The interest in heat acclimation for athletes has made a surge in recent years as it has proven to have many benefits, even beyond just preparing for racing in a hot environment. Following the successful series of research-informed Marathon des Sables (MdS) inspired articles over the past 2 years, this is the first of two more articles related to heat preparation strategies for the MdS Legendary and other ultra-endurance events taking place in 2025, such as the Badwater 135.
Previous articles can be found here:
- How Runners can Acclimate to Heat – https://ultra.coach/2018/03/16/how-runners-can-acclimate-to-heat/
- Running in the Heat – Thermoregulation, Heat Stress & Heat Acclimation – https://ultra.coach/2022/02/16/running-in-the-heat-thermoregulation-heat-stress-heat-acclimation/
- Heat Acclimation for Women – https://ultra.coach/2022/02/23/heat-acclimation-for-women/
- Heat Stress & Gut Function: Nutritional Considerations for Running in the Heat – https://ultra.coach/2022/03/10/heat-stress-gut-function-nutritional-considerations-for-running-in-the-heat/
The Marathon des Sables is renowned for being one of the most gruelling endurance events, typically held annually in the Sahara Desert. Since the 2024 edition, some changes have been implemented due to new ownership, but the underpinning concept remains the same, and heat training forms a crucial part of race preparations.
As with previous articles, I thank Drs Ash Willmott and Freya Bayne, PhD student James Barber and Professor Justin Roberts for their contributions and taking the time to provide evidence-informed recommendations.
How to Recover Effectively Between Heat Acclimation Sessions?
Effective recovery between heat acclimation (HA) sessions is crucial to maximize the benefits and minimize the risk of heat-related illnesses and/or overtraining type situations – particularly as though consecutive days of heat training can evoke successful adaptations, training at high exercise intensities and increased volume/load can potentially result in a state of over-reaching. Managing recovery during heat training is therefore critically important, especially as accumulated fatigue over the course of short and long-term HA protocols (ranging up to ~10-15 days) may impact the success of the programme and/or interfere with the quality of training/tapering. Here are some strategies to facilitate recovery if you decide (which we recommend) to undertake heat training, also known as heat acclimation (more found here in our previous article – https://ultra.coach/2022/02/16/running-in-the-heat-thermoregulation-heat-stress-heat-acclimation/ in the lead up to the Marathon des Sables:
1. Understand how the additional heat stress affects your body and what to do about it
Increased Cardiovascular Strain
When exposed to heat stress and exercising at similar (or potentially even lower) running intensities, your heart rate increases and works harder to help pump the blood to the skin surface for effective cooling (via sweating and subsequent evaporation) compared to exercise in temperate conditions. This increased workload on the heart can be stressful, especially in the early stages of heat acclimation – so we recommend you take it easy initially (e.g. training in zone 1 to 2), familiarise yourself with the additional heat whilst exercising and understand your threshold/training prescription methods may need to be altered to account for the higher heart rate.
Dehydration and Electrolyte Imbalance
One of the benefits of heat acclimation is an earlier onset of sweating to cool yourself down more effectively – a positive adaptation, however, this means that your overall sweat rate will increase over the course of heat acclimation. If not properly managed and replaced, this can result in acute and even long-term dehydration – a negative consequence that will affect health and training quality. Along with fluid loss, sweating depletes electrolytes (for example sodium, potassium, magnesium), which are essential to maintain effective muscle function and nerve signalling. An imbalance of these electrolytes can potentially lead to muscle cramps, fatigue, and more severe issues including heat cramps, heat exhaustion or heat stroke. Therefore, it’s essential you develop an effective hydration strategy (as outlined below) and importantly begin exercise reasonably hydrated – as demonstrated by a pale/light yellow, straw like urine colour (via the urine colour chart) or 1.011-1.020 arbitrary units (a.u.) on the refractometer for urine specific gravity, or 350-700 mOsm/kg on the osmometer for urine osmolarity (see our second article in this series for more details on this).
