Hydration and sports during a heatwave: what you really need to know.
You're told to hydrate well. But what does that actually mean? Should you keep training when it's 35°C? Is water really enough? Clear answers, no BS.
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The heatwave is here. Temperatures are exceeding 35°C, and the immediate reflex is well-known: drink plenty of water. But this advice, as true as it may be, isn't enough on its own. Hydrating properly during a heatwave, and especially when exercising, is more precise, more nuanced, and more important than one might think.
The summer of 2025 saw over 5,700 heat-related deaths in France between June and September, according to the Ministry of Health. These figures remind us that heat is not a benign discomfort. It's a real physiological constraint, with serious consequences if poorly managed.
Here, we answer the real questions, the ones every athlete asks when the thermometer soars.
1. Proper hydration: what does it actually mean?
The first reflex to disarm: waiting until you're thirsty to drink. Thirst is a late signal. When it appears, the body is already in a state of dehydration. During a heatwave, this deficit sets in much faster than usual.
The basic rule: drink regularly throughout the day, in small sips of 10 to 12 centilitres. Not large glasses one after another; the stomach will bloat, and the body won't absorb it better. Regularity trumps the amount consumed at once.
In terms of volume, scientific recommendations set basic needs at approximately 1.5 litres of water per day for an adult at rest. In hot weather and with one hour of training, this figure can rise to 3.5 litres per day, according to experts from Parisian hospitals. The body loses between 0.5 and 2 litres of sweat per hour of effort, depending on intensity and heat.
The best indicator of good hydration? The colour of your urine. Pale yellow: you are well-hydrated. Dark yellow or amber: drink now. It's simple, free, and reliable.
2. How to know if your body is well-hydrated?
Beyond urine colour, there are other signals the body sends when hydration is insufficient.
The first signs often appear disguised: unexplained fatigue in the mid-afternoon, persistent headaches, difficulty concentrating, cramps. These symptoms are not an unavoidable summer fate: they signal a need that can be anticipated.
A mere 2% drop in body weight due to dehydration is enough to degrade athletic performance: reduced endurance, difficulty concentrating, heavy legs. For an 80 kg man, this represents 1.6 litres of fluid loss: less than an hour of exercise in the heat.
Weighing yourself before and after exercise is a simple method to assess actual fluid loss. Each kilogram lost corresponds to approximately one litre of fluid to be recovered.
3. Is water really enough?
This is where opinions diverge on social media, and where precision is key.
For daily life, even in hot weather, plain water is sufficient in the vast majority of cases. Experts are clear on this: adding salt tablets or electrolytes to water is unnecessary for someone who eats a balanced diet. Westerners already consume, on average, double the recommended daily amount of salt.
But when you exercise, especially prolonged efforts, things change. When you sweat, you don't just lose water. You lose sodium, potassium, and magnesium. These minerals regulate muscle contraction, fluid balance, and nerve transmission. Without them, cramps occur, performance drops, and recovery takes longer.
The practical rule: for an effort of less than 1.5 hours under normal conditions, plain water is enough. Beyond that, or in hot weather with significant sweating, you need to compensate for lost minerals. Specifically during a heatwave, the threshold decreases, as heat accelerates sweating and mineral loss occurs faster. Even for a shorter effort, compensation becomes relevant as soon as you sweat heavily.
An important clarification: if someone advises you to take electrolytes at every session, regardless of duration: that's marketing. Supplement brands have an interest in making you believe you systematically need them. Science says otherwise: water is sufficient for short and moderate efforts. Beyond 1.5 hours or in hot weather, electrolytes become useful and not necessarily expensive.
Effective homemade recipe:
— 500 ml water
— 500 ml grape juice
— A pinch of table salt
Economical, simple, and as effective as most products sold in stores.
Beware of the opposite error: drinking very large quantities of plain water without salt intake can dilute sodium in the blood and cause hyponatremia. This is rare but real, and it mainly affects endurance athletes who drink a lot without eating. Nausea, headaches, confusion: do not confuse this with classic dehydration.
4. Exercise and heatwave: should you continue, reduce, or stop?
This is the divisive question, and for good reason, expert opinions are genuinely nuanced.
The short answer: yes, you can continue to exercise during a heatwave. But not just anyhow.
The French Athletics Federation published official recommendations in May 2026: shift sessions to before 9 am or after 7 pm, reduce intensity, plan regular breaks with hydration, and favour shaded areas or indoor spaces. The Ministry of Sports reminds that during a red heatwave alert, all intense outdoor effort should be avoided.
Santé publique France specifies in its 2025 epidemiological bulletin that training above 30-35°C poses real challenges for thermal regulation and can have dramatic consequences if intensity is maintained.
Concretely, here's what that means in practice:
— Train early in the morning or in the evening. Never between 12 pm and 5 pm.
— Reduce intensity by 20 to 30% compared to your usual. Your body is already working harder to regulate its temperature.
— Shorten the duration if the heat is extreme.
— Hydrate before you start, every 20 to 25 minutes during exercise, and immediately after.
— Listen to your body. Dizziness, nausea, confusion — stop. These are not signs of weakness. They are alarm signals.
5. Gym workouts or swimming: really better?
Training indoors with air conditioning is a good alternative when temperatures are extreme. The thermal load is reduced, the body expends less energy cooling down, and performance remains more stable.
Swimming is often cited as the ideal option. It is, with one condition: pool water is chlorinated and dries out the skin. It doesn't negate the need to hydrate properly. You sweat in the water without realizing it. Drinking before, during, and after remains essential.
For strength sports or intense efforts, an air-conditioned gym is the best option during the day. For trail running, general running, or outdoor disciplines, the morning or evening slot remains the non-negotiable rule.
In summary: what you remember
Drink before you're thirsty. Small, regular sips. Pale yellow urine as a reference. For a short effort, water is enough. For a long effort or in hot weather, compensate for lost minerals. Don't stop exercising: adapt it. Schedule, intensity, duration. And stop immediately at the first signs of discomfort.
A heatwave is not a reason to stop training. It's a reason to train smarter.
The Editorial Team - S COMME SPORT
SOURCES
Santé publique France — Heat and athletic performance: what means to limit the risks incurred? Weekly epidemiological bulletin, n°7, 2025. santepubliquefrance.fr
Ministry of Sports — Sport and heatwave: beware of heatstroke. info.gouv.fr, August 2025.
French Athletics Federation — Recommendations for athletes and meetings during periods of high heat. May 2026.
Franceinfo — Heatwave: sports to favour, hydration, recovery to adapt. June 2025. franceinfo.fr
RTBF Actus — Is the addition of electrolytes essential for hydration in case of high heat? May 2026. rtbf.be
Broadwhey — Athlete hydration: how much and what to drink during effort. 2025. broadwhey.com
Hydratis — Athlete hydration: what amount of water to stay at the top. November 2025. hydratis.co