Ideal intervention investigations are both for altitude training and heat acclimatization difficult to conduct, but it appears inappropriate that Minson & Cotter (2015) include studies lacking an appropriate control group as support for their statement. That heat acclimatization does not impair performance in cool conditions and may be a clever strategy for athletes if the competitive conditions are potentially hot does not allow physiologists to conclude that it improves performance in cool settings. Lessons from altitude training taught us that the ‘responder–non-responder issue’ could not be supported by robust responses during repeated exposures (Robertson et al. 2010), and double blinded, placebo controlled studies (Siebenmann et al. 2012; Robach et al. 2014) fail to support the 1% performance enhancement concluded from meta-analysis of altitude training studies (Bonetti & Hopkins, 2009). Although, heat acclimatization studies in truly elite athletes are lacking it is difficult to see that they should respond differently from the endurance trained athletes in recent studies (Karlsen et al. 2015; Keiser et al. 2015) as plasma volume expansion is ineffective when subjects possess high plasma volumes and a cardiovascular system adapted to several years of training (see Nybo & Lundby, 2015). Minson & Cotter (2015) speculate that we (Karlsen et al. 2015) may have missed an acclimatization effect due to travel fatigue; however, the outward travel did not preclude a steady performance improvement in the heat. Furthermore, the time-zone difference was only 2 h and post-tests were repeated twice over a 2 week period to minimize the risk of missing a performance effect. Blinding subjects to the environmental temperature is not possible and matching training in the heat with a cool control group will always be a matter of debate as the cardiovascular response is influenced by the environment and changes in the relative exercise intensity. Whether training should be balanced for absolute or relative intensity, maintained volume or indices of so-called ‘training impact’ is not clear. We suggest that training should be optimized for the given environment, and studies looking for truly additional effects of acclimatization should aim at including well-trained athletes and ensuring that subjects have conducted optimized training in their habitual environmental settings during the lead-in period. On the basis of current available studies fulfilling these requirements, we reiterate our conclusion: there is no evidence supporting superior effects of training in the heat as compared to training in cool climates in terms of improving exercise performance in a cool environment.
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