Finishing your marathon requires more than just physical training in the preceding months. Proper hydration before and throughout the 42.195 km makes all the difference. This practical guide explains exactly how to drink during exertion, which isotonic drinks to choose and which accessories to use, so you can turn your hydration strategy into a genuine performance asset.
Preparing your hydration before a marathon
Starting to hydrate well with mineral-rich water several days before the race is essential for building up water reserves in your body. This technique boosts your performance while avoiding the risk of hyponatremia. Thanks to this careful preparation, you will tackle the 42.195 km with optimal water reserves and a perfect mineral balance.
Controlled water loading
During the last two days before the race, runners should drink regularly to build up a slight water reserve. Pale yellow urine indicates optimal hydration—it is your best indicator before the start. This self-monitoring will naturally guide you toward the right balance.
If your urine remains dark, drink 2 to 3ml of water per kilogram of body weight 1 hour 30 minutes before the start. Be careful not to drink too much either: excess plain water can dilute the sodium in your blood and make you need to use the toilet. That's never a great experience during a marathon, and it wastes time. The goal is to start feeling light but perfectly prepared for the effort ahead.
Practical pre-race guidelines
4 hours before the start, drink 5 to 7ml of water or a lightly sweetened drink per kilogram of body weight. For a 70kg runner, this amounts to 350 to 500ml, giving your body enough time to absorb it without needing frequent toilet breaks during the race. The sodium in the drink helps your body retain water more effectively and prepares your muscles.
10 to 20 minutes before the start, top up with 150 to 250ml of isotonic drink rich in sodium, potassium and magnesium. This final intake stabilizes your electrolytes and gives you a little boost for the first few kilometres without weighing down your stomach.
Choose natural formulas without preservatives or other chemical additives. Always test your drink during long training sessions to estimate the right amount, check tolerance, and determine the spacing between servings. On race day, do not try anything new: this golden rule will help you avoid unpleasant surprises and allow you to reach the starting line with complete peace of mind and a perfectly controlled hydration plan.
Hydration and energy during the marathon
Hydration during a marathon requires a careful balance between fluid, carbohydrates, and electrolytes. A runner can lose up to one liter of water per hour through sweating, depending on running intensity and weather conditions. Properly managing your hydration helps prevent cramps, dehydration, and fatigue,
How much to drink and when
For effective marathon hydration, plan to drink 150 to 250 ml every 15 to 20 minutes, or 400 to 800 ml per hour depending on your sweat rate and the weather. This gradual approach maintains fluid balance without overloading the stomach.
- Ideal pace: Drinking small amounts regularly allows better absorption, reduces the risk of dehydration, and prevents a sloshing stomach—the sensation of liquid moving around in your abdomen when you have drunk too much.
- Amount per serving: 150 to 250 ml, or a few sips, is better absorbed than a large volume all at once.
- Climate adaptation: Aim for 600 to 800 ml/hour in hot weather, compared with 400 to 500 ml in cool weather.
- Personal test: Weigh yourself. The difference in weight before and after training indicates your fluid loss (1 kg lost ≈ 1 liter).
It is often stated that a 2% loss of water in the body can reduce energy levels, and therefore endurance capacity, by 10 to 20%, but no scientific study demonstrates this precisely.
Carbohydrates and electrolytes are essential
Drinking pure water during a long effort such as a marathon is not enough to maintain proper hydration for several physiological reasons.
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Osmotic imbalance: Pure water is not absorbed well by the intestine when it does not have the right concentration of salts and sugars. A good isotonic drink (with the same concentration as blood plasma) crosses the intestinal barrier more quickly when it contains 4 to 8% carbohydrates (40–80 g/liter). Ideally, consume 30–60 g of carbohydrates per hour (up to 90 g for trained marathon runners or triathletes), combining different types of sugars.
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Loss of minerals (electrolytes): When we sweat, we mainly lose water, sodium, potassium, and magnesium. Replacing only the water without these electrolytes dilutes the blood. This can cause cramps, nausea, or headaches. Electrolytes are therefore crucial for proper hydration during a race: 300–700 mg of sodium per litre, combined with other essential minerals
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Energy needs: Water provides no energy. During a marathon, however, the body uses carbohydrates. Sports drinks therefore contain simple sugars (maltodextrin, glucose, fructose) that fuel the muscles. However, when it comes to carbohydrate intake, water may be sufficient if you consume gels or energy fruit purées alongside it.
What is the difference between isotonic drinks and hydration drinks?
Many beginner runners wonder which type of drink to use during a marathon. It must be said that the range of products available is extensive and not always very clear.
- A hydration or electrolyte drink contains only minerals and vitamins. It may contain very few carbohydrates simply to mask the mineral taste. These drinks are used alone for short efforts lasting less than 1.5 hours, or during long efforts alongside energy gels or fruit purées to provide carbohydrates. Read the hydration drink comparison guide to compare the compositions of the main brands on the market.
- An isotonic, energy, or sports drink should contain both minerals and vitamins, as well as carbohydrates. It is used for efforts lasting more than 1.5 hours. Read the sports drink comparison guide to compare the compositions of the main brands on the market.
