Hydration in a marathon: guidelines, amounts, and key mistakes

Hydration in a marathon is not decided on race day, but in the weeks before, when the body learns to tolerate paces, heat, and aid stations without losing stability. Over 42 kilometers, every sip counts because sweat, breathing, and sustained effort slowly empty the internal reservoir of water and salts, and that wear and tear ends up showing in the legs, in the head, and in the ability to hold pace.

The key is not to drink out of anxiety or to trust thirst as if it were a precise clock. Thirst appears late, when there is already a certain degree of dehydration, and during a long effort it can also be blunted by the exercise itself. That is why a useful strategy combines volume, frequency, and the composition of what is ingested, with special attention to sodium and carbohydrates.

What happens inside the body over 42 kilometers

In a marathon, the body works like a factory that cannot be stopped. The muscle consumes energy at high speed, body temperature rises, and sweat becomes the main cooling system. That sweat, however, does not come free: it carries away water and electrolytes, especially sodium, an essential piece for maintaining fluid balance and neuromuscular function.

When fluid loss progresses, the blood becomes relatively more concentrated, the perception of effort increases, and thermoregulation becomes more complicated. The result can range from a drop in performance to more serious symptoms such as dizziness, cramps, confusion, or, in extreme cases, hyponatremia, which appears when blood sodium drops too much because of excess water and too little salt replacement.

The problem is not only how much is lost, but also how it combines with the duration of the race. A runner who sweats a lot on a humid morning does not face the same scenario as another who competes in cool air and at a more conservative pace. Sweating can vary significantly from person to person, and it also changes depending on body weight, heat acclimatization, intensity, and the material of the clothing.

That is why serious recommendations do not rely on rigid formulas applied to everyone in the same way. The best reference is the body’s own response, with simple data such as weight before and after long training runs, the amount ingested, and digestive tolerance. That information, properly interpreted, is worth more than any universal rule repeated without nuance.

How much to drink before the start

Preparation begins before the starting gun. Arriving at the start well hydrated allows the heart to work under less stress and heat to be managed with more margin. The usual reference in sports medicine places pre-race intake at around 5 to 7 milliliters per kilo of body weight, about two or three hours before the effort. For a 70-kilo person, that works out to roughly 350-490 milliliters.

If the urine is still concentrated or the morning is especially hot, a small additional intake can be added closer to the start, without turning it into a last-minute liquid meal. Drinking too little leaves the runner short on margin; drinking too much, right before running, can cause heaviness, the need to use the bathroom, and a feeling of a full stomach that ruins the first kilometers.

The composition matters as much as the volume. A drink with sodium helps retain part of the fluid and arrive with a better salt balance than water alone. This is especially useful if the start takes place in heat, if the runner comes from a lower-salt dinner, or if the race is expected to be long and demanding from the very first stretch.

In that pre-race window, it is also worth observing how the stomach responds. The 42 kilometers punish any improvisation, and a new drink, sweeter than usual, or with an unfamiliar texture can become a silent obstacle. Digestive tolerance is a performance factor, not a minor detail.

What should happen during the race

During the race, the logic changes completely. It is no longer about arriving well hydrated, but about losing as little as possible and replenishing in time so the deficit does not spiral. In a marathon, a reasonable guideline is usually around 400 to 800 milliliters per hour, although the real range depends on the weather, pace, and each runner’s sweat rate.

The best strategy is usually to split it up. Taking small amounts regularly tends to work better than drinking a lot at once at a single aid station. The stomach empties more easily with a spread-out intake, the intestine absorbs better, and the discomfort that appears when liquid builds up suddenly is reduced.

The type of drink also matters. In long-duration efforts, an isotonic drink with sodium and a certain amount of carbohydrates can help maintain energy and fluid replacement at the same time. It does not need to be very concentrated; in fact, drinks that are too dense can slow gastric emptying and create the feeling of having a block in the abdomen.

Heat changes the rules of the game. On mild days, some runners tolerate water better; on hard days, the body appreciates a more structured replenishment. The more you sweat, the more important it becomes to replace salts, because the problem is no longer just the water lost, but the full balance of the internal environment. Sodium matters more than it seems, and in a marathon it often makes the difference between efficient hydration and just a succession of cups.

