Running in the heat isn’t just about sweating more. Your body works harder to cool down, your pace may drop at equal effort, and your hydration needs change.
In trail running, heat is even more demanding: exposed paths, long climbs, backpack on your back, difficult water access, reflection… If you want to keep running in the heat without pushing too hard, you need to adapt your session, your gear, and your hydration method.
Running in the heat: what really changes
Heat increases fatigue and the risk of dehydration
When it’s hot, your body has to manage two efforts at once: moving forward and dissipating the heat produced by the effort. As a result, your heart rate can rise faster, your sensations become harder to interpret, and your pace may drop even if your effort remains high.
That’s why you shouldn’t compare a session done at 30°C with the same session run in cool weather. At equal effort, you run slower and your recovery time increases.
Heat, intense heat, heatwave: don’t confuse them all
Not all situations are equal. Running in hot weather can remain reasonable if you adjust the timing, pace, and hydration. Running in intense heat requires more caution: shorter session, reduced intensity, familiar route, and accessible water points.
During a heatwave, the logic changes. The priority is no longer to “complete the session,” but to decide if it still makes sense. Postponing, shortening, or replacing it with a gentler activity may be the best training decision.
You can run, but adjust your pace and monitor your sensations.
Favor a short outing, early in the morning or late in the evening, without high intensity.
The right decision may be to postpone, walk, do light cycling, or simply recover.
Exercise hyperthermia: the signal never to take lightly
Exercise hyperthermia corresponds to an excessive rise in body temperature during activity. In simple terms: your body can no longer dissipate enough heat produced by the effort.
Certain signs should make you slow down immediately or even stop: chills despite the heat, headaches, nausea, dizziness, confusion, feeling not clear-headed, very hot skin, or unusual fatigue.
In that case, don’t try to “finish the session.” Get into the shade, slow down or stop, drink gradually, cool down, and ask for help if symptoms persist.
How to adapt your training in the heat?
Choose the right time
The best time is often early morning, when the air and ground are still relatively cool. Evening can also work, but beware: after a hot day, paths, roads, and rocky areas can still radiate a lot of heat.
Avoid the most exposed times as much as possible, especially midday or early afternoon. In trail running, the forecast temperature isn’t enough: exposure, wind, shade, altitude, and terrain type greatly affect how it feels.
Reduce intensity
In the heat, save intense sessions for the coolest times. If you planned intervals, a tough hill session, or a long paced run, adjust. You can turn the session into easy endurance, reduce volume, or lengthen recoveries.
Also accept walking more uphill. It’s not a failure: it’s often the smartest way to control effort and avoid overheating.
Easy jogging, endurance, short outings, familiar routes, low to moderate intensity.
Hard intervals, long exposed runs, midday starts, strict pace goals.
Favor shaded routes or those with water points
When it’s hot, route choice becomes strategic. A loop in the forest, a route with fountains, rivers, or shelters, or a short outing with the option to return quickly will be better than a long route exposed to full sun.
If you’re heading to the mountains, watch out for ridges, scree slopes, and areas without shade. Even with wind, you can dehydrate quickly, especially with a pack and high intensity uphill.
Hydration and nutrition: avoid running dry
Drink before you’re thirsty
By the time you feel thirsty, you’re often already behind. In the heat, it’s better to drink regularly in small sips rather than wait until you’re very thirsty and drink a lot at once.
The goal is to maintain stable hydration. For a short run, water may suffice. For a long, hot, or sweaty run, you need to think more broadly: water, minerals, energy, and the ability to drink easily without stopping.
Why water alone isn’t always enough
When you sweat, you don’t just lose water. You also lose minerals, especially sodium. If you only drink water over a long period, especially in strong heat, it can be hard to maintain a good balance.
This is where electrolytes become useful. They support hydration, especially when you sweat a lot, the run is long, or aid stations are far apart.
Electrolytes, energy drinks, flasks: what to prepare?
The easiest approach is to adapt your hydration to the duration of your outing. For a short run, one water flask may be enough. For a longer or hotter run, you can carry one flask of water and another with electrolytes or an energy drink.
The key is to keep drinking easy. If your flasks are accessible, you’ll drink more regularly. If you have to stop or take off your pack each time, you might delay drinking.
A simple solution to include in your hydration strategy when it’s hot. Electrolytes are especially useful on long runs, efforts in high heat, or sessions where you sweat a lot.
Test them during training before using them in a race, especially if you’re preparing for a long trail or a summer ultra.
See electrolytes
What gear should you choose to run in the heat?
Breathable, lightweight, and quick-drying clothing
In the heat, your choice of fabric really changes how you feel. Avoid materials that retain moisture and favor breathable, lightweight clothing that dries quickly.
The most technical Raidlight textiles weigh as little as 65 g/m², with some shirts under 50 g. The goal: to keep a top that is very light, highly breathable, yet durable enough for trail running.
The right clothing must wick sweat, limit the feeling of wet fabric, and remain comfortable even with a hydration pack.
An ultra-light, breathable shirt made in France, designed for hot or intense efforts. It’s the type of top to favor if you want to run with maximum ventilation without compromising on technical features.
To choose for summer outings, fast sessions, or trails where heat management becomes a priority.
See the Ultra Drylight shirt
Cap, sunglasses, and sun protection
When you run under the sun, protection is not limited to the t-shirt. The head, eyes, neck, and exposed skin must also be protected.
A cap protects the face, limits direct exposure, and helps better endure long sections in the sun. A Saharan version provides extra neck protection, very useful in the mountains, on exposed terrain, or during extreme heat.
Useful protection for running long in the sun, especially when the route offers little shade. Neck protection quickly becomes important on hot outings or at altitude.
Sunglasses suited for exposed outings, to protect your eyes and maintain good terrain reading when brightness is strong.
In summary: the right reflexes for running in the heat
Running in the heat requires changing your benchmarks. You need to accept slowing down, choose the right time, drink regularly, adapt your route, and watch for warning signs.
In extreme heat, the priority is not to stick to your plan exactly, but to stay able to train over the long term. A shortened, rescheduled, or canceled session can be a very good decision if conditions become too demanding.
With proper hydration, breathable clothing, real sun protection, and a bit of common sense, you can keep running in the summer without suffering from the heat.
