Most Marathoners Fuel at a Third of What They Need

The Number That Should Worry Every Marathon Runner

A study published in 2025 tracked carbohydrate intake across hundreds of marathon finishers. The average? Just 21.7 grams of carbohydrates per hour.

To put that in perspective, current sports nutrition guidelines recommend 60 to 90 grams of carbohydrates per hour for events lasting more than 2.5 hours. Elite marathoners regularly hit 80 to 120 grams per hour. Some push even higher.

The average recreational runner is fueling at roughly a third of what the science says they need. And then they wonder why they bonked at mile 18.

This isn't a knowledge problem. Most runners who've trained for a marathon have read at least one article about gel timing. They own the gels. They packed them in their belt. But somewhere between the starting line and mile 15, the plan fell apart.

Understanding why — and how to fix it — might be the single most impactful thing you can do for your next race.

Why the Gap Exists

The under-fueling problem isn't about ignorance. It's a collision of physiology, psychology, and poor feedback loops.

Your Stomach Wasn't Ready

The most common reason runners eat less than planned is gastrointestinal distress. Gels that went down fine on a Saturday morning long run suddenly cause nausea, cramping, or worse on race day.

The difference? Race-day intensity. When you run harder, blood diverts from your gut to your working muscles. Your digestive system has less capacity to absorb carbohydrates. If you haven't trained your gut to handle fuel at race effort, your stomach vetoes the plan your brain made.

Research estimates that up to 90% of endurance athletes experience some form of GI distress during competition. For many, the response is simple: stop eating. That solves the stomach problem and creates the energy problem.

Race Day Nerves Change Everything

Pre-race anxiety elevates cortisol and adrenaline. Both suppress appetite and alter gut motility. Many runners feel too nauseous to eat their planned pre-race breakfast, which means they start the race with lower glycogen stores than expected. Then the same nerves make early-race gels feel harder to stomach.

By the time the nerves settle around mile 8 or 10, the fueling deficit is already significant. And catching up on carbohydrates mid-race is harder than staying on schedule.

The Timing Problem

Most generic fueling advice says something like "take a gel every 45 minutes." That sounds simple until you're running, navigating aid stations, managing pace, and trying to remember when you last ate. Without a structured timing plan tied to your actual race effort, the gaps between gels stretch. Forty-five minutes becomes an hour. An hour becomes "I'll take one at the next aid station." The next aid station comes and you feel sick, so you skip it.

The result: you've consumed three gels over three hours instead of the six or seven your plan called for.

You Don't Know Your Actual Number

Here's the deeper problem: "60 to 90 grams per hour" is a range that spans a 50% difference. A runner who needs 85 grams per hour but targets 60 is under-fueling by almost 30%. But most runners have never established their personal carbohydrate target — the amount their body can absorb and use at their race pace, in their conditions, at their fitness level.

That number depends on your body weight, your race pace relative to your threshold, your gut training history, the carbohydrate type in your gels (glucose-only vs. glucose-fructose), and even the weather. No generic guide can give you that number. It has to come from your own data.

What Happens When You Under-Fuel

The consequences of chronic under-fueling during a marathon are predictable and devastating.

Glycogen depletion. Your body stores roughly 1,500 to 2,000 calories of glycogen in muscles and the liver. At marathon effort, most runners burn 600 to 1,000 calories per hour. Without adequate external carbohydrates, glycogen runs out somewhere between miles 16 and 22 for most runners. That's the wall.

Pace collapse. When glycogen drops, your body shifts to burning more fat. Fat oxidation produces energy more slowly than carbohydrate oxidation at the same intensity. The result: you slow down involuntarily. What feels like a willpower failure is actually a fuel failure.

Cognitive decline. Your brain runs almost exclusively on glucose. As blood glucose drops, decision-making degrades, perceived effort spikes, and motivation craters. This is why the miles after a bonk feel not just physically hard but mentally unbearable.

Increased injury risk. Fatigued muscles that are running low on fuel absorb ground forces less efficiently. Late-race hamstring pulls, calf cramps, and ankle rolls are more common in under-fueled runners — not because their muscles are weak, but because their muscles have run out of the fuel needed to contract properly.

How to Close the Gap

Fixing the under-fueling problem requires changes in training, race preparation, and race execution. Here's the practical approach.

Step 1: Find Your Personal Carbohydrate Target

Stop working from the generic 60–90g range and find your number within it. The key variables are:

Body weight. Heavier runners burn more total calories and generally need more carbohydrates per hour.

