In a marathon, nutrition is not a comfort detail: it's a performance component just like training. A runner with solid physical preparation can see their race collapse at the thirtieth kilometer because their reserves are empty, or drop out due to perfectly avoidable digestive issues. Conversely, a well-constructed and tested strategy transforms the final ten kilometers.
This guide covers everything: what the body actually consumes over 42.195 km and why the wall exists, carbohydrate strategy before, during and after, hydration and sodium, then supplements. And for the latter, the same honest method as for all our guides: what has an authorized European claim, what is based on data without a claim, and what is pure marketing.
The central problem. Glycogen reserves, the storage form of carbohydrates in muscle and liver, are limited and insufficient to cover a marathon at sustained pace. This is the mechanism of the wall. The entire strategy consists in filling these reserves before the race, supplying carbohydrates during, and replenishing them after. Widely used benchmarks: 30 to 60 g of carbohydrates per hour up to two and a half hours of effort, up to 60 to 90 g per hour beyond that when combining multiple types of carbohydrates.
On the supplements side, four families have a relevant authorized European health claim: carbohydrate-electrolyte solutions (water absorption, maintenance of endurance performance), carbohydrates (restoration of normal muscle function after prolonged exercise that has depleted glycogen), proteins (maintenance and growth of muscle mass), and iron (oxygen transport, fatigue reduction), to which magnesium and vitamin D may be added depending on the situation. The rule that overrides everything : nothing is tested on race day, and the gut trains like everything else.
- What the body consumes during a marathon, and why the wall exists
- Carbohydrate strategy, before, during, after
- Hydration and sodium
- Training your gut
- Supplements with authorized health claims
- What has data, but no health claim
- The four-phase protocol
- The most common mistakes
- Frequently asked questions
What the body consumes during a marathon, and why hitting the wall happens
During exercise, the body draws on two main fuels: fats, available in virtually unlimited quantities but usable primarily at moderate intensity, and carbohydrates, stored as glycogen in muscle and liver. The higher the pace, the greater the proportion of carbohydrates used.
The problem boils down to one sentence: glycogen reserves are limited, and they are insufficient to cover a marathon run at a sustained pace. When supplies run out, the body is forced to slow down to rely more heavily on fats. This is hitting the wall : a sudden drop in pace, often in the second half of the race, frequently occurring around the thirty-kilometer mark, accompanied by a sensation of empty legs and sometimes mental fog.
It's neither a lack of character nor poor training. It's a question of supply and refueling. And that's good news, because it can be managed: you can increase your starting reserves, reduce the rate at which they deplete by managing your pace, and supplement them during the race with regular intake.
Two additional constraints apply over time. The loss of fluids and electrolytes, which increase with heat and exercise duration, and whose extent varies greatly from runner to runner. And digestive tolerance, which deteriorates as blood is mobilized toward the muscles: a stomach that accepts everything at rest accepts very little at the thirty-fifth kilometer.
Carbohydrate strategy, before, during, after
Before: maximize starting reserves
Over the two to three days before the race, the goal is to arrive at the start with full glycogen reserves. The principle is often misunderstood: it's not about eating more in volume, but about shifting the distribution toward carbohydrates, progressively reducing the proportion of fiber as race day approaches to limit digestive discomfort.
On race morning, a meal rich in carbohydrates, low in fiber and fat, consumed two to three hours before the start, completes liver reserves without overloading digestion. This meal should have been tested several times before long workouts, never tried for the first time on race day.
During: slow down the depletion of reserves
The widely used benchmarks in endurance nutrition are as follows:
| Exercise duration | Hourly indicative intake | Note |
|---|---|---|
| Up to 1 hour | Intake generally not necessary | Existing reserves are sufficient |
| 1 to 2.5 hours | 30 to 60 g of carbohydrates per hour | A single carbohydrate source is generally sufficient |
| Beyond 2.5 hours | Up to 60 to 90 g per hour | Combining multiple types of carbohydrates improves absorption |
Three essential clarifications. These figures are guidelines, not prescriptions : digestive tolerance varies greatly from runner to runner. Next, you must start early rather than waiting for the sensation of depletion: when the decline is felt, it's already too late, because absorption takes time. Finally, high intakes cannot be improvised and require a trained gut, the subject of the next section. [This is a continuation of item 15]
In terms of format, gels, carbohydrate drinks, fruit pastes, or solid foods can all work: the form matters less than the quantity actually absorbed and tolerated. With concentrated forms, remember to drink enough to facilitate absorption.
