What’s the Deal With Continuous Glucose Monitoring?
Ultracyclist Ryan Collins, 26, takes the Abbott Libre Sense Glucose Sport applicator, presses it against the back of his arm and pushes. Thunk. In goes the fine needle biosensor, which gets inserted under his skin, and the transmitter adheres to his arm. He adds a protective adhesive patch over the transmitter, fires up the Supersapiens app, and is ready to start experimenting.
First up: the gel trial. Collins grabs some fistfuls of gels and hops aboard the trainer for a seven-hour ride, the whole time watching his glucose levels rise and fall, aiming to stay in the literal sweet spot where his muscles have all the energy they need to fuel his marathon effort.
“I did the workout fuelled entirely by gels, and I could see that my glucose just went up and down and up and down for seven hours,” he says. “I would hit one and I’d feel great and then I would be like, ‘Boom, I need another gel.’”
That’s not ideal for Collins, who is the Six-Hour World Record Holder and the 2019 and 2021 USA National Champion in the 12-Hour Time Trial. He’s currently preparing for a season that includes a 12-hour race in Florida, defending his national championship, and other ultras. He needs steady energy.
“I started using a continuous glucose monitor because I was curious about myself. You hear all this advice about what and how often you should eat and that these special gels will make you go faster, and I’m always like, ‘Really?’ Now I can test that out myself,” he says. “Nutrition can be such a moving target, but this lets me get as close as I can to the optimal solution for a given timeframe.”
What are continuous glucose monitors?
A CGM lets you see your glucose levels in real time.
A CGM is a circular glucose monitoring apparatus about the size of a quarter that includes a hair thin needle and a transmitter that pairs with a companion app via Bluetooth.
You use a special applicator to insert the needle and press the disc onto the back of your upper arm (it feels more like a tap than a shot). The needle registers the concentration of glucose in milligrams per deciliter (mg/dL) that is circulating in your interstitial fluid, which is the fluid that surrounds every cell of the human body.
“If your cells are houses and your bloodstream is the highway, the interstitial fluid is the side streets the glucose travels along before it stops and enters the house, so an interstitial measure may be a better indicator of what’s happening in the houses or cells,” explains sports physiologist Allen Lim, Ph.D., founder of SkratchLabs, who has been testing a CGM for the past year.
The transmitter sends that information to your phone, so you can read your glucose levels in real time. (It also stores the information if you’re not near your phone.) Though there’s a five- or 10-minute delay between the blood sugar levels in your bloodstream and the levels in your interstitial fluid, research finds that the readings are very close.
Supersapiens also includes system integration with Garmin head units and watches, as well as a special Supersapiens Energy Band so you can monitor your glucose in real time while you work out. The company is also working on integration with Wahoo and Hammerhead units.
Each sensor lasts for 14 days. They also come with an adhesive patch that helps keep the sensor in place. Supersapiens comes with a water-resistant patch so swimmers and triathletes can use them for training and racing.
How continuous glucose monitors made monitoring blood sugar easy
People with diabetes (especially Type 1) have used continuous glucose monitors (CGMs) to manage their blood sugar for years. The disease makes managing blood sugar difficult and levels that are too high or too low put your health at risk. CGMs replaced the traditional finger stick blood test, which could only provide a snapshot of blood glucose at a point in time, while the CGM provides a full-length continuous documentary of your glucose levels. For those with Type 1—especially athletes who exert themselves and have a high demand for blood glucose—CGMs were a game changer (and potential lifesaver).
Now companies are bringing CGMs to the masses to help them manage their blood sugar and metabolic health to hopefully avoid Type 2 diabetes and other chronic disease, as well as improve fitness and performance. The three biggest players right now are NutriSense, Levels, and Supersapiens (which is not yet available in the U.S.).
Some experts are hailing CGMs as the gateway to better health for millions who have high levels of chronic metabolic disease. The UCI has banned them in cycling competition. Skeptics say they’re being oversold. Here’s the lowdown on what they can and cannot do.