What usually works in the race

A useful scheme, without turning it into dogma, is to use each aid station to give the body small, steady amounts. In runners who finish the race between 2 hours 45 minutes and 4 hours 30 minutes, the intake usually alternates between water and sports drink, adjusting the ratio according to pace, heat, and personal tolerance. What matters is not copying someone else’s pattern, but maintaining a stable routine.

When the effort goes on, it also becomes useful to link hydration with energy. A marathon is not run on water alone: after a certain point, glycogen drops and the brain starts to economize. Carbohydrates and fluids must coexist without fighting each other in the stomach, which is why carbohydrate drinks, gels, and some soft bars are part of serious race plans.

The runner who starts the race with a good glycogen store and a previously tested intake pattern usually has fewer ups and downs. The fluid goes along with the fuel, it does not replace it. That is one of the reasons long training runs are also a laboratory for testing bottles, gels, flavors, and amounts. Nothing should be tried for the first time at kilometer 30 if it can be tested before.

Sodium, that detail that decides more than it seems

Water restores volume, but it does not fully correct mineral losses. Sodium is the most relevant electrolyte in this context because it helps preserve fluid balance and prevents rehydration from being incomplete or excessively diluted. In runners who sweat a lot, ignoring it is like filling a tank with a leak open.

The exact concentration depends on each case, but in sports drinks it usually stays within moderate levels, designed to facilitate absorption and recover part of what was lost. The goal is not to turn every bottle into a salty soup, but to achieve a functional proportion. Flavor, in fact, is often a clue: when a drink tastes too sweet or cloying, it is often drifting away from the tolerance a long race demands.

Salt capsules, popular among long-distance runners, are not a universal solution. They can be useful in hot conditions or with very high sweat rates, but they do not replace a complete strategy and can cause digestive problems if taken without enough water or if the runner is already drinking too much. The combination matters more than the isolated supplement.

In a marathon, being obsessed with salts can be just as counterproductive as forgetting them completely. The right balance is usually somewhere in the middle: enough sodium to support replenishment, enough water to make it possible, and enough prior experience to know how the body responds in motion.

When water alone falls short

Plain water can work in a cool environment and for a runner with low sweat loss, but in a marathon it often falls short because it does not replace what is actually lost. Sweat is not distilled water; it carries minerals and, depending on the case, a significant amount of sodium. Replacing only fluid may ease the feeling of dryness, but it does not properly restore the internal environment.

Also, water alone does not provide energy. And in a race lasting several hours, the problem is not only getting to kilometer 30 in one piece, but also sustaining the brain and muscle all the way to the finish. Effective hydration in a marathon is often also an energy strategy. The body appreciates solutions that bring both together with moderation and without excess.

This explains why so many plans combine water, isotonic drink, and later on gels or race food. The advantage is not sophistication, but continuity. A simple plan, tested in long runs, usually performs better than a plan loaded with products the runner barely knows. The stomach, in a race, does not easily grant second chances.

It is also worth remembering that too much water can be a serious mistake. Drinking constantly out of obligation, without taking real sweat loss into account, can favor an unnecessary dilution of blood sodium. In a marathon, more is not always better; sometimes, drinking with judgment protects more than drinking in excess.

The role of carbohydrates within hydration

In long-distance running, fluid does not travel alone. Drinks with carbohydrates help sustain energy and can delay fatigue when the effort goes beyond the first hour. The right concentration depends on individual tolerance, but a drink that is too loaded can feel heavy, while one that is too light provides little fuel.

The physiological logic is simple: as glycogen drops, pace becomes more expensive. The runner feels as if they were pushing a door that gets heavier and heavier. Replenishing carbohydrates during the race does not replace training, but it softens that drop and gives air to the muscle and the nervous system.

Gels, energy drinks, and some soft bars fit here because they allow energy to be added without long chewing or complex digestion. By contrast, more solid foods can work for slower runners or very accustomed profiles, but for most marathoners, simplicity wins. The less extra work the digestive system has to do, the better.

A good plan usually spreads the intake so that fluid and energy arrive in layers, not all at once. That cadence helps avoid spikes and maintain a sense of control. In a long race, the quiet details matter more than the big gestures.