Adapt to the climate and your pace
In hot weather, slightly increase your water and sodium intake to compensate for increased sweating. Fast marathon runners (<3h) generally need more carbohydrates than those running in 4h or more, as intensity directly affects their energy needs.
Alternate between isotonic drink and plain water to avoid flavor fatigue. After each compote or energy gel, rinse your mouth with water. This simple tip optimizes nutrient absorption, supports hydration all the way to the finish, and prevents the sugary taste from lingering in your mouth, which can accelerate taste fatigue and promote tooth decay.
| Marathon duration | Recommended volume/h | Carbohydrates/h | Sodium/L |
| Under 3h | 600–800ml | 60–90g | 500–700mg |
| 3h–3h30 | 500–650ml | 40–60g | 400–600mg |
| Over 3h30 | 400–550ml | 30–50g | 300–500mg |
Smart fueling during the marathon
The end of water bottles at aid stations—a revolution!
The 2025 Paris Marathon will take place without bottles or cups, in an effort to protect the environment. Other road races will follow this approach. This means runners will have to manage their own containers, which is a real change!
Prepare your 500ml soft bottles or your 250ml soft bottles filled in advance with your favorite isotonic drink to have enough liquid for 10–15km. Their wide opening makes refilling easy and allows you to add energy powder, while the graduations will help you precisely manage your intake during the race.
A running belt or trail vest frees your hands and distributes the weight for greater comfort. Add a collapsible eco cup to a pocket for quick water refills at official aid stations. This well-organized setup will turn your hydration plan into an automatic routine and save you a lot of stress throughout the marathon.
The London Marathon at the forefront of innovation
Since 2019, the London Marathon has hydrated its thousands of participants without plastic, thanks to edible water bubbles. Yes, you read that correctly. The company Notpla has indeed succeeded in turning algae into edible packaging. What is surprising is that this revolutionary concept has not caught on to this day.
Recovering well after the marathon
As soon as you cross the finish line, restoring your fluid levels becomes the absolute priority to regain your physical fitness and prepare for future races. Proper rehydration helps restore electrolyte balance, replenish glycogen stores, and support kidney function. For optimal absorption, spread your fluid intake over the next 2 to 4 hours - this prevents overloading your digestive system and promotes muscle tissue repair after this intense effort.
Rehydrate and refuel
If possible, weigh yourself immediately after the race, then plan to drink approximately 1.5 times the weight lost over the following hours. This slight overcompensation offsets residual sweating and fully replenishes your water needs. For example, if you lose 1 kg, plan to drink 1.5 litres of water in regular small sips rather than all at once.
- The right calculation: comparing your weight before and after the race allows you to accurately assess your losses and adapt your hydration strategy.
- Key timeframe: make the most of the first 30 to 60 minutes after the race to absorb nutrients (bars, gels, fruit purées, drinks) that will speed up your recovery. This is the famous metabolic window.
- Ideal pace: choose 200–300 ml every 15–20 minutes rather than a large amount all at once for better absorption.
- Warning signs: dark urine, headaches, or unusual fatigue indicate persistent dehydration requiring more fluids.
Continue drinking regularly in the days following the marathon. This will reduce muscle soreness, accelerate muscle regeneration, and help you better understand your specific water needs. Feel free to note the amounts you drink and how you feel to refine your approach according to the effort and weather conditions.
Electrolytes and antioxidants
Within an hour of finishing, choose a balanced recovery drink with a 3:1 or 4:1 carbohydrate-to-protein ratio. This combination restarts glycogen synthesis and helps repair muscle fibers. Make sure it contains sodium and other electrolytes for effective rehydration.
Choose natural drinks with antioxidants (vitamins C and E) that reduce post-race inflammation.
Frequently asked questions
How much water should you drink during a marathon?
Hydrate regularly by drinking 150 to 250ml of water or an isotonic drink every 15 to 20 minutes during exercise. This amounts to approximately 400 to 800ml per hour, adjusted according to your sweating rate, the weather conditions, and your speed.
This amount helps maintain good hydration without overloading your digestive system. To assess your needs, weigh yourself before and after training: a loss of 1kg is equivalent to approximately 1 liter of sweat.
How should you hydrate properly before a marathon?
Start pre-hydrating 48 hours before the race by gradually increasing your water intake. Monitor the color of your urine: pale yellow indicates good hydration. Four hours before the start, consume 5 to 7ml of fluid per kilogram of body weight.
In the 10 to 20 minutes before the start, drink 150 to 250ml of an isotonic drink to provide sodium and electrolytes. However, be careful not to drink too much, as this could dilute your blood sodium level. Let your thirst and physical sensations guide your hydration intake.
When should you take energy gels during a marathon?
Consume an energy gel (20-25g of carbohydrates) every 30 to 45 minutes, depending on your pace and tolerance, together with your isotonic drink. To aid absorption, drink 150 to 250ml of water after each gel.
Most marathon runners need 30 to 60g of carbohydrates per hour, increasing to 90g for highly trained runners. Always test your energy gels during training sessions to find the right balance before race day.