Why should you care about glucose levels?
Anyone who’s ever bonked knows how important glucose levels are for your energy levels. When you’re working out at a high intensity, muscles break down glycogen (or stored glucose) for fuel. Meanwhile, the liver releases glycogen to raise your blood sugar. Both mechanisms give you the energy you need to perform. Fully stocked, you only have between 350 and 500 grams of glycogen (about 1 700 calories’ worth) of muscle and liver glycogen stores, so you need to keep fuel coming in if you want to keep turning your pedals.
Once your glycogen stores run dry, you start relying on fat, which can go a long way, but it can only be used at low intensities, so you have to dial down the pace—sometimes dramatically—and watch your competitors roll away. That’s why athletes carefully plan their nutrition for training and events and are told to eat 30 to 60 grams of carbs—and 80 to 100 for racing—to keep their energy levels high. These are, of course, general recommendations, and CGMs may help you pinpoint what you actually need.
“The elite athletes I work with are finding that they need fewer carbs than they thought when they’re not going hard, but when they’re going hard and competing, they need more carbs than they thought,” Lim says. “They may have been taking in 60 to 80 grams during racing. Now they’re trying to get 100 to 120. Ultimately, the amount of energy we need depends on our fitness…the fitter the athlete, the more energy is needed to fuel that fitness.”
Of course, you don’t need high-performance blood sugar levels when you’re plopped at your desk. In those circumstances, you want a nice stable, moderate glucose level. The Supersapiens app (which I personally tested the longest) has two scales that reflect this: Perform, which is between about 110 and 180 mg/DL and Recover, which is between about 70 to 140 mg/dL. The sensor captures glucose concentrations between 55 mg/dL and 200 mg/dL.
Your body regulates your glucose to maintain a relatively constant baseline. It does so through insulin and muscle contraction, Lim says.
“The brain and nervous system rely solely on glucose for their energy source and that glucose enters the brain and nervous tissue according to the concentration of glucose in the blood or interstitial fluid,” he says. “If glucose levels are too low, then the nervous system suffers. But if it’s too high, too much glucose can enter those cells causing damage. Muscles (and fat cells) on the other hand either need the hormone insulin to absorb glucose while you’re at rest, or they can uptake glucose by contracting. Either way, the body is always trying to keep blood glucose levels very constant.”
When this system falls out of balance, especially if your food intake isn’t matching your energy output, that can spell trouble for your glucose levels. Low levels (hypoglycaemia) bring about shakiness, irritability, weakness, and irregular heartbeat. High levels are problematic, too, especially over time.
The brain and nervous system rely solely on glucose for their energy source.
High levels of sugar overload the liver, increase inflammation, and lead to higher amounts of stored fat. “Bathing your cells in high glucose levels is damaging to them, which is where you get vascular disorders, cardiovascular disease, and neuropathy [numbness and pain, usually in the hands and feet from nerve damage],” says Lim.
You also don’t want your insulin to spike and crash over and over throughout the day, as it can when you eat simple carbohydrates without fibre and/or other macronutrients that slow digestion and absorption into the bloodstream. When you have a large spike, your body needs to release more insulin to bring those levels down. Repeat that process over and over and you can develop insulin resistance where the cells become unresponsive to insulin and let the excess sugar course through your system. That sets the stage for chronically high glucose levels and Type 2 diabetes.
On the general health front, a CGM can be a powerful tool, Lim says. “I don’t drink soda anymore. Liquid carbs crank up your blood sugar levels. That’s why Coke is so good when you’re working out, but so bad when you’re just sitting there on your sofa watching a movie. I also don’t eat carbs without protein or fat to stabilise the digestion and absorption of the sugar.”
The takeaway: You want your glucose levels high when you’re busting your butt, but to stay lower when you’re not. That requires looking at what you eat, but also when you eat, your state of mind, and your overall lifestyle.
