The authors provide a good glimpse into the latest research on the topic and discuss applying the strategies considered, including how to plan heat acclimatization before competitions. A great piece of advice for coaches and athletes!
In the 2004 Olympic Games in Athens, Paula Radcliffe, one of the favourites for the gold medal in the woman´s marathon, had to withdraw mid-race with abdominal pain, bloated stomach and an urge to go to the bathroom. The Athens marathon took place under conditions of 35°C and 31% relative humidity. During the previous week, Paula had taken NSAIDs (non-steroidal anti-inflammatories) to treat a leg injury, according to published data. The reason for her withdrawal could have been due to the use of NSAIDs or lack of acclimatisation to the heat. 
It is well known that the next Olympic Games in Tokyo in 2021 will be in high ambient temperatures (over 30°C) and high humidity. From a performance and health perspective, competing in these types of environments can lead to a notable decline in performance, severe gastrointestinal problems and heat stroke.
For many professional athletes and teams, it is vital to prepare and adapt to compete in high-temperature events such as the Tour de France, World Cup and World Championships. These events are held in locations with extreme temperatures where there can be a loss in performance yield of between 6%–16%. 
In this article we will delve into and analyse the best strategies to adapt to high temperatures, as well as the most relevant physiological aspects of a correct acclimatisation strategy.
Strategies for adapting to heat
Preparation should focus on heat-acclimatisation strategies, in order to maximise competition performance and avoid possible heat stroke.There are two main strategic approaches:
- heat acclimation – carried out in artificial environments (climatic chambers or saunas)
- heat acclimatisation – carried out in the open air (warm places).
The main advantages of strategies carried out in a climatic chamber or a sauna are:
- better control of environmental conditions
- easier to monitor thermoregulatory response
- stimulus for adaptation.
Strategies carried out in the open air have the following favourable aspects:
- can be carried out where the competition is being held
- easy to monitor the thermal response of the athlete.
The biggest problem with heat-acclimation strategies is that not all elite athletes and amateurs have access to a climatic chamber or sauna to implement them. Therefore, their only option is to carry out climate adaptation in outdoor locations.
Both strategies include a physiological adaptation, and they lessen the decline in performance caused by competitions in very hot environments. The main physiological adaptations occur in the body at different levels:
- blood adaptations (higher plasma levels)
- greater tolerance to compete in high-temperature environments
- lower intestinal permeability
- greater myocardial protection
- adaptations of thermoregulatory processes such as increased sweating rates
- cardiovascular adaptations
- better central and peripheral adaptations to temperature level.
A brief explanation of the thermoregulation adaptations induced by acclimatization strategies. Adapted from Gibson et al, 2020. 
Considerations for applying acclimatization strategies
There is a scientific consensus that acclimatisation strategies should be carried out daily for 60 minutes during training, for a minimum of one week, but ideally for two weeks to have the desired effects on thermoregulation and sports performance. 
The majority of scientific studies that have evaluated heat adaptation protocols have been carried out for short adaptation protocols (four to seven days). Despite these one-week protocols, there are two more types: medium, with a duration of 8–14 days, and long protocols, greater than 15 days. Physiologists and scientists recommend that the duration of the protocols be in accordance with the duration of the tests to be competed, as the duration of the adaptation protocol is more effective for tests with a longer duration. Thus, protocols of more than 15 days would be suitable for endurance athletes who compete in tests lasting longer than two hours in high-temperature environments.
It is important to bear in mind that the duration of any adaptation protocol will have an effect on the body until the dreaded physiological maladjustment occurs. Likewise, the effects of heat maladjustment begin to be evident within two to four weeks after the acclimatisation, depending on the length of the acclimatisation period. 
This reduction in the effects of acclimatisation has been observed to be lower in athletes with a higher level of training. Furthermore, carrying out refreshers of heat acclimatisation has proven to be an important strategy in retaining the physiological effects for weeks and consequently avoiding loss of physiological adaptations obtained over a longer period of time.
Heat acclimatization planning for competing
The considerations for a trainer or physiologist to bear in mind when implementing a good adaptation strategy are diverse, including:
- assessing the duration of the event and the temperature where the competition will take place to decide if the acclimatisation protocol should be shorter or longer
- assessing the athlete’s physiological response by evaluating objective and subjective parameters
- bearing in mind what means are available to monitor internal temperature (temperature sensors).
An example of heat training for an athlete who competes in hot climates would be to start an acclimatisation protocol of five to seven days, carried out two to three weeks before going to the competition, then a refresher of one to two days a few days before the competition. 
It is important to mention that the different protocols must always be individualised, taking into account the athlete´s tolerance and physiological response, since acclimatisation to heat represents a significant stress to the daily training load. It is vitally important to assess the duration and intensity of the protocol that is prescribed to the athlete and the correct assimilation, since it often causes noticeable fatigue, which leads to a significant decrease in the athlete´s performance. [6-7]
Different kinds of heat adaptation protocols (short and long) with reminders (HE), applied to a competition calendar. Image adapted from Saunders, 2019. 
- Competing in hot environments can lead to significant loss in performance (between 6%–16%) as well as withdrawal due to heat stroke and/or gastrointestinal problems.
- The longer the duration of an adaptation protocol, the longer it will have an effect on the athlete´s body.
- At a higher level of training, the physiological losses derived from adaptation to heat will be lower.
- Losses of adaptation to heat begin to be noticeable two to four weeks after the acclimatisation period.
- Carrying out heat-adaptation refresher sessions of one to two days is a good strategy to maintain the adaptation achieved.
When athletes compete in hot environments, it is important to make adaptations to ensure the heat affects performance as little as possible. Each coach or physiologist should study the best acclimatisation methods, duration and intensity so as not to compromise the athlete´s performance in competition.
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3: Oliver R. Gibson, Carl A. James, Jessica A. Mee, Ashley G.B. Willmott, Gareth Turner, Mark Hayes,and Neil S. Maxwell. Heat alleviation strategies for athletic performance: A review and practitioner guidelines. Temperature (Austin). 2020; 7(1): 3–36.
4: Racinais S, Alonso JM, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Scand J Med Sci Sports. 2015;25(S1):6–19
5: Weller AS, Linnane DM, Jonkman AG, et al. Quantification of the decay and re-induction of heat acclimation in dry-heat fol- lowing 12 and 26 days without exposure to heat stress. Eur J Appl Physiol. 2007;102(1):57–66.
7: From Lab to Real World: Heat Acclimation Considerations for Elite Athletes. Julia R. Casadio, Andrew E. Kilding, James D. Cotter, Paul B. Laursen. Sports Med (2017) 47:1467-1476