Continuous glucose monitors
Evidence: weak
There is no scientific consensus that watching glucose in real time improves performance in healthy, non-diabetic runners, and the marketing runs well ahead of the data. The device is also least accurate during the hard running when a runner would most want to trust it, so acting on the numbers can mislead fuelling rather than sharpen it.
Not medical advice
This is a general knowledge base, not medical advice. A continuous glucose monitor is a medical device; if you have diabetes or a medical reason to track glucose, follow the guidance of your doctor, not this page.
A continuous glucose monitor (CGM) is a small skin-worn sensor, with a filament sitting under the skin, that reports glucose continuously through the day. It was developed for people with diabetes and has been repackaged and sold to endurance athletes on the promise that watching your glucose lets you fuel perfectly and never hit the wall. For a runner without diabetes, that promise is unproven.
It measures the wrong compartment, with a lag
A CGM does not measure blood glucose. It measures glucose in the interstitial fluid, the fluid around the cells, which trails blood glucose by a lag that widens whenever glucose is changing quickly, which is exactly what happens during hard running (Bowler et al. 2023). The reading on the screen mid-session is therefore both delayed and, during exercise, unreliable.
That unreliability has been measured. Set against a laboratory glucose analyser, a CGM agreed reasonably well at rest but drifted badly during exercise: while running, its mean absolute relative difference from the true value was around 18 to 22%, well outside the 10 to 15% usually treated as clinically acceptable, and its limits of agreement were wide (Bauhaus et al. 2023). The authors warned plainly that relying on interstitial-glucose data during exercise can lead to mismanaging carbohydrate intake. The device is least trustworthy in the one situation the marketing is built around.
No performance evidence in healthy runners
Set accuracy aside and the deeper problem remains: there is no good evidence that acting on glucose data makes a healthy runner faster. A review by a sports-nutrition group framed the athletic uses as possible rather than demonstrated, and flagged both the interpretation problems and the risk that athletes develop an anxious relationship with the numbers (Bowler et al. 2023). Anti-doping and sports bodies have put it more directly: there is no scientific consensus that real-time glucose feedback produces measurable performance improvement, normal swings in glucose after exercise are physiological rather than a problem to fix, and misreading them could push a runner to over-restrict food (USADA).
Individual, but not therefore actionable
The seductive pitch is personalisation: everyone’s glucose curve is different, so watch yours and optimise. The individuality is real, but it is not the same as actionability. No study has shown that adjusting your fuelling to your personal curve improves any outcome that matters, and the pull to over-interpret a noisy, lagging signal is a real downside, particularly for runners already prone to disordered eating. The proven fuelling advice needs no sensor: match carbohydrate to the work, take carbohydrate in during long efforts at rehearsed rates, and train the gut to tolerate it.
The honest take
For a healthy runner, a CGM is an expensive way to generate an interesting graph. It measures the wrong fluid with a lag, is unreliable during hard running, and has no evidence of improving performance. As with the metrics on a GPS watch, the data can be fascinating and still not worth acting on. Until there is evidence that CGM-guided fuelling beats the well-established basics, it is a gadget in search of a use.