Most runners who blow up at kilometre 32 of a marathon blame their legs, their training, or their mental toughness. Almost none of them blame the thing that actually caused it: they ran out of fuel.
Hitting the wall is not a character flaw. It is arithmetic. Your body can store a finite amount of carbohydrate, a marathon costs more than that amount, and the gap has to be filled from the outside — in a stomach that is bouncing, under-perfused, and being asked to absorb sugar at a rate it has never practised. Get the numbers right and the last 10K is a race. Get them wrong and it is a survival exercise, no matter how well you trained.
Marathon race-day fueling, in short: Take 60–90 g of carbohydrate per hour (use a glucose–fructose blend above 60 g/h), drink 400–800 ml of fluid per hour adjusted to sweat rate and heat, and take 300–600 mg of sodium per litre of fluid. Start fueling by kilometre 5 — not when you feel empty. Carb-load with 8–12 g/kg/day for the 36–48 hours before the race, and practise the exact race plan on long runs for 6–10 weeks so your gut can absorb it.
The arithmetic of the wall
A running body costs roughly 1 kcal per kilogram of body weight per kilometre. That gives you the total bill for a marathon:
| Body weight | Marathon energy cost | Glycogen stores (carb-loaded) | Deficit |
|---|---|---|---|
| 60 kg | ~2,530 kcal | ~1,900 kcal | ~630 kcal |
| 70 kg | ~2,950 kcal | ~2,100 kcal | ~850 kcal |
| 80 kg | ~3,380 kcal | ~2,300 kcal | ~1,080 kcal |
Fully carb-loaded, a trained runner stores roughly 400–500 g of glycogen in muscle and another ~100 g in the liver. Call it 500–600 g total, or about 2,000–2,400 kcal. Fat provides the rest of the energy, and you have plenty of it — but fat oxidation cannot supply energy fast enough to sustain marathon pace on its own. Once glycogen runs low, pace collapses to whatever fat alone can support. That is the wall, and it lands somewhere around kilometre 30–35 for the majority of runners who take in nothing or take in too little.
The deficit column is the whole story. You need to import 600–1,000 kcal — roughly 150–250 g of carbohydrate — during the race. At 60 g/h over 3.5 hours you import 210 g. The numbers close.
I came to marathons from long-course triathlon, and after five Ironman finishes the thing I was most confident about was fueling — I had spent years taking on 80–90 g of carbohydrate per hour on the bike without a problem. Then I ran a marathon where I decided I would "keep it simple" and take three gels total, because a marathon is only three hours and the aid stations are close together. I was on pace through 30 km and finished the last 12 km at nearly a minute per kilometre slower, hollow and cold, doing the maths in my head about how much of that I had given away in the previous two hours by not eating. The engine was there. The tank wasn't.
How much carbohydrate per hour
The single most important number in race-day nutrition is grams of carbohydrate per hour. It is dictated by how fast your intestine can move sugar into your bloodstream, not by how hungry you feel.
| Expected finish time | Target carbohydrate | Why |
|---|---|---|
| Under 2:00 (half or elite marathon) | 30–60 g/h | Glycogen stores cover most of the effort |
| 2:00–3:00 | 60–90 g/h | Depletion becomes the limiter |
| 3:00–4:00 | 60–90 g/h | Longest total deficit to cover |
| Over 4:00 | 60–90 g/h, favour the lower end | More time to absorb, lower intensity |
The 60 g/h line is a physiological boundary, not a guideline. Glucose crosses the intestinal wall via the SGLT1 transporter, which saturates at roughly 60 g per hour. Anything beyond that simply sits in your gut and causes the sloshing, cramping, and nausea that runners interpret as "I can't take gels."
The workaround, established by Asker Jeukendrup's research on multiple transportable carbohydrates, is to use fructose alongside glucose. Fructose uses a different transporter (GLUT5), so a glucose–fructose blend in roughly a 2:1 ratio lets total absorption reach about 90 g/h. Nearly every modern gel and sports drink is formulated this way — check the label for maltodextrin plus fructose. If your gels are glucose-only, taking four an hour will not deliver more energy than taking three. It will just make you feel sick.
In practical terms, with a standard gel at 20–25 g of carbohydrate:
- 60 g/h = roughly 3 gels per hour, or 2 gels plus 500 ml of sports drink
- 90 g/h = roughly 4 gels per hour, or 3 gels plus 500 ml of sports drink
Elite marathoners now routinely train up to 120 g/h, and the research supporting that intake in well-adapted athletes is growing. It is not the place to start. Build to 90 g/h reliably before you go looking for more.