Race pace relative to threshold. If you're racing at 75% of your VO2 max, you're burning a different fuel mix than someone racing at 85%. Slower marathoners often burn through glycogen faster relative to their pace because they're working at a higher percentage of their aerobic capacity.

Gut absorption capacity. If you're using glucose-only gels, your gut can absorb roughly 60g per hour. With a glucose-fructose mix (most modern gels use a 2:1 or 1:0.8 ratio), you can push to 90g or even higher — but only if your gut has been trained to handle it.

Duration. A 3-hour marathoner needs to sustain their fueling rate for 3 hours. A 5-hour marathoner needs to sustain it for 5. The total volume of carbohydrates is dramatically different, even if the hourly rate is similar.

Sweatr calculates your personal carbohydrate target automatically. It uses your body weight, your training data from Apple Watch, Garmin, or Strava, and your target race pace to build a per-hour carbohydrate plan. No spreadsheet, no guesswork.

Step 2: Train Your Gut Systematically

Your gut is trainable. Like any other system in your body, it adapts to the demands you place on it. If you never practice eating during runs, your gut won't be ready to absorb 80g of carbs per hour on race day.

The protocol is straightforward. Start your gut training 8 to 12 weeks before race day. On your weekly long run, begin fueling from the first 20 to 30 minutes. Start at a dose you can tolerate — maybe 40g per hour — and increase by 10g per hour every two weeks.

Use the same products you'll race with. Gels, chews, drink mix — whatever your plan calls for. Your gut adapts to specific carbohydrate types and concentrations, not just volume.

By race week, you should have completed at least three long runs at or near your target intake rate without significant GI distress. If you're still struggling at 60g per hour, you're not ready for 90g on race day. Better to race at a sustainable rate than to aim for an aspirational one and blow up at mile 16.

Step 3: Build a Race-Day Timing Plan

"Take a gel every 45 minutes" is not a plan. A plan tells you exactly what to consume and when, mapped to your race.

For a 4-hour marathon targeting 75g of carbs per hour, that's 300g of carbohydrates over the race. If each gel delivers 25g, that's 12 gels — one every 20 minutes. If you're using a drink mix in a handheld that delivers 40g per hour, you need fewer gels but have to plan your fluid intake around the mix.

Map your fueling to aid stations where possible. Know which stations have water (you need it to wash down gels) and which have sports drink (which contributes to your carb total). Rehearse the plan in training so the execution is automatic.

Step 4: Start Early and Don't Fall Behind

The biggest fueling mistake is waiting too long to start. Many runners take their first gel at mile 6 or 7, believing they have enough stored glycogen to coast through the first hour. By the time they start fueling, they're already in deficit.

Take your first fuel within 20 to 30 minutes of the race start. Your body doesn't need the carbohydrates yet, but your gut needs time to absorb and process them. Early fueling also establishes a rhythm that's easier to maintain than trying to ramp up later.

If you miss a planned intake — a gel you couldn't stomach, an aid station you ran past — don't try to double up at the next one. That's a fast track to GI distress. Instead, take a slightly smaller dose sooner and get back on schedule gradually.

Step 5: Use Your Data to Refine

Every long run and every race is a data point. Track what you consumed, when, and how you felt. Cross-reference with your watch data: pace, heart rate, heart rate drift, and environmental conditions.

Over time, you'll build a picture of your personal fueling response. You'll know that at 28°C you need more fluid and less solid food. That above 75% of your max heart rate, gels need to be taken with more water. That caffeine gels work better after mile 15 than before.

This is exactly what Sweatr automates. It takes your training and race data, builds a fueling model specific to your body, and adjusts your plan for the conditions you'll face on race day. It replaces the spreadsheet and the guesswork with a plan that improves every time you run.

The Bottom Line

The average marathoner fuels at 21.7 grams of carbs per hour. The science says they need three to four times that amount. That gap — not fitness, not willpower — is why most bonks happen.

Closing the gap requires knowing your personal number, training your gut to handle it, building a specific timing plan, starting early, and using your data to refine. You don't need to be an elite athlete to fuel like one. You just need a plan built for your body, not a generic range from a blog post.

Sweatr builds that plan. Connect your Apple Watch, Garmin, or Strava, enter your race details, and get a personalised carbohydrate and hydration plan in minutes. Your fueling shouldn't be a guess. Make it a system.