After: Recovery
This is the most neglected moment, yet one of the best documented from a regulatory standpoint. Carbohydrates have an authorized claim for the restoration of normal muscle function after very intense or long-duration exercise that has caused muscle fatigue and glycogen store depletion. In practice: replenish reserves in the hours following completion. Proteins, with their claim on the maintenance and growth of muscle mass, provide a useful complement.
Hydration and sodium
Over three to five hours of effort, sweat losses become significant, and they vary enormously depending on heat, intensity, and individual physiology. Two symmetrical errors exist: drinking too little, and drinking massive amounts of pure water for hours, which can be problematic by diluting electrolytes.
The practical rule comes down to two points. Drink regularly in small amounts rather than a lot at once. And do not neglect sodium, the main electrolyte lost in sweat, which is precisely covered by carbohydrate-electrolyte solutions: they have authorized claims on theimprovement of water absorption during exercise and on the maintenance of endurance performance during prolonged endurance exercise.
A simple test to do during training: weigh yourself before and after a long workout, under the same conditions, to estimate your personal losses. This is much more useful than a general recommendation, because the difference between two runners can be considerable.
Training your gut
Digestive issues are among the leading causes of underperformance and dropout in marathons. Yet they are largely preventable, because the intestine trains like the rest of the body : its capacity to absorb carbohydrates during exercise improves with repetition.
The method comes down to three principles. Test during long runs exactly what is planned on race day, product by product. Increase gradually hourly quantities rather than aiming immediately for maximum intake. And note what works and what doesn't, to build a personal plan rather than copying someone else's.
Add to this a reduction in fiber intake in the twenty-four to forty-eight hours before the race, and the bulk of digestive risk is addressed. This approach costs nothing and prevents most unwelcome surprises.
Supplements with authorized health claims
| Ingredient | Authorized health claim | When it's relevant |
|---|---|---|
| Carbohydrate-electrolyte solutions | Improve water absorption during exercise and contribute to maintaining endurance performance during prolonged endurance exercise | During the race and long runs |
| Carbohydrates | Contribute to the recovery of normal muscle function after prolonged exercise that has depleted glycogen stores | In the hours following the finish |
| Protein | Contribute to the maintenance and growth of muscle mass | Recovery, throughout the preparation |
| Iron | Contributes to normal oxygen transport, normal formation of red blood cells and hemoglobin, and to reducing fatigue | Only based on blood work and medical advice |
| Vitamin C | Increases iron absorption and contributes to reducing fatigue | In combination with iron intake |
| Magnesium | Contributes to reducing fatigue, to normal muscle function and to electrolyte balance | Training load, sweat losses |
| Vitamin D | Contributes to the maintenance of normal bone health, normal muscle function, and normal calcium absorption | Winter preparations, high impact volume |
Iron is the most poorly addressed topic in distance runner nutrition. Its health claims are real and relevant for endurance, but excess iron is harmful. We therefore never supplement "just in case" or based on temporary fatigue: only a blood test prescribed and interpreted by a doctor makes it possible to know whether supplementation is justified, at what dose and for how long. If you feel abnormally tired during preparation, the right approach is to consult a doctor, not to make a purchase. After medical advice
normal transport of oxygen in the body , to the normal formation of red blood cells and hemoglobin, and toreducing fatigue . Our version usesbis-glycinate , a chelated form with amino acids renowned for its digestive comfort, a frequent concern with conventional iron salts. To be considered only after a blood test and your doctor's advice.View Bis-glycinate Iron
What has data, but no claimThe gray area, presented for what it is.
| What is being studied | Regulatory status | Caffeine |
|---|---|---|
| Very widespread use before and during exercise | No authorized performance claim; substance on the anti-doping monitoring program | Nitrates, beetroot juice |
| Studies on effort economy in endurance | No authorized claim | BCAA |
| Branched-chain amino acids, heavily marketed | No authorized claim | Beta-alanine |
| Buffering role in intense efforts lasting a few minutes | No authorized claim, and profile poorly suited to marathon running | Creatine |
| Authorized claim, but for a different type of effort | Targets series of very intense short-duration efforts, not prolonged endurance | The case of |
creatine deserves clarification, because it illustrates the method well. It has a perfectly valid claim, but this one targets successive series of very intense short-duration exercises . This is the profile of a hybrid format likehyrox l'hyrox, not that of a marathon. A product can therefore be excellent and off-topic: what matters is the correspondence between the claim and your effort.
