I first tried a continuous glucose monitor (CGM) on a whim before an eventful 200km ultracycling ride. I wanted to understand why, despite meticulous planning, I often “bonked” hard mid-ride: that sudden, dizzy, powerless feeling that turns forward momentum into a long, slow struggle. What I learned from wearing a CGM has changed how I plan nutrition, pacing, and recovery. Below I’ll share practical, experience-based guidance on how to use a CGM to prevent bonking during ultra-distance rides.
Why a CGM can help on ultra rides
A CGM gives you near-continuous insight into your blood glucose (BG) trends, not just one-off fingerpricks. For endurance rides, that matters because the problem is often timing and trend rather than absolute numbers. I can see when my glucose is steadily dropping, when carbs are upticking it, or when I’m overcompensating and heading towards reactive spikes followed by crashes. That early-warning trend information lets me act before the bonk hits.
Which CGM should you consider?
Popular options include the Dexcom G6/G7 and the Abbott FreeStyle Libre. I’ve used both. Dexcom tends to provide real-time streaming and custom alarms, which is great if you want immediate alerts while riding. Libre is often cheaper for short-term use and newer Libre models also offer on-demand scanning or optional real-time sharing. If you want alarm-based warnings mid-ride, choose a system that supports live alerts to your phone or a paired cycling computer.
Key settings and setup before you ride
- Set personalized targets: I set alarms not at clinical diabetic thresholds but at riding-specific trigger points. For me, an early alarm at 5.0 mmol/L (90 mg/dL) catches downward trends, and a more urgent alarm at 3.9 mmol/L (70 mg/dL) means immediate action.
- Enable trend arrows: Don’t just look at the number. A reading of 5.5 mmol/L with a double down arrow is different from a steady 5.5. I train myself to act based on arrows—down arrows = fuel now.
- Check sensor placement: I prefer the back of the upper arm for comfort and lower interference; cycling-specific alternatives are abdomen or lower ribs if your kit makes arm placement uncomfortable. Test locations during training first.
- Calibrate against a fingerprick when uncertain: Most modern CGMs are factory-calibrated, but if readings look inconsistent, I double-check with a meter before taking critical action.
How to interpret CGM readings during a ride
CGM readings lag behind blood by about 5–15 minutes because sensors measure interstitial glucose. I always account for lag: if I see a steep downward arrow, I assume my blood glucose is already lower. Conversely, a rapid rise might be catching up to blood levels. I use the trend, not just the number, to decide whether to eat, drink carbs, or rest.
Practical fueling strategy informed by CGM
Using a CGM transformed my fueling from “eat every hour” to targeted carbohydrate intake. A few practical rules that work for me:
- If trend arrow is flat and >6.0 mmol/L (108 mg/dL): maintain, consider smaller snacks.
- If arrow is trending down and between 4.0–6.0 mmol/L (72–108 mg/dL): take 20–40 g fast carbs (gel, sports drink) and reassess in 10–15 minutes.
- If arrow is double down or reading <4.0 mmol/L (72 mg/dL): stop, take 30–60 g quick carbs, sit for a few minutes, then ride slowly until stable.
- Combine carbs with small amounts of sodium and water—fluid and electrolytes affect absorption and perceived energy.
| CGM Range | Trend | Action I take |
|---|---|---|
| > 7.0 mmol/L (126 mg/dL) | Flat / rising | Maintain pace, small fueling (10–20 g carbs) if long effort planned |
| 5.0–7.0 mmol/L (90–126 mg/dL) | Flat / slight fall | Take 20–30 g carbs every 30–45 min depending on duration |
| 4.0–5.0 mmol/L (72–90 mg/dL) | Falling | Take 20–40 g fast carbs and reduce intensity for 10–20 min |
| < 4.0 mmol/L (<72 mg/dL) | Any | Immediate 30–60 g fast carbs, stop effort if symptomatic |
Choosing the right carbs
Not all calories are equal in terms of speed. I keep a mix of options on the bike:
- Fast sugars: gels, glucose tabs, sports drinks — for emergency quick boosts.
- Slower carbs: bars, bananas, or rice cakes — for sustained energy and to avoid frequent spikes.
- Pair carbs with a bit of sodium or caffeine when needed (I use caffeinated gels for late-race boosts). But I test combos in training—GI tolerance varies.
How much carbohydrate per hour?
For ultra rides I aim for a baseline of 60–90 g carbs/hour depending on intensity, rider size, and gut training. If my CGM shows steady decline despite that baseline, I increase to 90–120 g/hr using multiple carb types (glucose+fructose blends) which can improve absorption. I never try these higher rates for the first time on race day—always test in long training rides.
Use CGM data to personalize fueling plans
One of the best parts of using a CGM is post-ride analysis. I export or screenshot sessions and look for patterns: when do I dip (after long climbs, after long flats, with heat)? Which foods produce stable glucose vs big swings? I then adjust meal timing, carb composition, and pacing. For example, I learned I need a pre-ride 30–60 g carb snack starting 30–45 minutes before a morning ultra; without it my CGM showed a steep early drop.
Pacing and power management
CGM data has taught me that sustainable power is the best defense against bonking. If my glucose trends down during a hard effort, I either back off power or take carbs earlier. Overcooking the start of a ride creates prolonged oxidative stress and depletes glycogen faster—CGM alerts help me stay in a sustainable power zone.
Hydration, electrolytes and other factors
Low blood volume or dehydration can affect how you feel and how quickly carbohydrates work. If my CGM shows erratic swings, I consider hydration and sodium status: I’ll sip an electrolyte drink (with 400–800 mg sodium/L) rather than plain water. Heat also accelerates carb use—so I adjust intake upwards on hot days.
Training the gut and interpreting GI symptoms
CGM won’t remove GI issues, but it helps me distinguish energy-related symptoms from GI-related ones. If glucose is stable but I feel awful, it’s probably GI or hydration. I practice higher carb intakes on long training rides with the CGM so I can see what my gut tolerates. Over weeks, I’ve increased my hourly carbs without crashes because I trained my gut and monitored glucose stability.
Alarms, sharing, and safety
I set vibrating alarms on my phone or watch so I don’t rely solely on seeing my phone. For organized rides, live-share features let a support rider monitor trends and hand over fuel before I’m aware of the decline. If you’re using a CGM off-label (non-diabetic use), be aware of local regulations and the device’s intended use—treat the data as guidance, not medical diagnosis.
Common questions people ask me
- Is CGM accurate enough? For trend detection and preemptive fueling, yes. Absolute numbers can vary slightly; use trend arrows and confirm with a fingerstick if you’re about to make a medical decision.
- Can I prevent every bonk? No—illness, extreme weather, or mechanicals still happen. But CGM dramatically reduces unexpected crashes when used proactively.
- Will wearing a CGM change my mindset? Definitely. I became less anxious about “not eating enough” and more tactical—waiting for true downward trends or managing spikes to avoid later crashes.
- Should non-diabetics use CGMs? Many endurance athletes find them useful for optimization. But use responsibly: learn how to interpret data and consult healthcare professionals for persistent concerns.
If you want to try this yourself, start by using a CGM on a few long training rides—not on race day. Create alarm thresholds that feel conservative at first, test different fuel types, practice gut training, and use post-ride reviews to refine your plan. Over time, the CGM becomes less of a gadget and more of a trusted teammate on the bike—catching trends early and keeping you rolling farther, stronger, and with fewer dramatic crashes.