After crossing the finish line, the work continues

Finishing does not mean recovery is complete. During the hours afterward, the body keeps correcting losses of water, sodium, and energy, and that phase matters much more than it usually seems from the outside. Post-marathon rehydration should aim not only for the return of normal thirst, but for recovery to be real and complete.

A practical and widely used reference is to replace around 1.5 liters for every kilo lost on the scale, a figure that also covers the losses that continue after the finish line. For that, body weight before and after the race gives a very useful clue, as long as it is measured under similar conditions and without soaked clothing fooling the scale. The data is not perfect, but it gives a fairly clear direction.

In that window, sodium becomes important again because it helps retain the fluid ingested. Drinking large amounts of water without salts can cause part of the replenishment to be lost in the urine too soon. Effective recovery is usually gradual, not an immediate flood. The body does not refill like a bucket; it reorganizes like a city after a storm.

Carbohydrates and some easy-to-digest food also need to be added. The priority is to restore glycogen, rehydrate, and not irritate the stomach, which after the effort is usually more delicate. Anyone who turns post-race eating into an excessive celebration risks prolonging the physiological hangover more than necessary.

Mistakes that keep being repeated at the start and during the race

The most common one is starting with the idea that thirst will be enough of a guide. It is not. Another frequent mistake is arriving at the race without having tested what is later intended to be used, whether it is a new drink, a gel that is too concentrated, or some salts that seemed harmless in training and become a nuisance in the race.

It is also very common to drink only when an aid station appears, as if the stations marked the only chance to hydrate. The plan should adapt to the runner’s clock, not to the organization’s sign. In races with many stations, that may work better; in others, the density of aid stations forces you to think through each stretch more precisely.

Another mistake is switching from water to energy drink without transition, or mixing too many different products in a short period of time. The result can be slow digestion, nausea, or that bouncing feeling in the abdomen that ruins the stride. The marathon tolerates few improvisations and quickly punishes experiments without rehearsal.

And there is one last mistake, perhaps the quietest: waiting for the crash to appear before reacting. Preventive hydration works precisely because it acts before the problem. When the runner already notices a dry mouth, a foggy head, or a sudden drop in energy, part of the damage has already been done.

What changes depending on the weather, pace, and runner profile

There is no fixed guideline that works for everyone because the marathon is a test of variables. Heat increases losses, humidity makes sweat evaporation harder, and a high pace raises the demand for fuel and fluid. A 3-hour runner does not need exactly the same thing as another who goes beyond 4 and a half hours; nor does one who sweats buckets and another who barely soaks the shirt.

Heavier runners usually have a greater expenditure, but they may also tolerate certain reserves better. Lighter runners, on the other hand, sometimes suffer more from fluid excess if they drink too much relative to their sweat rate. Personalization is not a luxury; it is the only sensible way to get close to precision.

In urban marathons with lots of crowd support and frequent aid stations, logistics help. But the crowd can also distract, interrupt the drinking motion, or make the runner take half a cup in a hurry. In that context, practicing the gesture matters: grab, drink, let go, and keep going without losing too many seconds or throwing off the stride.

The runner’s experience weighs almost as much as physiology here. Whoever has tested their strategy in long runs enters the race with a kind of internal map. They know when to push, when to ease off, and when a sip is enough. That familiarity is worth gold when the legs are already running on fumes.

A marathon is won in the bottle too

Hydration in a marathon sums up the logic of long races well: it is not enough to run a lot, you have to arrive with the body prepared to sustain the effort without emptying out from the inside. Drinking before, during, and after is not an accessory gesture, but part of performance, just like pacing, rest, or pre-race nutrition.

The strongest formula combines water in the right amount, enough sodium to replace losses, carbohydrates to sustain energy, and a strategy tested in advance. No improvised heroes and no extreme solutions. Over 42 kilometers, the body rewards consistency and punishes excess, almost always with the same mathematical coldness.

That is why the best hydration is not the one that looks the flashiest, but the one that lets the runner keep moving with the pulse under control, the mouth without extreme dryness, and the head clear all the way to the finish. In the end, the marathon is a long conversation between effort and balance, and water, well accompanied, has a lot to say in that negotiation.

Medical reference: Fluid and electrolyte balance, MedlinePlus (U.S. National Library of Medicine).