The four-phase protocol
The most common mistakes
- Waiting until you're hungry to eat. When the energy drop hits, absorption comes too late. You must start early and stay consistent.
- Discovering a gel at a aid station. An untested product distributed along the course is a gamble on your digestion at the worst possible moment.
- Not training your gut. Aiming for 60 or 90 g per hour without ever practicing it on long outings leads straight to digestive issues.
- Confusing carb-loading with overeating. The goal is to shift the distribution toward carbohydrates, not to increase total volume to the point of arriving heavy at the finish line.
- Drinking only water for hours. Sodium is lost through sweat and is as important as fluid volume.
- Supplementing with iron without testing. The riskiest mistake on this list: excess iron is harmful.
- Neglecting the hours following arrival. This is when replenishing reserves is most effective, and it's the moment everyone forgets.
- Eating an unusual breakfast. Too heavy, too fatty, or too high in fiber: the result shows by the tenth kilometer.
No product replaces a tested plan. The strategy that works on race day is the one you've repeated on long runs, adjusted to your tolerance, and written down before departure. A miracle gel discovered at an aid station will never make up for absent nutritional preparation.
Frequently asked questions
The wall and energy
What is the marathon wall?
It's the sudden collapse in pace that many runners experience in the second half of the race, often around the thirtieth kilometer. Its main mechanism is the depletion of glycogen reserves, the form in which carbohydrates are stored in muscle and liver. These reserves are limited and insufficient to cover an entire marathon at sustained pace. The nutritional strategy aims precisely at delaying this moment.
How many carbohydrates should you take during a marathon?
Widely used benchmarks place intake around 30 to 60 g of carbohydrates per hour for efforts of one to two and a half hours, and up to 60 to 90 g per hour beyond that, provided multiple types of carbohydrates are combined. These figures remain benchmarks: digestive tolerance is highly individual and should be developed during training. A plan never tested on a long run won't hold up on race day.
Should you carb-load beforehand?
Increasing the share of carbohydrates over the two to three days before allows you to maximize glycogen reserves. It's not about eating more in volume but about shifting the distribution toward carbohydrates, progressively reducing fiber as race day approaches. Like everything else, this strategy should be tested before a long run.
Should you use a different strategy depending on your target time?
Yes, mainly due to exercise duration. A runner targeting three hours and another targeting five hours don't have the same total carbohydrate needs, the same hydration constraints, or the same heat exposure. The principle remains identical, but the total volume and number of servings increase with duration. The common rule: start early and stay consistent.
Hydration and digestion
How should you hydrate during a marathon?
Drink regularly in small amounts rather than a lot at once, taking into account heat, duration, and your personal sweat rate, which varies greatly. Sodium matters as much as water, as it's the main electrolyte lost in sweat. Drinking large amounts of plain water over several hours can actually cause problems: the rule is consistency, not volume.
How do you avoid digestive issues during the race?
Three levers. Train your gut by testing planned intake during long runs, gradually increasing amounts. Reduce fiber in the twenty-four to forty-eight hours before the race. And avoid anything new on race day. Digestive issues are one of the leading causes of withdrawal or underperformance in marathons, and they're largely avoidable.
Are gels essential?
No, the form matters less than the amount of carbohydrates actually absorbed and tolerated. Gels, sports drinks, fruit pastes, or solid foods can work depending on preferences. What counts: reaching your target hourly intake, drinking enough with concentrated forms, and having tested the exact same protocol during training.
What should you eat the morning of a marathon?
A meal rich in carbohydrates, low in fiber and fat, consumed two to three hours before departure, and most importantly, already tested several times before long runs. The objective is to top up liver glycogen stores without overloading digestion. Absolute rule: nothing new on race day.
Supplements
Which supplements have authorized claims for marathons?
Four main families. Carbohydrate-electrolyte solutions improve water absorption during exercise and contribute to maintaining endurance performance. Carbohydrates contribute to the restoration of normal muscle function after prolonged exercise that has depleted glycogen. Proteins contribute to the maintenance and growth of muscle mass. Iron contributes to normal oxygen transport and reduced fatigue. Magnesium and vitamin D complement these depending on the situation.