Thermoregulatory Strain
Exposure to heat stress raises your core body temperature, which triggers autonomic cooling mechanisms (i.e. vasodilation and activation of sweat glands). Initially, your body may struggle to maintain a stable core temperature, leading to thermal discomfort, higher heart rates and potential heat-related illnesses if not managed appropriately. Over time, your body adapts by lowering resting and exercising core temperature as well as improving sweat efficiency and circulation, but until these adaptations occur (which may take ~5-10 days or more), the body is under considerable physiological strain – thus, it is important to monitor core temperature during/following heat training, acknowledge hyperthermia thresholds ~38.5°C (and unsafe levels of >39.0°C which are associated with heat related illnesses) and to be supervised by a trained practitioner.
Metabolic, Mental and Psychological Stress
Heat exposure can increase metabolic rate, leading to higher energy expenditure. This puts additional stress on the body’s energy systems, requiring more efficient nutrient use and increased caloric intake to maintain energy balance. In addition to this, heat stress can increase the production of free radicals, leading to oxidative stress (an imbalance of free radicals and antioxidants in your body that could lead to cell damage). Furthermore, the sensation of overheating can cause significant discomfort and mental fatigue. Psychological stress can result from pushing through the discomfort of heat, especially during prolonged or intense sessions (~60-90 mins). Your cognitive function may also be impaired whilst running in hot conditions, because there are not sufficient resources to be allocated to both the exercising (e.g. running) and mental tasks (e.g. decision making on pacing, fuelling etc), making those tasks that require concentration more difficult and mentally draining. Adaptation to hot conditions through heat acclimation will improve your psychological tolerance to heat, lower the sensation of fatigue and improve thermal comfort as well as your cognitive function – allowing you to make informed decisions and complete mental tasks more easily compared to pre-heat acclimation. However, until these adaptations have been achieved (which may take ~5-10 days or more) we recommend avoiding any difficult or mentally demanding tasks immediately around your heat acclimation sessions (e.g. long drives, night shifts, challenging work commitments, etc).
2. Factors to consider when planning in your heat acclimation sessions
Time of Day
In order to track heat adaptations effectively and avoid the influence from daily changes in circadian rhythm, it is recommended that heat acclimation sessions be completed at the same time of day (<1-2 hours difference). This also allows you to get into a HA routine. If you are considering completing your heat acclimation sessions in the morning, you may be starting your sessions at a lower core temperature and heart rate, which is good for avoiding heat related illnesses, however, you will potentially have a higher cortisol level and be in a dehydrated state due to recently waking up in comparison to evening sessions. Depending on which protocol you follow, you may also need to exercise for longer and/or at a higher intensity to meet the desired targets of the session, for example, controlled hyperthermia where practitioners target an internal core body temperature of 38.5°C.
Periodisation, Tapering and Mixed-Methods
Whilst it is recommended that heat acclimation is undertaken in the months prior to the event to fully adapt, followed by subsequent “top up” sessions, due to time, accessibility and logistical constraints, it is often completed close to the date that you are flying out to compete, which may avoid decay of adaptions but place a higher workload and stress during tapering routines. Therefore, it is recommended that you complete your heat acclimation followed by a taper week that factors in heat exposure before you fly or, fly to the location of the competition and complete a taper week in the same conditions that you are likely competing in to gain further natural acclimatization. You may also wish to utilise passive acclimation strategies independently or combined with active strategies to reduce exercise load demands and make use of sauna exposures, hot water bathing and/or hot yoga practices.
Nutritional considerations during exercise heat stress
To reduce the negative effects of exercising in the heat, for example gastrointestinal issues, dehydration and syncope (or headaches/dizziness), consuming carbohydrates (CHO) in the form of fluid-based beverages, prior to (~15 g, 4-6% solution) and during heat acclimation (~30-45 grams per hour, 4-6% solution) have been shown to ameliorate endotoxemia and/or systemic inflammatory responses, which may also potentially contribute to improved recovery. Likewise, sodium intake of ~500-700 mg per litre of water is recommended during heat acclimation sessions lasting longer than ~60 mins. This strategy could be personalised based on personal sweat rates and sodium loss. However, its important to understand you own preference, dietary needs, and intolerances.
3. Physical recovery to complete outside of heat acclimation sessions
To minimise the cumulative effects of the additional physiological and thermal strain following daily or non-consecutive heat acclimation sessions, it’s imperative you focus on recovery to prepare for the subsequent session and maintain health status.