How can a CGM help with performance?
CGMs help you fine tune your fuelling strategy before, during, and after a ride.
It’s easy to think of a CGM like a fuel gauge for your car: when your numbers show your glucose levels are “full” you’re good to go; when they show you’re running low, it’s time to fuel up. But it’s not that simple.
While it is useful to be able to see where your glucose levels are and if they’re trending up or down (which the Supersapiens app shows), cycling physiology is complex and there are many factors that go into how you’re feeling at a given moment, including hydration, electrolyte status, and body temperature.
A CGM can be a good tool to help you understand what you’re feeling, though. If you’re cruising along several hours into a long event and you’re starting to feel off, checking your glucose levels can help you determine if you need food or if it’s something else. That way you don’t automatically shovel in a packet of energy blocks and find yourself with a case of gut rot.
It can also help you understand the lag time between the fuel you take in and the increase in glucose with different foods and drinks, which can help you time your fuelling better.
Overall, the real value of CGMs is just that: dialling in your fuelling plan during and around training rather than trying to use it in real time as a fuel gauge. That’s why the Supersapiens app focuses on three elements—prime, perform, and recovery. And here’s where CGMs can come into play in each of those settings.
Dialling in your nutrition before a ride or race
CGM technology is very useful for pre-ride and pre-race fuelling strategies, Lim says. When you eat, glucose levels rise, which triggers insulin to bring blood sugar down by moving it into muscle. That’s generally a good thing. But if you start exercising in that window, you also have your contracting muscles moving even more glucose out of the bloodstream.
“Then all of a sudden there’s not enough blood glucose to fuel the brain and nervous system, which absorb glucose based on its concentration in the blood or interstitial fluid,” Lim says. “So if you miss-time your pre-training or racing meal, which many athletes do, eating in a 60- to 90-minute window, you can feel really crappy as you begin. A CGM helps you figure out the timing and composition of those before-exercise meals and snacks.”
Some athletes find they’re better off eating two and a half or three hours before go-time, so they have less insulin circulating in their bloodstream. Then the key is not eating again until you’re on the line and ready to go, when insulin levels will stay low in response to exercise. Or if your event is super early, you could be best off just eating a little right before and fuelling as you go.
With experimentation, you can learn the fuelling strategy that works best for you as an individual, says former pro cyclist Brad Huff, global ambassador manager for Supersapiens. “Two athletes can eat the same pre-race meal and one will have a spike and a crash and the other won’t. Without this insight, it’s hard to know that.”
See how much food you need during racing and training
Though, again, it’s not a precise fuel gauge, a CGM can allow you to experiment with fuelling strategies during performance to see which works best for you.
It’s that kind of experimentation that has Collins hooked on his CGM. Along with his seven-hour gel experiment, Collins has pulled similar multi-hour trainer sessions using Skratch Superfuel, Clif Bars, and various combinations of pure carbs and mixed macronutrient energy foods.
“I’ve also experimented with the grams of carbs per hour,” he says. “I compared my glucose levels taking in 100 and 110 grams of carbs per hour at a steady work rate. My glucose levels really didn’t change, nor did the variability. So, I probably don’t need 110 grams. That’s good information because having less in my stomach means there’s less to digest, which lets blood flow go elsewhere in my body.”
He’s also experimented with “dripping versus dumping,” meaning taking in small bits of food at smaller intervals versus eating a larger amount every hour. “The steady drip strategy works best for me,” he says.
Learning how to eat to recover faster
Wearing a CGM also shows you the importance of early recovery nutrition, Huff says. “How often are you at the end of a training ride and you’ve got another 45 minutes or so and don’t bother eating? It’s easy to blow it off. But with this technology, you can see your blood sugar falling. And if you let it keep dropping it’s harder to recover than if you keep your energy levels stable,” he says.
