The Average Marathoner Fuels at a Third of What They Need

In Summary

The average marathoner takes in only ~22g of carbs per hour — a third of the recommended 60–90g.
Under-fueling causes the bonk at mile 18, not lack of fitness.
Gut training over 8–10 weeks lets most runners tolerate 60–80g per hour comfortably.
Your target depends on body weight, pace, and sweat rate — not a one-size-fits-all number.

You Didn't Hit the Wall Because You Were Unfit

You ran the miles. You hit your long runs. Your watch said you were ready. And then somewhere between mile 18 and mile 22, your legs turned to concrete, your brain went fuzzy, and you shuffled to the finish wondering what went wrong.

It probably wasn't your training. It was your fueling.

A widely cited study found that the average marathoner consumes roughly 22 grams of carbohydrate per hour during a race. The current evidence-based recommendation is 60 to 90 grams per hour. That means most runners are fueling at about a third of what their body actually needs to sustain marathon-pace effort over three to five hours.

That gap is the bonk. And closing it is simpler than you think.

Why 22 Grams Isn't Enough

Your body stores about 2,000 calories of glycogen in your muscles and liver. At marathon pace, you burn through roughly 60 to 100 grams of carbohydrate per hour depending on your pace, body weight, and intensity.

If you're only replacing 22 grams, you're running a calorie deficit from the first mile. By mile 15 or 16, your glycogen stores are nearly empty. By mile 18, they are empty. That's the wall.

Here's what makes it worse: the faster you run, the higher proportion of carbohydrate you burn relative to fat. But even slower runners (4:00 to 5:00 marathon pace) burn significant carbohydrate — and because they're running longer, they need more total fuel across the race, not less.

A 4:30 marathoner needs more gels than a 3:00 marathoner. That surprises almost everyone.

The Absorption Ceiling Most Runners Don't Know About

Your gut can only absorb about 60 grams of glucose per hour through a single transport pathway. This is why early sports nutrition advice capped recommendations at 60g/hour.

But there's a second pathway. Fructose uses a different transporter (GLUT5) than glucose (SGLT1). When you combine glucose and fructose — ideally in roughly a 2:1 ratio — your gut can absorb up to 90 grams or even more per hour.

This is why gel formulation matters. A gel made from maltodextrin (which breaks down into glucose) plus fructose gives you access to both pathways. A gel made from glucose alone hits a ceiling at 60g no matter how many you eat.

Check your gel label. Look for both a glucose source (maltodextrin, dextrose, or glucose syrup) and fructose. If it only lists one sugar source, you're leaving absorption capacity on the table.

How to Build Up to 60–90g Per Hour

You can't jump from 22 grams to 90 grams overnight. Your gut needs training, just like your legs.

Here's a practical 8-week protocol you can layer onto your existing marathon training:

Weeks 1–2: Baseline (40g/hour) Start taking one gel every 30 minutes on your long runs. This puts you at roughly 40–50g per hour depending on the gel. Focus on tolerating it without GI distress.

Weeks 3–4: Build (50–60g/hour) Add a sip of carbohydrate drink mix between gels. You're now hitting 50–60g per hour from combined sources.

Weeks 5–6: Push (60–75g/hour) Increase to one gel every 25 minutes, plus drink mix. Or switch to larger-format gels (some brands offer 40g servings). Target 60–75g per hour.

Weeks 7–8: Race simulation (75–90g/hour) On your final two long runs before taper, simulate race-day fueling at your target rate. This is your dress rehearsal. If anything causes GI issues, you have time to adjust the product, timing, or concentration.

Two rules throughout this process:

Always take gels with water. A concentrated gel without water sits in your stomach and pulls fluid from your bloodstream to dilute it. That's what causes nausea.
Start fueling early. Take your first gel at 20–25 minutes, not at mile 6 or 7. By the time you feel like you need fuel, you're already behind.

Your Number Isn't Everyone's Number

The 60–90g range is a population guideline. Your actual target depends on:

Body weight: A 55kg runner and a 85kg runner have different glycogen stores and different burn rates.
Pace: A 3:15 marathoner burns carbohydrate at a different rate than a 4:45 marathoner.
Sweat rate: Higher sweat rates mean more fluid loss, which affects how well your gut absorbs carbohydrate.
Training history: A gut that's been trained on high-carb intake during runs can absorb more than one that hasn't.
Conditions: Heat and humidity increase sweat rate and reduce gut blood flow, lowering your effective absorption ceiling.

This is why copying someone else's fueling plan doesn't work. Your training partner's plan was built for their body, their pace, and their sweat rate. It might be completely wrong for you.

The most reliable way to find your number is to combine your wearable data — training load, heart rate zones, estimated calorie burn — with your sweat rate and body weight. That gives you a personalised carbohydrate target per hour that you can test and refine in training.

Sweatr calculates this automatically from your Apple Watch or Garmin data. It builds your carb and fluid targets per hour based on your actual numbers, not a generic chart. And it adjusts as your fitness and conditions change through your training cycle.

The Race-Week Check

With fall marathons approaching in the next two to four weeks, here's a quick audit:

Do you know your carb-per-hour target? If not, calculate it from your body weight and expected pace, or let your wearable data do the math.
Have you tested your gels at race pace? The gut tolerates fuel differently at easy pace versus marathon pace. Test at the intensity you'll actually race at.
Do your gels use dual-transport carbs? Check the label for both glucose/maltodextrin and fructose.
Do you have a timing plan? Know exactly when you'll take each gel relative to the aid stations on your course.
Have you practised taking gels with water? This sounds basic, but fumbling a gel packet while running at pace is a skill you need to rehearse.

The Bottom Line

The wall at mile 18 is not inevitable. It's a fuel problem with a fuel solution.

Most runners don't bonk because they can't run the distance. They bonk because they consume a third of the carbohydrate their body needs. Fix the fueling and the last 10K of your marathon becomes a completely different race.

Start gut training now. Build up gradually. Find your personal carb target — not the generic one from a chart, but the one calculated from your data. And test everything before race day.

Your legs did the work. Make sure your fueling plan doesn't waste it.

Sweatr builds your personalised marathon fueling plan from your Apple Watch and Garmin data — carb targets, fluid targets, gel timing, all mapped to your race. Try it free.