Cooling
Outside of heat acclimation, resting in cooler conditions and avoiding additional heat exposure (especially prolonged sun light/UV exposure) is advised. Simple and effective cooling techniques, such as facial mist sprays, neck cooling collars, fans, and ice slurries (which could include nutritional needs) may also minimise discomfort, lethargy and/or unwillingness to continue after heat training, whilst also promoting rehydration, enjoyment and some “heat relief” following the hard sessions. Likewise, whilst it may appear counter-intuitive, if you undertake heat training late at night, sleep quality may be negatively affected, therefore you may wish to consider these post-exercise cooling strategies as part of your recovery, which has been shown to reduce the negative affect on sleep quality
Rehydration and Electrolytes
Ensure that you drink sufficient water before (e.g. approx. 500 mL around an hour before training or as applicable), during (e.g. 200-500 mL per hour pending individual requirements), and after each session (e.g. 150% of sweat loss) to replace fluids lost through your sweat. Where sessions are longer, consideration to individual gut tolerance may be needed. This typically isn’t an issue for lower volumes, but can become problematic at higher volumes for some individuals. The practitioner completing your heat acclimation service should be able to inform you of your sweat rate after each session so that you know how much you need to replace throughout the rest of the day/before your next session – typically 100-150% of your overall sweat loss during the acute session. However, it is advised to avoid a large intake of fluid over a short time period (<30 min), with intake spread over 90-120 min following the heat acclimation session.
In addition to this, you should be consuming drinks containing electrolytes to replenish those lost during sweating, particularly sodium, potassium, and magnesium. However, with such a range of individual variation in sweat loss (0.2 to > 3.5 litres per hour) and sweat sodium concentrations (0.1 to >1.0 g per litre of sweat), it may be advisable to complete a specific sweat test, especially “salty sweaters”, to determine your individual sweat sodium loss per litre of sweat – this information will guide and inform you on how much sodium you will likely need to replace throughout the rest of the day/before your next session.
Balanced Diet
Ensure that you maintain a balanced diet rich in high quality proteins, carbohydrates (particularly from wholefoods), and healthy fats (such as oily fish, nuts and seeds) to support muscle repair, fluid retention and energy replenishment. This should include foods high in antioxidants (e.g., fruits and vegetables) to help reduce oxidative stress that may be caused by the additional heat exposure. It’s advisable that CHO following exercise in heat stress is similar to exercise within normothermic environments for recovery methods, such as 1 gram per kg of body mass per hour (during the acute phase of recovery i.e. for around 4 hours post exercise NOT just immediately after), or 0.8 gram per kg of body mass per hour when co-ingested with protein.
You may also wish to chose foods high in water content that are also more palatable aside of drinking lots of water, these may include watermelon, cucumber, tomatoes, applies, celery, and peaches.
Sleep
Due to the additional stress of heat acclimation on the body, you may experience disturbances in your sleep quantity and quality. Therefore, you may wish to improve sleep hygiene practices and aim for more sleep than usual and if needed cool down after evening HA sessions.
For example:
- aim for >7-9 hours of quality sleep each night to promote effective recovery,
- implement a “sleep routine”,
- adopt regular sleep-wake hours,
- ensure you have a comfortable sleeping environment in cool, dark and quiet settings,
- avoid caffeine at least 4-6 hours before bed, and,
- put down electronic devices (mobile phones, tablets and laptops) and refrain from blue light ~1 hour before bedtime.
You can also support this with short naps (~20-30 minutes) during the day if you’re feeling particularly fatigued after your sessions. Most smart watches and wearable devices, such as WHOOP Straps, Ōura Rings, Muse S, Fitbit, Apple Watch, Garmin Watches, etc, can track and monitor useful information to give insight into your sleep quality.
Rest Days
The nature and timing of heat acclimation often means that you will be repeating consecutive days of heat training sessions to maximise adaptation and avoid decay within the time prior to departure. Therefore, cumulative fatigue may occur and needs to be considered in your training schedule. For example, you may wish to incorporate rest days and complete non-consecutive heat acclimation, lighter training days to prevent over-training/reaching or chose passive protocols such as hot water immersion and sauna bathing to allow for extended recovery and maintain training quality.
