Heat Adaptation in Military Personnel: Mitigating Risk, Maximizing Performance

REVIEW published: 17 December 2019 doi: 10.3389/fphys.2019.01485 Heat Adaptation in Military Personnel: Mitigating Risk, Maximizing Performance Iain T. Parsons 1,2* , Michael J. Stacey 1,3 and David R. Woods 1,4 1 Academic Department of Military Medicine, Research and Clinical Innovation, Royal Centre for Defence Medicine, Birmingham, United Kingdom, 2 School of Cardiovascular Medicine & Sciences, Faculty of Life Sciences & Medicine, King’s College London, London, United Kingdom, 3 Department of Diabetes and Endocrinology, Imperial College Healthcare NHS Trust, London, United Kingdom, 4 Department of Sport and Exercise Endocrinology, Carnegie Research Institute, Leeds Beckett University, Leeds, United Kingdom Edited by: Caroline Sunderland, Nottingham Trent University, United Kingdom Reviewed by: Ren-Jay Shei, Mallinckrodt, United States Simona Mrakic-Sposta, Institute of Clinical Physiology, Italian National Research Council, Italy *Correspondence: Iain T. Parsons ; Specialty section: This article was submitted to Integrative Physiology, a section of the journal Frontiers in Physiology Received: 26 August 2019 Accepted: 21 November 2019 Published: 17 December 2019 Citation: Parsons IT, Stacey MJ and Woods DR (2019) Heat Adaptation in Military Personnel: Mitigating Risk, Maximizing Performance. Front. Physiol. 10:1485. doi: 10.3389/fphys.2019.01485 Frontiers in Physiology | www.frontiersin.org The study of heat adaptation in military personnel offers generalizable insights into a variety of sporting, recreational and occupational populations. Conversely, certain characteristics of military employment have few parallels in civilian life, such as the imperative to achieve mission objectives during deployed operations, the opportunity to undergo training and selection for elite units or the requirement to fulfill essential duties under prolonged thermal stress. In such settings, achieving peak individual performance can be critical to organizational success. Short-notice deployment to a hot operational or training environment, exposure to high intensity exercise and undertaking ceremonial duties during extreme weather may challenge the ability to protect personnel from excessive thermal strain, especially where heat adaptation is incomplete. Graded and progressive acclimatization can reduce morbidity substantially and impact on mortality rates, yet individual variation in adaptation has the potential to undermine empirical approaches. Incapacity under heat stress can present the military with medical, occupational and logistic challenges requiring dynamic risk stratification during initial and subsequent heat stress. Using data from large studies of military personnel observing traditional and more contemporary acclimatization practices, this review article (1) characterizes the physical challenges that military training and deployed operations present (2) considers how heat adaptation has been used to augment military performance under thermal stress and (3) identifies potential solutions to optimize the risk-performance paradigm, including those with broader relevance to other populations exposed to heat stress. Keywords: heat acclimation, heat acclimatization, heat adaptation, heat stroke, heat syncope, heat illness, heat stress INTRODUCTION Heat stress, through occupational exposure to strenuous physical exercise and/or environmental extremes of heat and humidity, presents a perennial challenge (Hancock et al., 2007) to military personnel. The consequences of incomplete or inadequate heat adaptation may be fatal; either directly, as with heat stroke, or through impaired physiological functioning and increased 1 December 2019 | Volume 10 | Article 1485 Parsons et al. Heat Adaptation in Military Personnel susceptibility to military hazards, including combat. Augmenting physical and mental performance through heat adaptation may be pivotal to military operational success (Nindl et al., 2013). Over the last two decades, many Western militaries have conducted land-based campaigns in climatically-severe regions (World and Booth, 2008; Cox et al., 2016a). These operations frequently evolved into mature missions allowing for targeted physical training prior to deployment and large-scale graded acclimatization practices upon arrival to operational theaters. In more recent history, several militaries have contributed to internationally-sponsored security projects including the protection of internally displaced civilian populations (Bailey et al., 2019) and the containment of infectious diseases outbreaks, such as Ebola (Dickson et al., 2018). Achieving peak occupational performance and developing and maintaining resilience in the face of thermal threat has been a critical endpoint in all such circumstances. Routinely, the ‘special population’ fulfilling these roles is drawn from the general population of its parent nation and allied entities. Historically, the study of military personnel has provided insights into acclimatization mechanisms and strategies that have been generalizable to other occupational groups and the wider civilian population (Conn, 1963). The concept of relative climatic adaptation has been attributed to the renowned British Naval officer and clinician-scientist, Sir James Lind (Lind, 1771; Périard et al., 2015). As research findings on heat adaptation in this special population may be applied in other groups, so those learned elsewhere may have potential to prove militarily advantageous. Illustrated by research data from uniformed personnel observing traditional and more contemporary acclimatization practices – and with particular reference to recent United Kingdom (UK) military experience – this article (1) characterizes the physical challenges that modern training and operations present (2) considers how heat adaptation has been used or may be applied to protect and augment the health and performance of military personnel and (3) identifies potential solutions to optimize the risk-performance paradigm that operates both during and after heat acclimatization. THE THERMAL CHALLENGE OF MODERN MILITARY OPERATIONS, TRAINING, AND CEREMONIAL DUTIES In a volatile and increasingly complex world, governments of globally-facing nations foster contingency for a wide variety of overseas commitments (Ministry of Defence, 2018). These may be conducted through changing weather and elevation, over a range of timescales and set across challenging geographies: from short notice deployments to more enduring overseas operations (Edholm, 1969; Shapiro et al., 1981). Performance requirements are equally diverse arising from the scope of potential missions confronting the military and associated bodies, such as host nation civilian agencies and non-governmental aid organizations. Dedicated military taskings may include training and mentoring to allied forces; counter-terrorism/insurgency work; and fullscale war, including ‘peer-to-peer’ conflict conducted at lar (...truncated)