The Marathon des Sables (MDS) is a multi-stage, ultra-endurance event that takes place across the Sahara Desert, Morocco. Open to runners and walkers, the foot race consists of six stages, with the fourth stage being the longest, and runners are self-sufficient over a distance of ~250km. The event is often named “the Toughest Footrace on Earth”, attributed to its extreme heat and exposure to harsh, uneven, and rocky terrains. Although, the conditions were not as extreme as those experienced in previous years (>50°C), exertional heat illness may still occur due to dehydration, prolonged exercise, compromised heat dissipation and/or a combination of these factors. Therefore, education through heat acclimation is vital prior to an endurance event in extreme conditions, namely the Marathon des Sables. This blog post captures the experiences of the MDS athletes, but also MSc Applied Exercise Physiology students delivering the support.


The heat acclimation (HA) support provided by the University of Brighton’s Environmental Extremes Lab draws on research of two former University of Brighton PhD students, Dr Jess Mee and Dr Ash Wilmott, but is a cumulation of over 20 years researching heat acclimation in the field. The protocol consisted of a total of 6 sessions which were completed in our environmental chamber set at a temperature of 40°C and a relative humidity of 40%: 2 heat tolerance tests (pre- and post-HA sessions), and 4 HA sessions. The purpose of the heat tolerance tests were to determine how the athletes physiologically and perceptually responded to steady state exercise in the heat. As for the heat acclimation sessions, the athletes cycled for the first 30 minutes, aiming to reach a target core temperature (Tcore) of 38.5°C. Following this, the athletes completed a combination of cycling, running, walking, and resting to maintain their Tcore at 38.5°C for a further 60 minutes. In order to ensure the HA sessions replicated the anticipated conditions of the race, the athletes were allowed to consume water throughout and were also given the opportunity to practice running and walking with their race kit, which aided in improving their pacing strategies. Following each session, the athletes were naturally cooled, and aggressive cooling methods were avoided as they are known to reduce the heat adaptations of the sessions, however, were available to us if required. The athletes did not leave the environmental extremes laboratory until the lead experimenters were satisfied with their recovery.

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Key Takeaways

Throughout the HA sessions, the athletes were consistently reminded of the importance of hydration and the classic phrase “your nose knows” was relayed to each of them repeatedly. One of our athletes recounted his experience with dehydration during the 80 km long stage…

“I only had one hydration issue, and it was a biggy! On the long day I suddenly felt incredibly sluggish. I walked for a little while and stopped to urinate – it was a really dark red colour. I toyed with not speaking to medics in case they kicked me off the race (confusion?), but thankfully did – they told me I was dehydrated. I walked three km drinking water and started to run again. And what a run it was!”

Another simple, but key point that the athletes found particularly useful, was to carry out regular head-to-toe checks. This ensured they checked in with themselves and how different parts of their body were coping with the heat! One of our athletes stated…

“I learned so much to keep me safe in the desert and was given tools to self-monitor and was also really pleased with my acclimation results.”

On top of the HA support completed inside the Environmental Chamber, the athletes were provided with at home HA methods that was intended to be used as “top up” sessions prior to their departure.


The classic adaptations we would expect from athletes undergoing our HA support, are reductions in Tcore, heart rate, and perceptual measures (i.e., thermal sensation, thermal comfort, thirst, and rate of perceived exertion). Some would be expected to see changes in sweat rate as well, but as we were using short-term HA, this was not always expected, especially amongst those who already had a well-developed sudomotor (sweat) system. Adaptations to Tcore and heart rate of one MDS athlete are illustrated in the left-hand graph below with pre- and post-HA values indicated from the running heat tolerance test, alongside their peak physiological strain. Here you can see really good evidence of the HA. Increases in sweat rate were also observed, whilst avoiding excessive increases of above 3 L.min-1 as too large of a sweat rate would lead to excessive fluid and electrolyte loss, making it difficult to replenish said stores. Furthermore, it was vital for the athletes to understand their sweat rate prior to the race as it is useful in determining fluid loss during exercise and how much is required to replenish hydration status and electrolyte stores. Previous HA studies suggest that females require additional days/sessions to achieve the same magnitude of physiological adaptations as males, and this was discussed with the female athletes prior to the HA.

The right-hand graph was a plot of an MDS athlete’s rating of perceived exertion and thermal sensation during the heat tolerance test pre and post HA, again showing some clear reductions. The peak heat illness strain index captured heat illness symptoms that the athlete experienced, reinforcing that the HA has helped lessen them.

The race

The 2021 race was plagued with the Norovirus and scorching temperatures (~50°C) which led to a 50% drop-out rate. However, the 36th edition by contrast was the opposite, with much cooler days and average day-time temperatures of mid-20°C, requiring additional layers and the use of down jackets. For those who raced too light, they were unable to recover properly as their nights were spent trying to remain warm. Out of 900+ athletes, 105 withdrew from the race with 60 athletes unable to complete the second stage. However, we are very pleased to report that all our athletes completed the MDS, with all placing within the top 44% and with one athlete stating…

“The week was a huge success. I smashed all my targets, met lots of great people and learnt a hell of a lot about myself during the process”



Based on the athlete’s experiences, this year’s MDS highlighted the importance of hydration during ultra-endurance events and how dehydration can negatively affect endurance performance.  The heat acclimation provided an opportunity for the athletes to improve their pacing strategies and assess how their physiology changes during exercise in the heat. More than that though, our HA support programme is as much about education as physiology adaptation, collectively giving heat awareness to our MDS athletes. As the athletes gained more knowledge on their own physiology, their confidence also grew prior to their departure which meant they could add self-belief to their heat awareness toolbox!

This year’s HA support continues to demonstrate the importance of heat acclimation prior to an extreme ultra event, such as the MDS! One of our athletes recounted…

“I am very confident that I benefited from all the acclimation work we did though – some blogs refer to the high temperatures in the dunes; I never felt this, and managed to run throughout, only walking on hills.”

With another…

“I’ve gone from couch to MDS in 2 years and think that the heat acclimation process has been the final icing on the cake.”


Thank you to all the leaders and students who delivered a successful MDS support program and provided a professional service.

Thanks to MDS Leaders: Kayane Castro Alencar and Karolayne Castro Alencar

Thanks to our MSc Applied Exercise Physiology and BSc Sport and Exercise Science Students: Annabelle Bryan, Anya Gough, Harry Mounter, Evie Winterton, Niki Sapalidou, Billy Norton, Yang Yu, Kate Buntine

Thanks to all the athletes: It was a pleasure working with each of you and watching you progress through your MDS journey! Thank you for sharing your photos and key memories with us.

Final Word from Environmental Extremes Lab Lead – Dr Neil Maxwell

I just wanted to congratulate MSc Applied Exercise Physiology students, Kayane and Karolayne on delivering another successful MDS heat acclimation support programme here at the University of Brighton. They took on these lead roles as part of their SE709 – Personal and Professional Development  (placement) module and were exemplary in their preparation and delivery, with me receiving great feedback from the MDS athletes. This opportunity reinforces the value of practitioner-focused learning that is an integral element to how we develop our students to be effective practitioners while on the course.