Cycling with continuous glucose monitors
Continuous glucose monitors (CGMs) are no longer only for clinical use. Many endurance cyclists now wear them to refine fueling around training and racing. Used well, a CGM can help you avoid mid-ride dips, practice higher carb intakes, and match fueling to the demands of your power targets and training zones. Used poorly, it can become a distraction. Here’s how to turn glucose data into better decisions on the bike.
What a CGM shows—and what it doesn’t
- It measures interstitial glucose, not blood or muscle glycogen. There’s a 5–15 minute lag, which grows during rapid changes (hard efforts, fast-absorbing carbs, cold).
- Glucose is influenced by more than food. Adrenaline, intensity spikes (VO2 work), and caffeine can raise readings even without eating. A spike during 5-minute efforts isn’t automatically “bad.”
- Performance still rules. Prioritize power (watts), HR, and RPE. A “flat” glucose line with fading watts is worse than a wavy line with strong legs.
- Accuracy varies. Modern sensors are generally good at steady state, less so during surges, dehydration, or when the sensor is compressed (lying on it). Verify with a fingerstick if symptoms and numbers disagree.
- Medical note. CGMs are medical devices. For non-diabetic athletes, they’re used for insight—not diagnosis. If you manage diabetes or experience recurrent low glucose symptoms, speak with a clinician.
Turning CGM data into better fueling
Think of CGM as a feedback tool for carbohydrate timing and dose. Your aim is to keep glucose reasonably stable for the session goal while meeting energy needs and protecting the gut. Start with known best practices, then use CGM trends to fine-tune.
| Session type | Carb target (g/h) | Common CGM pattern | What to do |
|---|---|---|---|
| Endurance (Z2) 1.5–3 h | 30–60 | Gentle decline or steady; may drift low late | Start early (within 20 min). If trending downward with rising RPE, take 20–30 g fast carbs; aim small, frequent sips. |
| Tempo/threshold 1–2 h | 60–90 | Spikes with efforts, dips between | Keep a steady drip (10–20 g every 10–15 min). Don’t chase every spike; evaluate legs and power, not the graph alone. |
| Long rides or races 3–5+ h | 80–100+ (up to 120 with gut training) | Stable early, risk of late dips | Front-load slightly and maintain consistent intake. Mix glucose + fructose. If readings slide late and watts fade, increase by 20–30 g/h. |
| HIIT/VO2 60–90 min | 40–80 | Adrenaline spikes during intervals | Pre-fuel well. Small carb doses between sets. Mouth rinse can help if GI feels heavy. |
Key principles to apply
- Fuel the work, not the line. Match carb intake to planned power and duration first; let CGM confirm you’re on track.
- Start early. Don’t wait for a dip. Begin within the first 15–20 minutes and dose regularly.
- Use multiple transportable carbs. Glucose + fructose (roughly 1:0.8 ratio) supports higher intakes (90–120 g/h) with training.
- Pre-ride nutrition matters. A mixed carb breakfast 2–3 h pre-ride (low fiber/fat) with a small top-up 15–30 min before can reduce early drops and GI stress.
- Hydration supports absorption. Aim roughly 500–1000 ml fluid per hour depending on heat/sweat rate, with adequate sodium.
Good CGM use looks like: consistent carb dosing, steady legs, and predictable glucose trends that match the plan for your FTP-based training zones.
Practical tips, pitfalls, and a two-week plan
Common pitfalls
- Chasing a flat line. Small rises after gels or during hard pulls are normal. Don’t overcorrect.
- Over-fueling in response to a lagging dip. Remember the 5–15 min delay. Re-check symptoms and performance before doubling intake.
- Ignoring the gut. Your GI tract sets the ceiling. Progress carb intake gradually across weeks.
- Compression lows and cold weather artifacts. Avoid sleeping on the sensor; keep the site warm during winter rides.
- One-size-fits-all breakfasts. Test what stabilizes your glucose and power: oats + banana vs. rice + eggs, etc.
A simple two-week CGM protocol
- Day 1–2: Baseline. Wear the CGM during normal eating and an easy Z2 ride. Note typical pre-ride, mid-ride, and 30–60 min post-ride trends.
- Day 3–5: Pre-ride breakfast tests. Try two options on similar 90–120 min Z2 rides. Record: power, HR, RPE, GI comfort, carb intake, and glucose trend. Keep fueling at 30–40 g/h.
- Day 6–7: Tempo/threshold session. Plan 2×20 or 3×12 at ~90–100% FTP. Fuel 60–80 g/h in 10–15 min doses. Expect interval spikes; watch for between-interval dips and adjust +10–20 g/h if legs feel flat.
- Day 8–10: Gut training block. On 3 h endurance rides, step intake from 60 → 80 → 90 g/h using glucose+fructose. Use CGM to verify you’re not crashing late; check comfort.
- Day 11–12: Race rehearsal. Long ride with hard surges. Start fueling in the first 15 min. If glucose trends downward in the final hour and watts fade, add 20–30 g/h and ensure adequate sodium/fluid.
- Day 13–14: Review. Compare sessions: which breakfast produced stable legs and fewer dips? What dosing cadence felt best? Lock in a plan for your next event.
Calibration for real-world decisions
- When numbers and feel disagree: trust symptoms and power first; take a small fast-carb dose if you suspect a true low.
- For sprints and VO2 sets: don’t read too much into spikes; evaluate repeatability of efforts and recovery between sets.
- For long gran fondos: the goal is consistent watts and minimal late fade; use CGM to verify you’ve avoided prolonged dips, not to micromanage every minute.
Bottom line: CGM can sharpen your fueling—especially for long days and threshold work—if you use it alongside watts, heart rate, and RPE. Start with established carb targets, practice them, and let the glucose trace confirm that your strategy supports the work.
