Response.

Abstract

Dear Editor-in-Chief: We sincerely thank Drs. Brocherie, Debevec, and Millet for their comments on our recent review (1) and for continuing the conversation on the topic of heat versus altitude training in elite-level performance (2). We agree that Brocherie et al. (2) raise several key points that should be considered, and they provide excellent guidance for future research directions as it pertains to developing best practices for using heat and/or altitude to optimize endurance performance in elite-level athletes. In a hypothesis-driven format, the goal of our review was to compare the effects of heat and altitude training, independently, on endurance performance and synthesize best practices for each as they relate to training and competition. That goal remains prudent and timely given the common belief within the endurance sport community that heat training may offer equivalent temperate, sea-level performance benefits compared with altitude training. We agree that comparing the independent effects of heat and hypoxia is challenging to interpret due to heterogeneity in training protocols (e.g., exposure duration, stressor severity, performance test timing). However, we chose to focus on physiological mechanisms that most directly translate to endurance performance enhancements (e.g., plasma volume expansion, oxygen carrying capacity), which have been studied extensively in highly trained athletes (3–5). Given the goal of our review and our approach (i.e., directly comparing heat and altitude training), expanding our focus to further include the combined effects of heat and altitude training (and other mechanisms/underlying adaptations such as heat shock proteins, HIF-1α, etc.) goes beyond the scope of this review. The purpose of our review was at least partially rooted in the fact that terrestrial high-altitude residences tend to be cooler climates, and studying the combined effects of heat and hypoxia is difficult, such that it requires an altitude and/or heat chamber. As noted by Brocherie et al. (2), it is clear that similar molecular pathways are activated by both heat and hypoxia, largely stemming from the effects of heat shock proteins, which lends to the potential for cross tolerance. Yet, results of multiple heat and hypoxia experiments (6,7) indicate that the translation of potentially complementary molecular pathways stimulated by heat and hypoxia on endurance performance remains largely unclear. Importantly, however, this should not be interpreted as there being no benefit of combined heat and altitude training on endurance performance. Rather, given the promising data from in vitro and animal studies (8), we believe this previous work provides the foundation for future studies. Again, we thank Dr. Brocherie and colleagues for highlighting our review and for raising the possibility for potential additive benefits of combined heat and hypoxic adaptations for endurance exercise performance. We agree that further studies are warranted to examine the pairing of heat and altitude on both the hematological and nonhematological responses to this “hotpoxic” stressor. We hope that our review and this exchange promote the development and testing of additional hypotheses that lead to the optimization of heat or altitude (or a combination of both) for high-level human performance. Marissa N. Baranauskas Department of Kinesiology School of Public Health-Bloomington Indiana University, Bloomington, IN Keren Constantini School of Public Health Sackler Faculty of Medicine and Sylvan Adams Sports Institute Tel Aviv University Tel Aviv, Israel Hunter L. Paris Division of Natural Sciences Pepperdine University Malibu, CA Chad C. Wiggins Department of Anaesthesiology and Perioperative Medicine Mayo Clinic Rochester, MN Zachary J. Schlader Robert F. Chapman Department of Kinesiology School of Public Health-Bloomington Indiana University Bloomington, IN [email protected]