Continuous Glucose Monitors (CGM) 101

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A continuous glucose monitor, or CGM, is one of the most interesting pieces of wearable tech to move from medical use into fitness and nutrition. It is a small sensor that sits under the skin and tracks how much glucose (or sugar) is in the fluid around your cells. This fluid is called interstitial fluid.

Unlike a blood test, a CGM does not draw blood. Instead, it reads glucose levels in that surrounding fluid, which naturally reflects blood sugar with a slight delay. Because glucose needs time to move from the blood into this fluid, the readings usually lag behind by about 5-10 minutes. That delay matters most after meals, during hard workouts, or right after training when glucose levels change quickly.

The science under the skin

Every CGM has three main parts: the sensor, the chemistry, and the algorithm.

The sensor is a thin, flexible filament inserted just under the skin, usually on the arm or abdomen. Glucose molecules drift from nearby blood vessels into the fluid around the sensor. An enzyme called glucose oxidase reacts with the glucose. 

That reaction produces a tiny electric signal. The stronger the signal, the higher your glucose level. The CGM’s algorithm converts that signal into a number that represents your glucose level in milligrams per deciliter. The software also filters out noise and accounts for the few minutes of delay between blood and interstitial fluid.

When you open your app and see a line moving up or down, it is showing you the result of chemistry, electronics, and computation all happening under your skin.

How accurate are they

Accuracy is measured using something called MARD, which stands for mean absolute relative difference. It is just a way to compare how close the sensor is to a lab-grade blood test. The lower the number, the better the accuracy.

The Dexcom G7 and Abbott FreeStyle Libre 3 both have MARD values around 8%, which means they are usually within a few points of a blood reading. That is accurate enough for daily use and for learning how your glucose moves throughout the day. These sensors are also factory calibrated, meaning you do not need to prick your finger to set them up.

From medical device to everyday tool

Until recently, CGMs were only available by prescription to people with diabetes. That changed in 2024 when the FDA approved the first over-the-counter versions.

Dexcom launched a device called Stelo, and Abbott followed with Libre Rio. Both are designed for adults who are managing their glucose through food and lifestyle rather than insulin. This shift opened the door for people who want to understand their metabolism, energy, or recovery, not just those managing a medical condition.

The companies shaping the space

Dexcom and Abbott still make the main hardware, but many new companies are building experiences around their sensors.

Levels uses Dexcom sensors to show how your meals and habits affect your energy and focus. Nutrisense pairs similar data with one-on-one coaching from dietitians. January AI takes it further, using photos of your meals and wearable data to predict how your glucose might rise and fall, even without a CGM.

For athletes, Supersapiens uses Abbott sensors to track fueling during endurance training. And Oura, known for its sleep and recovery ring, now integrates glucose data through a partnership with Dexcom. These companies are turning glucose tracking into a view of metabolic health.

Why athletes and lifters are paying attention

For people who care about building muscle or performing well, glucose is fuel. Your muscles store glucose as glycogen and use it during both training and recovery. Watching how your glucose rises, peaks, and falls helps you understand how your body handles that fuel.

A CGM can show how a pre-workout meal affects your energy, how quickly you recover after a session, or how certain foods lead to big spikes and crashes. Over time, those insights help fine-tune carb timing and meal choices for steadier energy and better recovery.

What to keep in mind

There are a few important things to remember. Because CGMs measure glucose in interstitial fluid, they always show a slightly delayed picture. If your glucose graph suddenly drops, it might reflect what happened a few minutes ago, not what is happening right now. Use the data for trends, not quick reactions.

Also, noninvasive glucose sensors that claim to measure glucose through the skin or sweat do not exist yet in any approved form. The FDA has specifically warned consumers about smartwatches or rings that make such claims.

Lastly, cost and coverage are still evolving. Over-the-counter sensors are easier to buy, but insurance does not always cover them for people without diabetes. Prices range widely.

The frontier

Researchers and companies are already working on the next generation of sensors. Some are testing optical and biochemical patches that could measure more than one molecule, combining glucose with other markers of stress, recovery, or hydration. The future will likely combine CGM data with movement, heart rate, and sleep to give a full picture of how your body uses and recovers energy.

Why you should care

Muscle is the largest glucose sink in your body. When you train and build muscle, you improve your ability to use glucose efficiently, which keeps energy stable and supports recovery.

Using a CGM helps you see how food, sleep, and training affect that process. It gives you real feedback about how your body responds, letting you personalize your nutrition and fueling habits. For anyone curious about performance, energy, or long-term metabolic health, understanding glucose is one of the most direct ways to understand how your body runs.

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