To give you the big picture, Supersapiens gives you a Glucose Score from 1 to 100 for every workout. When the number is higher, it means you were able to maintain and control your glucose levels during exercise. When it is low, you may need to re-evaluate your fuelling strategy to make sure that glucose availability isn’t impeding your ability to perform. But again, it is not a strict measure of energy. You can have a low score and feel great or a high score and perform poorly. There are many factors that affect performance.
What else can a CGM tell you?
You’ve undoubtedly heard how bad stress and too little sleep is for you. You can see that in real time with a CGM.
Picture your liver as a big reservoir of glycogen. When your brain senses you need energy, stress hormones like adrenaline and cortisol open the gates and let stored sugar gush out so you can punch your way out of trouble. When you’re chill, you don’t need all that energy, so your glucose levels are more like a gentle rolling tide, swelling up when you eat and down as your body pulls the energy into the cells.
Eating with others is good for my blood sugar
Problem is, much of our daily stress doesn’t involve fight or flight, but sitting and stewing. So you experience stress, your liver lets out that gush of stored sugar, and you have no way to burn it off. Making matters worse, cortisol inhibits the action of insulin, so your blood sugar will stay high when you don’t need or want it to be.
Stress also can impact how you respond to any given meal, Lim says. “When I have a breakfast and I’m taking my time and looking at the mountains, my blood sugar response is nice and gently rounded. But if I get in the car and eat it on the way to the airport, my blood sugar is going to shoot up and stay elevated. The same happens when I’m eating out with friends and the meal is nicely paced and we’re having a conversation versus sitting at my desk shovelling food down. Eating with others is good for my blood sugar.”
Sleep has a similar effect. When you’re sleeping soundly, your overnight glucose and resting daytime glucose levels are stable and in a lower range. When you’re not, you become less insulin sensitive, so you don’t clear glucose as well.
Wearing a CGM can reveal this info, telling you how your glucose levels are responding to high stress or low sleep moments, so you can take steps to reign it in.
The Downsides to CGM Tech
The biggest problem I ran into while testing CGMs was physical and technical glitches. For the life of me I could not get a sensor to last the full, manufacturer promised 14 days. Some of it was my own doing: I would knock it off walking through a doorway (apparently, I cut corners in my house really close). During other trials the bandage would start rolling up around the edges after a few days and eventually the whole thing would pull off when I took off an arm warmer. Other times the sensor would simply die before the expiration date, which could have been from the device coming loose overtime because of those adhesive issues.
I also didn’t love the notion of generating so much plastic waste by tossing sensors and applicators every two weeks.
Continuous Glucose Monitors for Athletes: The Bottom Line
CGM use for performance in otherwise metabolically healthy people is new and there’s a lot that even the scientists are still learning. The real value comes from the companion apps that give context to the numbers you see and allow you to dial in your nutrition, exercise, and behaviour to maintain healthy levels without big spikes and drops.
To that end, the Supersapiens system is the best one for cyclists, because the app is focused on how your glucose levels impact performance and you can watch them in real time. But it is not yet available and not the only game in town.
In addition to Supersapiens, I also did a trial run with the NutriSense unit, which doesn’t give a graph in real time—you must scan the device when you want to check. But it delivers glucose scores and graphs based on your blood sugar response to your diet and exercise bouts. As soon as you onboard with the app, you get a note from one of NutriSense’s in-house dieticians who can help you interpret what you’re seeing and help you reach your goals. I have not used Levels, but the set up is similar to NutriSense, you get matched with a medical expert and the app provides metabolic scores and optimum zones, and advice on how to personalise your diet and exercise to achieve the healthiest levels.
Regardless of what system you choose, I think everyone could benefit from a few weeks of wearing a CGM. In fact, it doesn’t really take more than a trial period to experiment with your nutrition, get insights into how you respond to various fuelling strategies, and then proceed CGM free, knowing that you’ve dialled in what works best for you.