Start early, stay ahead
The most common execution error is not the amount — it is the timing. Runners wait until they feel low and then start eating. By then it is too late: absorption takes 15–20 minutes, and once you are genuinely depleted your blood is diverted to the legs and away from the gut, which slows absorption further.
Take your first gel by kilometre 5, when you feel completely fine and taking it seems unnecessary. Then keep to a fixed interval — every 20–25 minutes for 60–90 g/h — rather than reacting to how you feel. Setting a repeating alarm on your watch is not overkill; feel is a lagging indicator by design.
The last useful gel is around 30–35 minutes from the finish. Anything later has no time to reach the bloodstream.
Fluid and sodium
Fluid is simpler than the internet makes it sound, and the failure mode has flipped.
Aim for 400–800 ml per hour, toward the higher end in heat and for larger runners. The target is losing no more than 2–3% of body mass over the race — you can estimate your own sweat rate by weighing yourself before and after a one-hour run in similar conditions and adding back whatever you drank.
The more dangerous mistake now is overdrinking. Exercise-associated hyponatremia — dangerously diluted blood sodium — is caused by drinking more fluid than you lose, and the consensus guidance from the international expert panel on the condition is blunt about it: drinking to thirst is protective, and finishing a marathon weighing more than when you started is a red flag. Mild dehydration slows you down. Hyponatremia sends people to hospital.
For sodium, 300–600 mg per litre of fluid covers most runners. Sweat sodium concentration varies enormously between individuals — from around 200 to over 2,000 mg per litre — so if you finish long runs with salt crusted on your face and a history of cramping, sit at the top of that range or above it.
Carb loading, done properly
The old depletion-then-load protocol — a miserable week of low-carb training followed by a binge — has been obsolete for two decades. The current evidence supports something much simpler.
For the 36–48 hours before the race, take in 8–12 g of carbohydrate per kilogram of body weight per day. For a 70 kg runner that is 560–840 g of carbohydrate daily — considerably more than most runners realise, and hard to reach with "healthy" high-fibre foods. This is the moment for white rice, pasta, bread, potatoes, juice, and sports drinks. Cut fibre and fat, which crowd out carbohydrate and leave you with GI issues on the start line.
Expect to gain 1–2 kg. That is water bound to stored glycogen, roughly 3 g of water per gram of glycogen, and it is the point of the exercise — not a problem to correct.
Carb loading lands in the final days of your taper, when training volume is already low and you have the appetite capacity for it. If you are not sure how the last two weeks should be structured, see The Running Taper Guide.
Race morning
Three to four hours before the start, eat 1–4 g of carbohydrate per kilogram — for a 70 kg runner, somewhere between 70 and 280 g depending on tolerance and timing. Most runners land well at the lower end: oats, a bagel with honey, white bread with jam, a banana.
Keep it low in fat, fibre, and protein, all of which slow gastric emptying. Keep it identical to what you have already eaten before long runs. Race morning is not the time to discover that a new breakfast disagrees with you.
Caffeine is the one legal ergogenic aid with genuinely strong evidence behind it: 3–6 mg per kilogram taken 30–60 minutes before the start, which is 210–420 mg for a 70 kg runner. If you never use caffeine, do not introduce it on race day. If you use it daily, you do not need to abstain beforehand — the evidence for a "caffeine taper" is weak.
Train your gut like you train your legs
This is the part almost everybody skips, and it is the reason race-day nutrition plans fail even when the numbers are right.
Your intestine's capacity to absorb carbohydrate is trainable. Jeukendrup's work on training the gut showed that repeated exposure to high carbohydrate intake during exercise upregulates the transporters, increases how much you actually absorb, and reduces gastrointestinal distress. A gut that has never seen 80 g/h will reject 80 g/h on race day, no matter how carefully you planned it.
The protocol is straightforward:
- 6–10 weeks out: start taking carbohydrate on every long run. Begin at 40–50 g/h.
- Every 2 weeks: add roughly 10 g/h until you reach your race target.
- Use the exact products you will use on race day, at the exact intervals.
- Do at least two long runs at full race intake before the race — ideally at marathon-pace effort, since absorption gets harder as intensity rises.
Your long runs are the only realistic laboratory for this, which is one more reason not to treat them as a pace test. For how to structure them, see How to Do a Long Run.
