Most marathoners eat about 35 g of carbs an hour. The research says 60 g/h or more. In a 28-runner study, the group that took 60 g/h from gels finished in 3:38:31. The group that ate what they felt like finished in 3:49:26. That’s about 11 minutes, or 4.7%, from the in-race plan.
In short: you’re probably under-eating, and it costs you minutes. This is a full marathon fuelling plan. It covers how much to take by finish time, how to turn grams into gels, and how to train your gut over 12 to 16 weeks.
How many grams of carbs per hour for your finish time
The standard guidance runs in tiers. About 30 g/h works for events of 1 to 2 hours. Aim for 60 g/h in the 2 to 3 hour range. Events beyond 2.5 to 3 hours may benefit from up to 90 g/h (Burke 2011; Jeukendrup 2014). Translation: the longer you’re out there, the more fuel you need.
Here’s how that maps onto marathon finish times. These are practical targets built from those tiers, not numbers from a single trial.
Slower runners often get this wrong. Race plans written for elites assume a 2-hour effort. If you’re out for 5 hours, you burn through your stored carbs and need a steady supply, not a few gels at the end.
Most people under-eat. The control group in that marathon study averaged about 35 g/h, far below the 60 g/h target. Heat, nerves and a bouncing stomach all make you skip gels. A fixed schedule beats a feeling.
Gel and drink math: from g/h to a gel every few kilometres
A typical gel holds 25 g of carbs. So 60 g/h is a gel every 25 minutes, and 75 g/h is one every 20. You can swap in sports drink or chews. Just count the grams on the label.
| Finish | Plan (g/h) | Gel (25 g) every | Roughly every | Total carbs |
|---|---|---|---|---|
| 3:00 | 75 | 20 min | 4.7 km (2.9 mi) | 225 g |
| 3:30 | 70 | 21 min | 4.3 km (2.7 mi) | 245 g |
| 4:00 | 70 | 21 min | 3.8 km (2.3 mi) | 280 g |
| 4:30 | 60 | 25 min | 3.9 km (2.4 mi) | 270 g |
| 5:00 | 60 | 25 min | 3.5 km (2.2 mi) | 300 g |
| 5:30 | 60 | 25 min | 3.2 km (2.0 mi) | 330 g |
Totals are plan g/h times finish time. Slower runners eat more in total, even though the hourly number is lower. The simplest trick is to tie each gel to a watch alarm or to an aid station you know is coming.
Water matters too. The study group drank about 0.75 L per hour and took 60 mg of sodium per hour with 90 mg of caffeine. It had no significant stomach problems at 60 g/h.
Why 60 g/h is the ceiling for a single sugar
Think of your gut as a highway with one lane for glucose. That lane carries about 60 g/h. Push more glucose in and it backs up like traffic at a toll booth. The backup sits in your stomach and turns into cramps.
Fructose uses a second lane. A 2:1 glucose to fructose mix can lift carb burning by up to 65% compared with glucose alone, and it works up to about 90 g/h. Above that, you’d want more fructose.
Check the label on your gel. If you plan to go past 60 g/h, it should list both sugars. Want the extreme end? Our guide to training for 120 g of carbs per hour covers that ceiling. For a marathon, 60 to 90 is enough.
Lanes widen with use. Which brings us to training.
How common is stomach trouble in a marathon?
Less than you think. In one study of 221 endurance athletes, about 4% of marathoners reported serious symptoms. For a 70.3 it was about 14%, and for an Ironman about 31% (Pfeiffer 2012). The exact figures vary by study, but the pattern holds.
So don’t scare yourself into eating nothing. Marathon pace is easier on the gut than a long bike ride followed by a run. Most trouble comes from trying something new on race day.
A 12 to 16 week gut-training schedule
Your gut adapts like a muscle. In one trial, runners did a 2-hour run at 60% VO2max taking 30 g of carbs every 20 minutes (90 g/h). After two weeks of repeating that challenge, stomach symptoms fell about 60%. A 1-hour distance test improved 5.2% on gels and 4.3% on food, with no change on placebo.
Translation: practise eating while running, and the stomach complains less.
You don’t need to start at 90 g/h. Here is a ladder you can drop into your weekly long run. It’s an example schedule built from those studies, so adjust it to your race number.
| Phase | Weeks | Long-run fuel | What to check |
|---|---|---|---|
| Baseline | 1-4 | 30-35 g/h, one gel type | Any stomach upset? Note it |
| Build | 5-8 | 45-55 g/h | Add the sports drink you’ll use on course |
| Push | 9-12 | 60-70 g/h, 2:1 mix | Practise at goal-pace segments |
| Rehearsal | 13-15 | Your race number, race gels | Two full dress rehearsals |
| Taper | 16 | Light, normal food | Don’t try anything new |
Score each long run from 1 to 5 for stomach upset. A 1 means no trouble at all. A 5 means cramps or stops. If you hit a 4 or 5, hold the number the next week instead of climbing. Don’t rush the ladder.
Here’s an illustrative composite, not a real athlete. Call him Dan, 41, a self-coached runner with a 4:09 marathon. He ate about 35 g/h, mostly when he felt like it, and he faded after 32 km. Over 14 weeks he climbed to 70 g/h in his long runs, one gel every 21 minutes. He logged a stomach upset score after each run and stayed at 55 g/h an extra week when one scored a 4. On race day he ran 3:58, with even splits late.
Rehearsal beats courage.
Caffeine for marathon day
The research is clear. Take 3 to 6 mg per kg of body mass about 60 minutes before the start (ISSN position stand, 2021). A meta-analysis found a 3.03% boost in power output and a 2.22% cut in time-trial time (Southward 2018). Higher doses add side effects, not speed.
A 70 kg runner would take 210 to 420 mg. Start near 210 mg and test it in training. Then, late in the race, take 1 to 2 mg/kg (70 to 140 mg) from a caffeinated gel. The marathon study used about 90 mg per hour.
Coffee is the thermostat here, not the furnace. A small, steady dose keeps you warm. A huge one just makes you jittery.
Breakfast and the final hours
Common guidance is 1 to 4 g of carbs per kg of body mass in the 1 to 4 hours before the start (Thomas 2016). A practical pick is about 2 g/kg three hours out. For a 70 kg runner, that’s roughly 140 g. Think a big bagel with jam, a banana and a glass of juice. Skip high-fibre and high-fat food.
Our guide to carb-loading before a marathon covers the two days before. If you’re thinking about low-carb training days, see train low, compete high. And if you’re running on too little food overall, read about under-fuelling and energy availability.
What to do when your stomach revolts
Slow down for a minute. Switch to water and small sips of drink. Wait for the cramp to ease, then restart with half a gel. Don’t quit fuelling completely. A bonk is harder to fix than a cramp.
Life will also mess with your plan. You’ll miss a long run, travel, or sleep badly. AthleteOS adapts your training plan when you miss sessions, sleep badly, travel or get sick, and explains each change. Keep your fuel rehearsal on whichever day the long run lands. Start a plan on AthleteOS and build your race-day number from there.
Fuel early. Practise often. Race what you’ve rehearsed.