Why is iron a topic for distance runners?
Because iron participates in oxygen transport, the central element of endurance performance. It has several authorized claims: normal oxygen transport, normal formation of red blood cells and hemoglobin, fatigue reduction. Be careful, however: you don't supplement with iron on principle. Excess is harmful, and only a blood test prescribed by a doctor can determine whether supplementation is justified.
Does vitamin C help absorb iron?
Yes, and it's an authorized claim: vitamin C increases iron absorption. Combining a source of vitamin C with iron from plant sources or a supplement improves its assimilation. Vitamin C also has an authorized claim for fatigue reduction. This doesn't eliminate the need to first verify, through a blood test, whether iron supplementation is justified.
What role does vitamin D play for a marathoner?
Vitamin D has authorized claims for maintaining normal bone health, normal muscle function, and normal calcium absorption. These elements interest distance runners, whose skeleton absorbs a significant volume of impacts. In France, skin synthesis is very low from October to March, a period covering much of winter training preparations.
Does creatine do anything for a marathon?
Its authorized claim targets successive series of very intense short-duration exercises, which doesn't match a marathon profile. Creatine's validated effect doesn't apply to prolonged endurance. It's not a product to prioritize for this distance, unlike hybrid formats. Better to focus efforts on carbohydrate strategy, hydration, and recovery.
Should you take BCAAs for a marathon?
Branched-chain amino acids have no authorized health claims in the European Union. For a runner whose daily protein intake is already sufficient and well-distributed, their additional benefit is minimal. For marathon running, nutritional priorities are very clearly focused on carbohydrates and hydration.
Does caffeine improve marathon performance?
It is widely used, but no performance claims have been authorized in the European Union. It is not prohibited in sports, while being included in the World Anti-Doping Agency's monitoring program. Two precautions: never test an unusual dose on race day, and be wary of products that combine caffeine with other stimulants.
Does beetroot juice improve performance?
Dietary nitrates are the subject of research in exercise physiology, particularly regarding effort economy, but they have no authorized health claims in the European Union. This is a documented lead, to be considered as such, and to be tested during preparation if you wish to explore it, never on race day.
Can a supplement cause a failed anti-doping test?
Yes, it is one of the most frequent causes of involuntary positive tests, through cross-contamination in production or undeclared additives. The principle of strict liability applies: the runner is responsible for what is found in their body. The solution is to read all labels, choose products tested by an independent laboratory, and avoid opaque purchasing channels.
Recovery and Getting Started
How to recover after a marathon?
In the hours that follow, combine carbohydrates and protein: carbohydrates have an authorized claim for the recovery of normal muscle function after prolonged exercise that has depleted glycogen stores, and proteins have a claim for the maintenance and growth of muscle mass. Rehydrate by accounting for sweat losses, then allow yourself several days of very gradual resumption.
Where to start if you're preparing your first marathon?
With gut training during long runs, well before choosing products. Test a drink and hourly carbohydrate intake, increase gradually, note what works and what doesn't. This approach costs zero euros and prevents most unpleasant surprises. Supplements come later, and only to address an identified need.
Glossary
- Glycogen
- Form of carbohydrate storage in muscle and liver, the primary source of energy rapidly mobilizable at sustained pace.
- The wall
- Sudden drop in pace linked to the depletion of glycogen stores, often described in the second half of the race.
- Carbohydrate loading
- Increase in the proportion of carbohydrates in the days before the race, intended to maximize starting reserves.
- Gut training
- Repetition during training of the intake planned during the race, in order to improve tolerance and absorption.
- Carbohydrate-electrolyte solution
- Drink combining carbohydrates and minerals, with authorized claims on water absorption and maintenance of endurance performance.
- Authorized health claim
- Formulation evaluated and then authorized at European level, the only one legally usable to attribute an effect to a nutrient.
Sources
- European Commission. Register of authorized nutritional and health claims, particularly for carbohydrate-electrolyte solutions, carbohydrates, proteins, iron, vitamin C, magnesium and vitamin D. ec.europa.eu
- World Anti-Doping Agency. List of prohibitions and monitoring program. wada-ama.org
- The hourly carbohydrate intakes cited correspond to benchmarks widely used in endurance nutrition; they require individualization and prior digestive training.