One caveat worth stating plainly: the fitter your aerobic base, the more of your race energy comes from fat and the less pressure there is on the fueling plan. Consistent easy running is a nutrition intervention as much as a fitness one — see Easy Run Pace: How Slow Is Slow Enough?.
The full race-day plan on one page
| When | What |
|---|---|
| 48 h before | Begin carb loading: 8–12 g/kg/day, low fibre and fat |
| 3–4 h before | 1–4 g/kg carbohydrate breakfast, tested in training |
| 30–60 min before | Caffeine 3–6 mg/kg (if you use it); 300–500 ml fluid |
| Km 5 | First gel — before you feel you need it |
| Every 20–25 min | Gel or drink to hit 60–90 g/h |
| Every aid station | 150–250 ml fluid; sodium 300–600 mg/L |
| Last 35 min | Final gel; nothing after this point |
Print it, tape it to your fridge, and rehearse it on two long runs. A fueling plan you have never executed is a hypothesis, not a plan.
Where the evidence comes from
Two sources do most of the work here. The joint position statement on Nutrition and Athletic Performance from the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and Dietitians of Canada, published in Medicine & Science in Sports & Exercise (2016), is the reference document for carbohydrate loading targets, in-race intake ranges, and fluid guidance. And Asker Jeukendrup's work — particularly A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise (Sports Medicine, 2014) and Training the Gut for Athletes (Sports Medicine, 2017) — is the origin of the 60 g/h transporter ceiling, the glucose–fructose workaround, and the evidence that the gut adapts to training.
On fluid, the expert consensus statement on exercise-associated hyponatremia (Hew-Butler et al., 2015) is worth reading if you are someone who has historically drunk aggressively at every aid station. The guidance has changed.
Fueling is a training variable
The reason race-day nutrition gets treated as an afterthought is that it does not feel like training. It has no pace, no heart rate zone, and it does not show up in your weekly volume.
But it is the difference between running the marathon you trained for and running 32 kilometres of it. Every runner who has faded badly in the last 10K has, in some form, an energy accounting problem — and unlike VO2max or lactate threshold, it can be fixed in a single training cycle by anyone willing to practise it.
Do the arithmetic. Practise the plan. The legs you built over sixteen weeks deserve fuel to run on.
Zarkus builds race-focused plans that treat fueling as part of the training block — gut-training progressions built into your long runs, and a race-day plan calculated from your weight, your goal pace, and your expected conditions. Join the waitlist to try it before launch.
Frequently Asked Questions
How many carbs per hour should I take during a marathon?
For a marathon lasting longer than 2.5 hours, aim for 60–90 grams of carbohydrate per hour. Below 60 g/h you will not offset glycogen depletion in the final 10K. Above 60 g/h you need a glucose–fructose blend (roughly 2:1), because glucose alone saturates its intestinal transporter at about 60 g/h. In practice, 60–90 g/h means roughly 3–4 standard gels per hour, or a mix of gels and sports drink.
What causes hitting the wall in a marathon?
Hitting the wall is glycogen depletion. Your muscles and liver store roughly 500 grams of glycogen (about 2,000 kcal) when fully loaded, but a marathon costs a 70 kg runner close to 3,000 kcal. That deficit of 600–1,000 kcal is why the wall arrives around kilometre 30–35. It is an arithmetic problem, not a mental one, and it is solved by taking carbohydrate in during the race.
What should I eat the morning of a marathon?
Eat 1–4 grams of carbohydrate per kilogram of body weight, 1–4 hours before the start. A practical version for a 70 kg runner is 70–105 g of carbohydrate about 3 hours before the gun — white bread with honey, a bowl of oats, or a bagel and a banana. Keep fat, fibre, and protein low, and eat only foods you have already tested in training.
How much should I drink during a marathon?
Most runners do well on 400–800 ml per hour, adjusted for heat and sweat rate, with the goal of losing no more than 2–3% of body mass. Do not force fluid beyond thirst: drinking so much that you finish heavier than you started is the main cause of exercise-associated hyponatremia, which is more dangerous than mild dehydration.
Can you train your stomach to handle more carbs?
Yes. Intestinal carbohydrate transporters are trainable — repeated exposure to high carbohydrate intake during exercise increases how much your gut absorbs and reduces GI distress. Practise your race fueling on long runs for 6–10 weeks before the race, starting at 40–50 g/h and building to your race target. The gut adapts on roughly the same timescale as your legs.