CGM and carb fasting
A continuous glucose monitor (CGM) is a small sensor that tracks your blood sugar around the clock so you can see it rise and fall instead of guessing. The FDA cleared the first over-the-counter CGM, Dexcom Stelo, in 2024, and others followed. On carb fasting, a CGM lets you watch a fast day flatten your glucose curve. It gives feedback but can't diagnose you.
A continuous glucose monitor (CGM) is a small sensor, usually worn on the back of the arm, that reads your blood sugar every few minutes and sends it to your phone. A finger-prick gives you one number. A CGM shows the whole curve, including the rises after meals and the falls between them.
For a long time CGMs were prescription-only and aimed at people with diabetes. That changed in March 2024, when the FDA cleared the first over-the-counter CGM, Dexcom Stelo, which doesn’t require a prescription. Other consumer monitors followed. For carb fasting, that matters: a CGM turns an invisible metabolic effect into something you can actually watch.
Time in range: the metric worth knowing
A single glucose reading is a snapshot. “Time in range” shows the share of the day your glucose stays inside a target band rather than spiking and crashing. International expert consensus established common CGM targets and the time-in-range framework so that readings could be compared and interpreted meaningfully, instead of everyone inventing their own thresholds.
| Term | What it means |
|---|---|
| Glucose curve | The full rise-and-fall of your blood sugar across the day |
| Spike | A sharp post-meal rise, usually after carbs, followed by a crash |
| Time in range | How much of the day your glucose stays inside a target band |
The reason this is useful for carb fasting is simple: on a normal-carb day you’ll see peaks; on a fast day, with the carbs removed, that curve tends to flatten out. You’re watching metabolic flexibility happen in real time.
What a fast day looks like on a CGM
This is the part people often find motivating. Eat carbs and your monitor draws a hill; skip them on a fast day and the line goes calmer and flatter. The protocol’s creator has watched this in his own CGM data as his body shifted toward fat-burning after roughly 24 carb-free hours. That’s one person’s data, so it can’t promise your result. It does make the mechanism visible.
How to read the data
CGMs are good at spotting your own patterns over time. They are not a diagnostic tool, and if you don’t have diabetes you should read the numbers cautiously. The cut points and time-in-range targets were validated mostly in people with diabetes; a healthy body produces glucose swings that are completely normal, and a single high reading after a big meal is not a verdict.
Use a CGM as a feedback loop to see whether a fast day flattens your curve. If a reading worries you or you’re managing a condition, take the data to your clinician. A CGM can’t replace proper testing and care.
