In this article, elite runner Jacob Simonsen shares his experiences with heat training and preparation for hot runs. Jacob is, among other things, the Danish record holder in marathon and half marathon.
You practise the distance. You test energy gels. You plan your hydration down to the smallest detail. But one thing most overlook: the heat. Many just hope the temperature on race day will be pleasant. If it isn't, they take it as it comes. But heat is not just something to endure. It's something you can prepare for.
Is it really necessary? The short answer is yes to heat adaptation, but no to advanced heat blocks for most.
Why you run slower in the heat
Running in heat feels harder because the body has to both propel the muscles forward and use energy to cool itself down. More blood is sent to the skin, sweat production increases, and the heart has to work harder to maintain the same pace. The result is often a higher heart rate and a greater perceived effort at the same speed.
That's why it's completely normal to run slower on a hot day, even if your fitness is the same.
How I prepared for a hot World Championships in Tokyo
Leading up to the World Championships marathon in Tokyo 2025, I knew the heat could be a decisive factor. Therefore, I chose to travel to Japan 14 days before the championship. The goal was not to train harder, but to let the body get used to the conditions. I used air conditioning sparingly and consciously placed some of my training sessions at the warmer times of the day. They weren't the hardest training weeks, but they were targeted.
When the marathon started, I felt comfortable in the conditions. In the second half of the race, I saw several runners clearly struggling with the heat and falling significantly behind. Whether it was solely due to my heat adaptation is impossible to say. But I am convinced it was a significant part of the reason I could maintain a high level all the way to the finish - and even gain many positions in the second half of the race, moving from top 60 to top 30 and finishing 29th.
This illustrates the point well: Heat is a performance factor on a par with hills, wind, and terrain. If you expect to encounter it on race day, it makes sense to prepare for it.
Should all runners do heat training?
The short answer is: no. When talking about heat training, it's important to distinguish between two different things:
- The first is heat adaptation. Here, you typically spend 7–14 days gradually getting the body used to training in hot conditions.
- The second is actual heat blocks, where over several weeks you conduct 5-6 weekly heat sessions to create physiological adaptations that in some cases resemble those sought in altitude training.
It's the first strategy that I believe makes sense for most. Heat adaptation doesn't require special equipment or a complete overhaul of your training. It's about exposing the body to heat in the days leading up to your run. The second strategy with dedicated heat blocks is a tool primarily relevant for elite athletes or very serious amateurs who already have all the basics covered.
Training is a pyramid
I usually see training and performance-enhancing measures as a pyramid:
- At the bottom, you find the things that matter most: continuous training, enough sleep, and a good, varied diet.
- On top of that come things like recovery, strength training, treatment, and more specific training forms.
- Only at the very top do you find tools like altitude training and advanced heat blocks.
The problem is, many want to jump straight to the top.
If you're not already training consistently week after week, getting enough sleep, and have the basics covered, you'll likely benefit more from focusing on those than sitting in a sauna and suffering for 50 minutes five times a week. To increase something, you often have to decrease something else. Heat training isn't free. It's an extra stress on the body, and it needs to fit into the overall training load. Even when I use heat blocks as a professional athlete, it's something that requires planning and adjustment of the rest of the training.
How to heat adapt before an important run
As an amateur, it can make sense to do a simple heat adaptation before an important, hot run. If you have a marathon, half marathon, or fun run in August or September, it can be beneficial to spend the last week or two gradually getting the body used to the heat. It doesn't have to be complicated.
You can schedule some of your training sessions at the hottest time of the day, wear a bit more clothing during selected workouts, or supplement with a sauna after training. The body responds surprisingly quickly, and after just a few days, you start to see adaptations. Therefore, summer's hot training sessions can be an advantage instead of something you actively try to avoid.
Remember hydration in the heat
When you train in the heat, hydration becomes even more important. One of the adaptations seen with heat training is an increase in the body's plasma volume. This means the body becomes better at handling the load and transporting heat away.
To support this process, it's important to show up to training well-hydrated and replenish your fluid stores afterwards. At the same time, sweat loss increases significantly in hot conditions, and during longer training sessions, it may also be relevant to consider electrolytes.
My advice: Keep it simple
Heat is a factor that can significantly affect your performance. Therefore, it makes sense to prepare for it. Especially if you expect hot conditions on race day. But for most, it's not about advanced heat blocks or complicated protocols. It's about giving the body a chance to get used to the heat.
In short: Yes to heat adaptation. No to making it more complicated than it needs to be.
Ready for your next summer run?
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