35°C on the gauge, not a breath of wind, the tarmac shimmering in the heat. You’re barely halfway through your long ride and already something feels off: your head is spinning, your legs are cottony, your heart rate is climbing for no reason. I see this scenario come back every summer with the cyclists I work with. The cause is always the same: poorly planned hydration.
Summer heat is a fantastic playground, but it’s also the number-one trap when it comes to dehydration. When the thermometer exceeds 30°C, your body enters intensive cooling mode, consuming enormous amounts of water and minerals. If you don’t compensate, it’s your performance that drops — and sometimes your health that suffers.
Here, I’ll explain what heat does to your body, why water alone isn’t enough, and how to structure your hydration before, during, and after a ride in extreme heat.
What heat does to your body when you ride
When you ride in the sun, your body has to manage two simultaneous challenges: producing the energy needed for pedalling and dissipating the heat generated by your muscles. The main cooling mechanism is sweating — and that’s where things get complicated.
In hot weather, your fluid losses can reach 1 to 2 litres per hour, depending on effort intensity, humidity, and your personal sweat profile. In practical terms, on a three-hour ride in blazing sun, you can lose between 3 and 6 litres of water. That’s massive.
The problem is that dehydration doesn’t warn you. Research shows that from a loss of just 2% of your body weight, your physical abilities already begin to decline significantly. For a 70 kg cyclist, that’s barely 1.4 litres — less than an hour of intense effort in the heat if you don’t drink. Beyond 3% loss, your aerobic capacity plummets, with a drop that can reach 30% in both cognitive and muscular performance.
But you don’t just lose water. Your sweat also contains electrolytes, and primarily sodium. This mineral plays a central role in fluid retention, muscle contraction, and nerve transmission. When your sodium reserves drop, your body no longer retains water properly, which accelerates dehydration and increases the risk of cramps. It’s a vicious circle that heat amplifies.
Another overlooked point: heat disproportionately increases your heart rate. Your heart works harder to send blood to the skin. This is cardiac drift, and it worsens with dehydration.
Plain water or electrolyte drink? The right choice in extreme heat
This is the question I get most often in summer: “Is water enough?” The short answer: no, not when it’s really hot and you’re riding for a long time.
Plain water hydrates, obviously. But it doesn’t replace the electrolytes lost in sweat. And here’s where it becomes counter-intuitive: drinking a lot of water without sodium intake can actually make things worse. By diluting the sodium concentration in your blood without replenishing it, you risk hyponatraemia — a dangerous drop in blood sodium levels. This phenomenon, well documented in the scientific literature, is particularly common among endurance athletes who drink large quantities of plain water during prolonged efforts in extreme heat.
Drinking a lot of water without sodium intake can actually make things worse. The risk of hyponatraemia increases linearly with ambient temperature. Aggravating factors include excessive consumption of hypotonic drinks, effort duration exceeding two hours, and heavy sweating. That’s exactly the profile of a long summer ride.
An isotonic drink — a beverage whose salt and sugar concentration matches that of your blood — rehydrates while maintaining electrolyte balance. The sodium it contains promotes intestinal water absorption and helps your body retain it. It’s the difference between filling a leaky bucket and a watertight one.
For rides under one hour in moderate heat, water may suffice. Beyond that, a drink containing electrolytes — sodium, potassium, magnesium — becomes far more relevant. This isn’t marketing — it’s physiology.
Sports dietitian Nicolas Aubineau points out that a complete sports drink should combine the main electrolytes in appropriate amounts, particularly in extreme heat and during long-duration efforts. The goal isn’t just to hydrate, but to maintain all the mineral balances that support performance.
How to hydrate before, during, and after your ride in extreme heat
Before the ride: anticipate, don’t catch up
Hydrating for a hot-weather ride doesn’t start when you get on the bike. It starts well before — ideally at least 90 minutes before departure. The goal is to arrive at the start in an optimal hydration state, not in “catch-up” mode.
Drink regularly in the hours before, in small sips, preferring lightly mineralised water. Avoid gulping half a litre just before you leave: your body won’t have time to absorb it, and you risk gastric discomfort. A good indicator: if your urine is pale yellow, you’re well hydrated.
During the ride: consistency and common sense
On the bike, don’t wait until you’re thirsty — thirst is already a sign of dehydration. A few sips every 10 to 15 minutes is far more effective than a big gulp every half hour. Here are the key principles:
- Alternate your bottles: one bottle of plain water to cool yourself (pour on your head, neck) and one bottle of electrolyte drink to consume
- Ideal temperature: aim for a drink between 8 and 13°C. Too cold slows gastric emptying; too warm is less pleasant and less well absorbed. Consider insulated bottles or ice cubes at the start
- Break up your intake: small regular amounts rather than large spaced-out gulps
- Adapt to the terrain: on climbs, sweating spikes. Take advantage of descents or flat sections to drink more easily
- Monitor your weight: the dual weigh-in technique (before and after the ride) is the best tool to calibrate your real needs. Each kilo lost represents roughly one litre of fluid deficit
After the ride: replenish the stocks
After a ride in extreme heat, you need to replace the water and electrolytes lost. Mineral-rich water or a recovery drink containing sodium is ideal within the two hours that follow. Useful benchmark: drink roughly 1.5 times the volume of weight lost during the ride, always in small, regular amounts.
Warning signs you must know
Knowing how to hydrate also means knowing when things are going wrong. Here are the signals to watch for:
Early signals (act immediately):
- Marked thirst: you’re already behind — drink right away
- Dry, pasty mouth: classic sign of fluid deficit
- Unexplained power drop: when watts fall for no reason on flat terrain, think hydration
- Abnormally high heart rate: cardiac drift worsens with dehydration
- Dark urine or no urge to urinate: your body is holding on to everything it can
Emergency signals (stop riding):
- Dizziness or visual disturbances: don’t take any risks — get to shade immediately
- Widespread cramping: likely sign of sodium depletion
- Confusion or disorientation: symptom of severe hyponatraemia or heatstroke — this is a medical emergency
- Sweating stops despite extreme heat: sign of heatstroke — call emergency services
The best strategy remains prevention. If you start well hydrated, drink regularly with the right electrolytes, and adapt your effort to the heat, you’ll never reach these critical thresholds.
Personalise your hydration with Ograal
You’ve understood: hydration principles are universal, but the precise quantities depend on you — your weight, your sweat profile, your ride intensity, and the weather conditions of the day. That’s exactly why I designed Ograal.
The app calculates a personalised hydration plan tailored to each ride: how much to drink, when, with what composition. No more guessing or following generic recommendations that don’t account for your real-world conditions. Discover your hydration plan on Ograal.
Because riding in extreme heat is possible — and even very enjoyable — as long as you don’t let dehydration catch up with you. Discover your hydration plan on Ograal and head out this summer with peace of mind.
Ingrid Gallerini — Sports Dietitian, founder of Ograal
