I started using a continuous glucose monitor (CGM) during marathon training because I was tired of the unpredictable “wall” that could turn a confident 20-mile training run into a crawl. Over several months I learned to read my body in a new language — numbers and trends — and to translate that into smarter fueling decisions. If you want to stop bonking and fine-tune marathon fueling, a CGM can be one of the most practical tools in your toolkit when used thoughtfully.

Why a CGM for endurance athletes?

Most of us judge energy by how we feel: strong, tired, getting shaky. A CGM gives objective, continuous feedback. Instead of guessing whether your next gel will save you, you can see the direction and speed of your glucose change and act proactively. I used devices like the Freestyle Libre and Dexcom in training, and I experimented with athlete-focused platforms such as Supersapiens that combine CGM data with nutrition guidance.

Understand the data: trends matter more than single numbers

One instantaneous glucose value is just a snapshot. What matters are the trend arrows and the rate of change. Here’s how I read common patterns:

  • Rising steadily — Carbs are being absorbed; you have energy coming. Good zone for sustaining pace.
  • Flat — Steady-state. If you’re running long, flat can be fine, but beware of a slow downward drift.
  • Rapid decline — Warning sign. If glucose is dropping quickly, take carbs now to prevent a crash.
  • Low and steady — Hypoglycemia risk; immediate carbs recommended.

    • Most CGMs report interstitial glucose, which lags blood glucose by ~5–15 minutes. That means you must treat trends, not raw numbers, especially during fast changes like after taking a gel.

    Set your personal thresholds

    There are no universal “good” numbers for every runner. I established my own thresholds during easy long runs and simulated race efforts. Tracks I used:

  • Baseline (resting) morning glucose — helps understand your non-exercise starting point.
  • Pre-run zone — I aimed for a moderate level before long runs (not overly high), so I wasn’t chasing immediate spikes.
  • Fuel-now trigger — For me, a sustained downward trend or a drop below a personalized threshold helped me decide when to take a gel.

    • Example personal rule I adopted: if my CGM showed a steady downward arrow for more than two readings, or a drop of >1 mmol/L (≈18 mg/dL) in 15 minutes, I’d take 30–45 g of carbs (a gel + water or a small sports drink).

    Design fueling around your CGM

    Using CGM data allowed me to move from a one-size-fits-all fuelling schedule to a responsive plan. Here's the framework I used:

  • Pre-run — Eat a moderate-carb meal 2–3 hours before long runs. Check CGM: if trending down sharply close to start, add a small carb snack 30–45 minutes prior (30–40 g).
  • During long runs and marathon pace workouts — Instead of rigidly taking a gel every 30–45 minutes, I watched my trend arrows. If stable or slightly up, I could extend the interval; if dipping, I took carbs earlier.
  • Types of carbs — Rapidly absorbable carbs (glucose, maltodextrin) produce quicker rises. Sucrose and fructose blends can increase total oxidation and reduce gut distress. I alternated gels with dilute sports drinks to balance absorption and hydration.

    • Practice gut training while monitoring glucose

    One huge benefit of pairing CGM with fueling practice is confidence to increase carbs without fear of GI fallout. During long training runs I gradually raised hourly carbohydrate intake and watched how my glucose responded. If numbers stabilized and I didn’t get GI symptoms, I kept the higher intake. If the CGM showed erratic spikes then crashes, I backed off and changed form (liquid vs. gel) or dose.

    Race-day strategy using CGM insights

    You probably won’t wear a CGM in major races (check race rules), but your training with CGM builds an evidence-based plan. From months of data I knew:

  • My ideal pre-race meal timing and size.
  • My best carbohydrate types (gels vs. chews vs. drinks).
  • My individualized hourly carb target — often 60–90 g/h for long efforts, depending on gut tolerance and training.

    • I used CGM to practice taking the first gel before signs of a drop — around mile 10–12 in marathon pace efforts — rather than waiting to feel drained. That proactive approach prevented the sudden energy loss that felt like bonking.

    Account for confounders

    CGM readings are influenced by more than carbs. I learned to interpret context:

  • Heat — Can increase carbohydrate oxidation and change sweat/electrolyte needs. You may require carbs earlier in hot races.
  • Hydration — Dehydration can amplify fatigue even if glucose looks fine.
  • Stress and caffeine — Both can cause transient glucose spikes; watch the trend not single spikes.
  • Intensity — High-intensity surges temporarily increase glucose; the subsequent drop can be steep, so refuel cautiously after sprints or steep climbs.

    • Common pitfalls and how I avoided them

    When I started, I made mistakes. Here’s what to watch out for:

  • Overreacting to single readings — I once ate an extra gel because a single low number scared me; later I realized it was a transient dip and I got a surge then felt sluggish. Now I wait for a trend before acting.
  • Ignoring sensor lag — Remember interstitial lag; when you take carbs, expect the CGM to show improvement after 10–15 minutes.
  • Relying only on numbers — Always combine CGM with perceived exertion. If your glucose looks fine but you feel off, consider hydration, electrolytes, or fatigue.

    • Practical tips and a simple fueling table

    Here’s a table I used as a starting template. Adjust based on your CGM trends, body size, and gut tolerance.

    Situation Trigger (CGM) Action
    Pre-run Downtrend before start Small snack 30–45 min before (30–40 g carbs)
    During steady long run Stable or slight rise 30–60 g carbs every 45–60 min or extend interval if very stable
    During long run Steady downward trend 30–45 g carbs now; reassess 15 min later
    Rapid decline or low Drop >1 mmol/L in 15 min or low reading Immediate 30–45 g fast carbs + small sip of sports drink

    Safety and legal notes

    If you have diabetes or metabolic conditions, consult your clinician before using or changing medications. For non-diabetic athletes, CGMs are a tool — not a replacement for medical advice. Also, verify race rules: some federations or events restrict wearable devices during competition.

    Using a CGM transformed how I approach marathon fueling. It taught me to be proactive instead of reactive, to trust data and sensation together, and to iterate a plan that fits my physiology. If you’re curious, start by monitoring a few long runs, look for consistent patterns, and gradually test changes. For more ideas and race-ready strategies, I share regular updates and examples on Sport News at https://www.sport-news.uk.