Trail Run Spain.

Heat Acclimatization

Competing in any race is hard relative to the amount of effort you put in and your level of fitness. Whether it is a road half marathon or a multi stage ultra across remote and hostile terrain the one thing which can really affect your chances of finishing is the temperature.

Races over marathon distance such as ultras tend to be something that you progress to, but the one type of event which defies this logic is the multi-stage, multi day ultra across remote and exotic landscapes. The granddaddy of these is the Marathon des Sables and after 25yrs and despite the high entry fee it now attracts 800 entries, many are first time ultra runners and the highest entry numbers after the French, (the event is organized by the French) come from the UK.

The UK and northern Europe in general isn’t noted for having a particularly warm and dry climate, summers are short and temperatures of over 20C are rare. It is hardly surprising that to enter a race in which temperatures of 20C (68F) would be considered cool and 30C (86F) more likely with maximums of 38-45C (100-115F) quite possible, needs some proper preparation.

Unfortunately the failure to prepare for the heat is probably the biggest reason that people fail or struggle to complete this type of race. Months of training in cold, wet, dark, frost, wind and often snow mixed in with the odd nice day isn’t the best way to go into an early spring race (the MdS) across the Moroccan Sahara, July in Andalucia (Al Andalus) or October in Egypt (Sahara race). No amount of money spent on getting lighter and lighter equipment and clothing, comfortable shoes, hydration systems, dehydrated meals plus reading and ‘chatting’ on forums is going to prepare you for the biggest test of all, the heat.

So what can you do? Well you may be surprised to know that heat acclimatization takes only about 2 weeks, even so, going to a race two weeks before the start isn’t an option for the majority of runners with jobs, families and holiday limitations. An additional 2 weeks in the sun would also add to the financial burden and for something like, for instance, the MdS this will already have cost close to 5000 pounds when taking into account the entry fee, equipment, training and travel.

Heat Acclimatization is the process through which the body deals with being introduced to a hot environment. Exercise intensity and duration should be gradually increased over the first two or three days as this is the time period in which most serious cases of heat illness occur. A minimum daily heat exposure of about 2 hours (which can be broken into two 1 hour sessions) combined with physical exercise that requires cardiovascular endurance such as jogging or running can be increased in intensity or duration each day. Some research also suggests that as long as your training is fairly intense, maybe 85% of your max heart rate, running just 30 minutes a day for about a week will acclimate you to running in hot weather.

You can also simulate heat by overdressing, wear more layers than usual when training, maybe wear a none breathable (cheap) waterproof jacket (more for those going to races like Jungle and Indo ultra where high humidity is also a problem), run indoors on a treadmill wearing layers and with room heating on, work out in a sauna and have a hot bath each evening.

Warning: Before heat training get a medical check up. Many races require you to submit an ECG dated within a month of the race. A month before the race would also be a good time to begin the heat training. Make sure someone knows you are heat training, a gym is a safe environment as there should be staff around but out in the hills you should try to have someone with you. If this isn’t possible then train on a small circuit with water/electrolytes available each lap, maybe stashed at points around the circuit.

The body’s sweat rate increases after 10 to 14 days of heat training. Because of this a greater fluid intake is required after acclimatization. In addition, increased sodium intake may be necessary during the first 3 to 5 days since the initial sweat rate will cause more sodium loss. After 5 to 10 days the sodium concentration in sweat will decrease and additional sodium supplementation shouldn’t be needed although I would still recommend you take Eletewater Tablytes after training and with meals before and during your race.

A high sweat rate can’t be maintained for hours so acclimatized athletes need to start each day pre-hydrated and not wait until they feel thirsty. However, an acclimatized athletes cells will become far better at picking up salt, and even though sweat flow may be four times the normal, the cells will absorb most of the salt. The kidneys also improve their ability to hold on to salt so that a fully acclimatized person in heat will need no more salt than an un-acclimatized person in temperate conditions.

Heat acclimatization is a very important part of an athletes preparation, it isn’t difficult to do and costs relatively nothing but it will make a huge difference to your race performance. However, remember the benefits of acclimatization are only retained for about 2 weeks.

For many reading this your first encounter with hot weather racing will be the 2012 MdS, (assuming that the 2009 races cool/wet conditions were a one off) so I hope this article has been useful. For those looking at doing Al Andalus ( in July chances are it will be even hotter so make sure you are heat training prior to coming out.

For those who are planning for a future MdS, Al Andalus, Gobi Challenge, Libya Challenge, Augrabies, Sahara race or any one of the many great races around the world please also consider attending one of our Desert Runner Training Camps which we hold throughout the year. They will make a huge difference to your preparation and you will soon find out how well you perform in high temperatures. Just make sure you are ‘’heat acclimatized’’ before arriving in order to gain the full benefit of training everyday in the heat.

IN PRESS- EUROPEAN JOURNAL OF SPORT SCIENCE: Heat acclimation protocol laboratory tested specific for ultra-endurance runners.

Costa, R.J.S., Crockford, M.J., Moore, J.P., & Walsh, N.P. (2012). Heat acclimation responses of an ultra-endurance running group preparing for hot desert based competition. European Journal Sport Science, (iFirst 1-11).

Conclusions : Two exercise-heat exposures of 2 hours running at 60% VO2max in 30 degrees C ambient temperature was sufficient to initiate heat acclimation in all ultra-endurance runners. Further heat acclimation responses occurred with increasing exercise-heat to 35 degrees C ambient temperature.

Prevention of exertional heat illness and optimising performance outcomes in ultra-endurance runners may occur with exposure to at least 2 hours of exercise-heat stress on at least two occasions in the days leading up to multi-stage ultra-marathon competition in the heat.

Putting the science into practice- "the heat acclimation recipe": 2 hour steady run in the heat (at least 30 degrees C) on Wednesday, 2 hour steady run in the heat (at least 30 degrees C) on Friday, then rest Saturday and Sunday at the race location (race locations ambient temperature), race starts on Monday. As the ultra-marathon progresses, the body appears to adapt with any shifts to the local ambient temperature (e.g. if the ambient temperature increases from Monday to Friday the body appears to cope with the increased change).

Paul Bateson

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