What happens to your body when you run in 40C heat

running in a heat chamber

by Paul Halford |

Back in the spring, Paul Halford visited a heat chamber to try to understand the impact of running in extreme temperatures

The sweat is pouring off me. The temperature reaches 40°C and my heart rate is telling me I’m doing a speed session. Yet this isn’t the Sahara. This is England in late March – when UK seems to get its winter nowadays – and I’m on a treadmill at a supposedly easy pace.

My run was in a heat chamber as part of a ‘Marathon des Sables Experience’ put on by Precision Fuel & Hydration (PFH) at Silverstone race circuit. The MdS itself was due to start in a few days’ time. Hundreds of hardy souls were ready to take on the six-day 250km race across the Sahara desert. The previous year was a killer edition in which competitors faced up to 56°C temperature – not made any easier by a stomach bug going around camp.

PFH had initially promised 50°C for my time in the heat chamber. “Sounds horrific. OK then,” I said as I accepted the challenge. Why did I agree? Aside from merely experiencing what it was like to run in extreme temperature, the afternoon would allow me to learn about the effect of heat and sweating on my own running and would perhaps enable me to tweak my training accordingly.

So it was with more than a little trepidation that I went along to the Porsche Human Performance Centre (PHPC) at Silverstone.

After a quick introduction to the centre and their work from PFH’s Abby Coleman, I headed to the lab. What I naively thought would be a 10-minute treadmill run turned out to be two sessions of half an hour each. The first first part of the experiment would be as a control in normal lab temperature – 18°C. PHPC lead sport scientist Jack Wilson recorded my height, weight and body composition data and then I did the half-hour at 12km/h before being weighed again.

As a means of taking a breather, I then underwent a sweat test back upstairs in the office. This was a painless procedure done in minutes without a blood test, using straps applied around my arm to collect samples. Abby and colleague Greg Thew ran me through a standard questionnaire as I sat there sweating away. “Are you a salty sweater?” I was asked, and I had to admit I was not really sure – despite having run for 25 years. “Does your sweat taste salty or do you get white stains on your skin after a hot run?” Greg asked. “I think so,” I responded.

That was confirmed by the small, portable machine analysing the chemicals of my sweat when it showed up the reading – 1617mg of sodium per litre of sweat. That put me at the low end of the very high category – at 70% of the highest PFP have seen and 70% higher than average.

The sodium content of sweat can vary by more than 10 times between athletes. Indeed, the lowest PFP have seen is 204mg/l. As you exercise, you’re losing sodium as well as other electrolytes, to a lesser extent, so just replacing sweat like for like with water to rehydrate may not be enough. Based on that information, I received a hydration plan on email which recommended their PH1500 product before a race and their PH1000 during racing or training.

Low sodium levels can hinder your performance. In everyday life, we will usually have enough, but exercise can deplete it as we are losing it via sweat. Using water alone to rehydrate can make it worse as you dilute your levels. The other thing to bear in mind is how much sweat you lose during exercise, which varies from person to person and, of course, according to the temperature. This can be worked out yourself relatively simply by weighing yourself nude immediately before and after a run (see right). On the initial trial in normal temperature, I lost 0.4l over 30 minutes. However, bear in mind that I wouldn’t have been sweating at all for the first 10 minutes or so, so my sweat rate is likely more like 1l per hour. When I considered I could lose almost 4% of my bodyweight on a two-hour long run – composed entirely of magic ingredients such as water and electrolytes, it made me realise how important it is to think about hydration.

That’s just under the typically comfortable conditions we get in this country. What about in more hostile conditions? I was about to find out! I was genuinely daunted. The very phrase ‘heat chamber’ was perhaps responsible for more of the fear than the temperature itself. “How much danger could I be in?” I asked Jack. The potential results of hyperthermia are very serious, including dizziness, vomiting, seizures or even death.

However, Jack reassured me he would take my temperature every five minutes and pull me out if I went above 39.2°C. The other journalist involved in the study, who was in the chamber just before me, reached 39.2°C at the final check, at 25 minutes.

My general experience in the chamber confirmed I had little to worry about. In fact, I found the heat uncomfortable rather than excruciating. But the stats were more revealing. Between five and 10 minutes in, my temperature soared by 0.9°C – virtually halfway towards the danger zone. After that, it was more gradual but it crept up and up so it was 39°C by 25 minutes. I made it to the end of the 30 minutes but the final reading of 39.2 showed how close it was.

body temperature graph

My heart rate also showed a big leap from five to 10 minutes before a steady progression. By the end it was up to 170bpm. I would expect a peak of around 175bpm when doing a set of speed intervals but here I was doing an easy pace. I produced 0.65l of sweat in the heat test compared to 0.4l in the control, mostly in the final 20 minutes.

I had always considered myself to be a reasonably good performer in the heat. Others had told me this, and I also noticed doing better than expected against my peers in hot races. However, there was nothing here to suggest that was the case.

This was after just half an hour. If I had run for longer, at some point I would have had to ease up, stop or cool myself down somehow. Our bodies’ cooling mechanisms are remarkable and the way humans sweat makes us best suited of all animals to long-distance running. But there is only so much we can take.

UK-based Frenchman Pierre Meslet had been advised by PFH on his way to ninth in that horrific MdS of last year. He joined us by video call at Silverstone to talk about what he surprisingly described as an “awesome” experience – just as I was still recovering from my own half-hour in Saharan temperatures.

He explained: “What I struggled with was the cooling down. You had to take more water than normal so you had to use some of that on the head, the neck and arms because the body temperature was going through the roof quite quickly. When you look at 56°C and 3% humidity, it sounds horrendous but when you’re out there and a little bit used to the heat for 10-14 days, your body sweats a bit differently. Less salty, a bit quicker sweating and just more comfortable in that environment.” His carefully calculated hydration plan was crucial, he believed. “I thought it was essential to my success even more so as it was such a hot year,” he said.

Perhaps you’re not about to run across the Sahara, but most runners could benefit from learning more about their sweat. Making sure your body has enough electrolytes and is adequately hydrated is vital, but even when you’re not racing it can be crucial for recovery. If you pay attention to the details of hydration, performing to your best – even in the heat – could be no sweat!

This article appears in the brand new edition of Trail Running.

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