Health and Safety
To minimise any ill-effect of the additional heat stress and training, it’s advised the athlete, or more routinely the practitioner leading the heat acclimation, to monitor and ensure core temperature and heart rate returns to near-starting resting values that fit within the normal ranges. An elevated heart rate for longer periods of time following heat acclimation, a higher than normal resting heart rate and/or increased heart rate variability can indicate insufficient recovery and/or overtraining. It’s therefore advised this simple measure is routinely tracked through heat acclimation with session difficulty/intensity and rest days adjusted where necessary. The athlete should be familiar with the symptoms of heat related illnesses so that they can self-monitor (with support of the practitioner) for symptoms throughout each heat acclimation session. Symptoms should be checked on a regular basis throughout heat acclimation sessions to ensure prevention of heat related illnesses (e.g. every ~5-10 mins). We also recommend ensuring expert advice is sought prior to heat acclimation, especially trialling “home based” strategies such as sauna bathing or hot water immersion. Active recovery is also recommended in the hour(s) preceding heat acclimation, where athletes may wish to engage in light activities such as walking and/or stretching to facilitate blood flow and reduce muscle stiffness.
4. Sensations of Fatigue and Mental Recovery
Exercising in heat stress, such as that during heat acclimation, has been shown to increase the sensations of fatigue, including symptoms such as general, physical, emotional and mental fatigue. Whilst in the long-term these symptoms can improve as you adapt to the heat stress, mental recovery is advised outside of heat acclimation sessions to switch off from the added stress of training and race preparation.
Relaxation and Mindfulness Techniques
Practicing relaxation techniques such as deep breathing, meditation, yoga, walking in green spaces (e.g. nature), journalling may promote mental recovery and reduce stress of heat acclimation.
Summary
In summary, it is important to recognise that training in hot conditions will result in increased physiological (i.e. cardiovascular, thermoregulatory, and induced hypohydration and electrolyte loss) and psychological (i.e. fatigue and discomfort) strain. This increased strain needs to be managed to minimise accumulated fatigue and associated over training to avoid negatively effecting your MdS legendary performance. To achieve this we recommend education around the physiological and psychological strain that you will be experiencing whilst adapting to the heat, as well as understanding the symptoms of heat related illnesses to ensure faster response and prevention. You should also employ scientifically supported physical and mental interventions to aid your recovery between heat acclimation session such as cooling (e.g. face mist spray, cooling collars, fans, ice slurries etc), nutrition (e.g. balanced diet, and electrolyte and water replacement), active recovery (e.g. walking and stretching), relaxation and mindfulness techniques (e.g. deep breathing, meditation, walking in green spaces, yoga, journalling), and rest days and sleep (e.g. factoring in rest days after heat acclimation sessions, and fostering sleep hygiene and routines).
References and useful resources
Daanen, H.A., Racinais, S. and Périard, J.D., 2018. Heat acclimation decay and re-induction: a systematic review and meta-analysis. Sports Medicine, 48, pp.409-430.
Driller, M. and FBASES, C.P., BASES Expert Statement: Methods to monitor athletes’ sleep.
Gaoua, N., de Oliveira, R.F. and Hunter, S., 2017. Perception, action, and cognition of football referees in extreme temperatures: impact on decision performance. Frontiers in psychology, 8, p.1479.
Heathcote, S.L., Hassmén, P., Zhou, S. and Stevens, C.J., 2018. Passive heating: reviewing practical heat acclimation strategies for endurance athletes. Frontiers in physiology, 9, p.1851.
Ihsan, M., Périard, J. D., & Racinais, S. (2021). How to integrate recovery during heat acclimation. British journal of sports medicine, 55(4), 185-186.
Ihsan, M., & Choo, H. C. (2024). Recovery During Exercise Heat Acclimation: Will Post-Exercise Cooling Enhance or Interfere with Adaptation?. Journal of Science in Sport and Exercise, 1-6.
Willmott, A. G., Hayes, M., James, C. A., Gibson, O. R., & Maxwell, N. S. (2020). Heat acclimation attenuates the increased sensations of fatigue reported during acute exercise-heat stress. Temperature, 7(2), 178-190.