prescription to access some CGMs. Most other countries have them all available over-the-counter. For example, if you live in the United Kingdom or Canada, the G7 and FreeStyle Libre will be covered or less expensive if prescribed by your doctor. However, anyone can walk into the drug store or chemist and purchase it off the shelf for a higher retail price.
When you do an internet search for “continuous glucose monitor” or how to buy it, you’re going to see a lot of different images. Many will show the actual sensors from Abbott or Dexcom on someone’s arm. It will be a white device that’s round or oval and about an inch in diameter. Others will show a big patch with a logo. That patch is like a bandage that covers the CGM. It’s an optional extra layer of protection. The tech companies that offer custom apps will put their logo on that ‘bandage.’ Know that the CGMs underneath the patch are the same ones we are talking about here. They’re all using either a Dexcom G6/G7 or a FreeStyle Libre CGM. Today, this includes tech companies like Signos, Nutrisense, Levels, and Zoe.
Stay with me. In a few more chapters, we get into the weeds with how the modern CGMs work, and how to optimize their use.
References
Didyuk O, et al. Continuous Glucose Monitoring Devices: Past, Present, and Future Focus on the History and Evolution of Technological Innovation. J Diabetes Sci Technol. 2020 Jan 13;15(3):676–683. doi:10.1177/1932296819899394