The continuous glucose monitor is entering endurance sport
You've probably seen that small white disc stuck on a pro triathlete's triceps at the start line of an Ironman. It's not a fashion accessory — it's a continuous glucose monitor (CGM), and it's changing how endurance athletes think about fueling. Long reserved for people with diabetes, this type of biosensor has opened up to sport with devices like Abbott's Libre Sense, designed specifically for non-diabetic athletes.
In practice, this sensor measures your interstitial glucose every minute for 14 days, worn on the back of the upper arm. The value: seeing in real time how your body responds to a gel, a carb drink, or a portion of rice before a long ride. That's exactly the type of data the Ograal app complements with its Daily Insight feature and training calendar sync: the sensor tells you what's happening in your blood, Ograal tells you what to eat and when to anticipate the rest of your training week.
Definition: a continuous glucose monitor (CGM) is a skin-worn device that continuously measures glucose concentration in interstitial fluid, without finger pricks, and transmits the data in real time to a mobile app.
For athletes without diabetes, the goal isn't to detect a pathology but to optimize fueling: understanding the glycemic dips that precede a bonk, adjusting the timing of carbohydrate intake, and avoiding reactive hypoglycemia after an overly concentrated gel.
What does a triathlete see on their glucose graph during an effort?
On the screen of the app paired with the sensor, the glucose curve tells a fairly readable story once you know how to interpret it. Here are the three signatures that show up most often in endurance athletes:
- The pre-bonk dip: a progressive drop in interstitial glucose 15 to 20 minutes before the sensation of heavy legs or mental fog sets in. It's a signal that carbohydrate intake is arriving too late relative to muscular demand.
- The post-gel spike: a rapid rise after ingesting a high-glycemic-index gel, sometimes followed by a reactive hypoglycemic rebound if the body over-responds by secreting insulin.
- The stable plateau: the signature sought by athletes well-trained in fueling, with glucose oscillating within a narrow range despite prolonged effort.
A study published in 2025 in the European Journal of Sport Science by Ross Hamilton's team (Swansea University) followed nine ultra-endurance athletes over two 28-day periods, comparing a low- versus high-glycemic-index carbohydrate-rich diet while continuously measuring interstitial glucose (Hamilton et al., 2025, DOI: 10.1002/ejsc.70092). Result: the low-glycemic-index diet reduced glycemic variability and cut time spent below the recommended threshold in half (2% versus 4% of time in mild hypoglycemia), with no loss of endurance capacity. In other words, the type of carbohydrate matters as much as the quantity.
Libre Sense: one measurement every minute for 14 days
The Abbott Libre Sense Glucose Sport Biosensor illustrates this technical shift well. Worn on the back of the upper arm, it captures an interstitial glucose value every minute over 14 days, with no finger-prick calibration needed. A June 2026 article highlighted how much this two-week autonomy has changed practice: athletes can now track several consecutive long sessions, an entire training camp, or even a full race, with a single sensor.
For a cyclist or triathlete, this data density is a game-changer compared to older methods (single-point capillary glucose from finger pricks). You go from an isolated snapshot to a full minute-by-minute film of the metabolic response, which lets you spot trends invisible to the naked eye — for example, a recurring glucose drop consistently 90 minutes after the start, regardless of the course.
Key takeaways
- Libre Sense measures interstitial glucose every minute, for 14 days, with no finger pricks.
- The Hamilton et al. (2025) study shows a reduction in time spent in mild hypoglycemia from 4% to 2% of total time with a low-glycemic-index diet over 28 days.
- Ironman pros like Cam Wurf consume more than 200g of carbohydrates per hour on the bike, a level made possible by progressive gut training and close monitoring of glycemic responses.
- A glucose dip often precedes the feeling of a bonk by 15 to 20 minutes: anticipatory fueling beats reactive fueling.
Case study: how Kat Matthews fine-tunes her Ironman fueling
Kat Matthews, the British triathlete ranked n°3 in the IRONMAN Pro Series women's standings in 2026 and a two-time series champion (2024, 2025), has become a reference for data-driven nutrition in long-distance racing. Her fueling strategy during her record-setting performances (8:10:34 at Ironman Texas 2025, the fastest women's time ever recorded over the distance) combines high-carbohydrate gels on the bike with drinks providing 90 to 100g of carbohydrates per hour, finely adjusted according to gut feedback (Triathlon Magazine, 2025).
This approach fits into a broader trend among endurance pros: pushing carbohydrate intake well beyond the old recommendations of 60-90g/h. The most spectacular example remains Cam Wurf, who set the fastest bike split in Ironman history in 2025 while ingesting more than 200g of carbohydrates per hour — close to 28 gels over the bike leg (Cyclingnews, 2025). That kind of carbohydrate volume doesn't happen by chance: it relies on progressive gut training and, increasingly, on a fine reading of individual glycemic response to calibrate the right balance between intake and absorption.
How to use your own glucose data in training
You don't need to be a pro to benefit from a CGM. Here's a simple method to get started:
This is exactly where the complementarity with a planning tool like Ograal becomes valuable: once you've identified your own glycemic dips thanks to the sensor, you can structure a fueling plan consistent with your training calendar, rather than reacting under pressure mid-effort. To go further on carbohydrate periodization across training phases, check out our article on train low, race high.
On the hydration side, remember to compensate for sodium losses during long sessions, especially if you're processing a lot of carbohydrates: sugar-free electrolyte lemon tablets 40x4g let you adjust sodium intake independently of your carbohydrate strategy.
Limitations to know before you start
Sports CGM isn't a magic wand. A few points of caution deserve attention before drawing hasty conclusions:
- Interstitial glucose lags 5 to 15 minutes behind capillary blood glucose, which can distort readings during rapid fluctuations.
- Studies have shown CGM can underreport capillary glucose during exercise, with an average discrepancy that can exceed 1 mmol/L depending on conditions.
- Clinical hypoglycemia thresholds, designed for people with diabetes, don't directly apply to a healthy athlete mid-effort: interpretation should remain contextual and individualized.
Used with these precautions in mind, CGM remains a great learning tool for understanding your own physiology under effort — as long as you cross-reference the data with your sensations and training history.
FAQ
Is a glucose sensor useful if I don't have diabetes?
Yes. For athletes without diabetes, it works as a learning tool to visualize glycemic response to different foods and adjust fueling timing, not for medical use.
How long does a Libre Sense sensor last?
14 days of autonomy with a measurement every minute, no finger-prick calibration, worn on the back of the upper arm.
Should I aim for 200g of carbs per hour like Cam Wurf?
No, that figure is an extreme case from a pro trained for years to reach that digestive tolerance. Most athletes progress effectively between 60 and 100g/h with progressive gut training.
Does CGM replace a structured nutrition plan?
No, it complements one. The sensor shows what's happening in your blood; a tool like Ograal translates that data into a concrete action plan aligned with your training calendar.
Sources
- Hamilton R.A. et al. (2025), "Glycaemic Impact of Low- and High-Glycaemic Index Carbohydrate Diets in Ultra-Endurance Athletes: Insights From Continuous Glucose Monitoring", European Journal of Sport Science, 25(12):e70092 — DOI: 10.1002/ejsc.70092
- Abbott, "Libre Sense Glucose Sport Biosensor" — libresense.abbott, June 2026
- Triathlon Magazine Canada (2025), "200g of carbs per hour? Ironman pros break records with extreme fuelling strategies" — triathlonmagazine.ca
- Cyclingnews (2025), "28 gels and over 200g of carbs per hour: Cameron Wurf reveals insane fuelling strategy" — cyclingnews.com
