Sport in the heat: Doctor and MTB trainer Stefanie Mollnhauer gives advice
Laurin Lehner
· 02.07.2025
The fact is - exercise in high temperatures really gets your circulation sweating and harbours risks. But giving up sport altogether? Not a solution either. How you can train safely, effectively and even improve your performance in extreme heat - and how you can recognise when it's time to stop: Find out in this article.
Interview with doctor and coach Steffi Mollnhauer
Biking in the heat - what happens in the worst case?
The body tries to keep the temperature constant between around 36 and 37.5 degrees. This is no longer possible during prolonged exercise at high temperatures, even though the body does a lot to achieve this. For example, a lot of energy goes into cooling alone. The main cooling mechanism is sweating. Incidentally, this evaporative cooling requires energy. This means that one litre of sweat that evaporates from the skin removes around 580 kilocalories from the body. The result: less power on the pedals.
Of course, this process is very individual for athletes - depending on how much you sweat. In addition to temperature, humidity also plays an important role.
What are the classic symptoms of overheating?
Headaches, nausea, muscle cramps or a significant drop in performance. Then it's time to stop immediately, seek shade and drink plenty of water - ideally with electrolytes. Cooling the forehead and neck is particularly helpful.
Dehydration is usually the cause of loss of consciousness. In this case, the skin is not sweaty, but dry and warm.
There is a myth about the midday heat. However, it is often hottest between 4 and 5.30 pm. What puts more strain on the body - the direct sunlight at midday or the higher air temperature in the afternoon?
Direct sunlight generally increases heat stress significantly - especially unpleasant when the blazing sun beats directly on the helmet.
We keep hearing about water intoxication, i.e. athletes who collapse from drinking too much water. What happens there?
I don't have any concrete figures, but it feels like every one of these incidents makes the press. I personally have never experienced this. The probability is low - we are talking about several litres of water in a very short time, without any minerals.
In the case of water intoxication, the sodium level in the blood drops dangerously. The cells swell up - which can be particularly critical for the brain.
The likelihood of you getting water intoxication is minimal compared to the risk of dehydration.
So, if I drink a litre of water at a time during exercise, I don't have to worry?
No, but it is safer if you drink several small amounts during exercise. This way, the fluid is distributed more evenly through the gastrointestinal tract and can be absorbed continuously. You should always drink more in summer. Also important: we lose up to one litre of fluid during the night while we sleep. If you start your training in the morning with just a coffee, you are already putting a strain on your body with a deficit. It is therefore best to drink a large glass of water straight after getting up - it is easier to drink lukewarm.
Jumping into a cold mountain lake is particularly appealing when overheating on a bike. Many people have reservations about this - and rightly so?
Yes, definitely. It can come as a shock. You have to imagine it like this: The body is in high power mode to bring the temperature down - like air conditioning in a car with the windows open.
The evaporation of sweat from the skin is one thing. When you are very overheated, the blood vessels are wide open to release heat. Cold water then leads to a sudden contraction of the blood vessels - they constrict at lightning speed. Blood pressure rises abruptly and the heart has to pump against more resistance. This can lead to cardiac arrhythmia or circulatory collapse.
Something similar happens when you go to the sauna. Extreme heat, ice bath and extreme heat again.
When you go to the sauna, something similar happens: extreme heat, ice bath and then extreme heat again.
True - but the cardiovascular system is stressed differently. During sport, the cardiovascular system works at full speed: the pulse is increased, blood pressure fluctuates and blood is redistributed to the muscles. The muscles produce lactic acid, the stress hormones are increased and the "sympathetic nervous system" is activated. The body is programmed for "performance" - it has developed a working hyperthermia. In short: the warming of the body's core temperature comes "from within". After the sauna, the pulse is also slightly elevated, but this is passive heating "from the outside", usually with a lower core body temperature (unless you completely overdo it with the duration of the sauna session!) The body is in a state of relaxation, not in fight-or-flight mode - i.e. without any particular sympathetic activation. The heart is stressed, but in a controlled way, without additional metabolic demands.
You watch a lot of amateur athletes. What mistakes do many make when exercising in the heat?
Many people wear too much or too tight clothing - often in black. This is unfavourable because black fabrics absorb the sun's rays and thus generate even more heat. Although many people avoid the blazing midday sun, they then ride on the southern slope on heated mountain pass roads. Not a good idea.
Do you have any other tips?
Yes: Take small, savoury snacks with you - like nuts or pretzel sticks. After all, you lose salt through sweating. Half fill your water bottle with water and put it in the freezer a few hours beforehand. Top up with fresh water just before training - this way you have a cool drink straight away.
Or you can wrap the bottle in a damp merino sock - here you can feel the evaporative cooling. Some athletes rub cooling menthol gel on their neck and forearms - this provides short-term relief.
All about training in the heat
🕶️ Before training
- Train early or late (in the morning or evening - not in the midday heat)
- Plan routeshady routes, possibly with drinking stations
- Think about clothinglight-coloured, breathable functional clothing; helmet with good ventilation
- Sun cream Apply with a high SPF (also neck, ears, hands!)
- Supply electrolytes in advance (e.g. in water or food)
🚴 During the training
- Drink enough500-1000 ml per hour (depending on intensity and temperature)
- Supplying electrolytese.g. via isotonic drinks, salt tablets or gels
- Watch your body for overheatingSymptoms such as dizziness, goose bumps, nausea = warning signs!
- Take breakscool down in the shade, especially on long journeys
- Adjust performancelower intensity, keep an eye on your pulse
🌡️ After training
- Rapid fluid and electrolyte intake (e.g. water with salt/lemon or recovery drink)
- Coolinge.g. with a damp towel, lukewarm shower
- Replenish your diet: Carbohydrates and proteins for regeneration
- Take body signals seriouslyIf there are signs of heat stroke/overheating, seek medical help if necessary.
Laurin Lehner
Editor
