Seasonal Heat Acclimatisation in Healthy Adults: A Systematic Review

Sports Medicine https://doi.org/10.1007/s40279-022-01677-0 SYSTEMATIC REVIEW Seasonal Heat Acclimatisation in Healthy Adults: A Systematic Review Harry A. Brown1 · Thomas H. Topham1 · Brad Clark1 · James W. Smallcombe2 · Andreas D. Flouris3 · Leonidas G. Ioannou3 · Richard D. Telford1 · Ollie Jay2 · Julien D. Périard1 Accepted: 20 March 2022 © The Author(s) 2022 Abstract Background Physiological heat adaptations can be induced following various protocols that use either artificially controlled (i.e. acclimation) or naturally occurring (i.e. acclimatisation) environments. During the summer months in seasonal climates, adequate exposure to outdoor environmental heat stress should lead to transient seasonal heat acclimatisation. Objectives The aim of the systematic review was to assess the available literature and characterise seasonal heat acclimatisation during the summer months and identify key factors that influence the magnitude of adaptation. Eligibility Criteria English language, full-text articles that assessed seasonal heat acclimatisation on the same sample of healthy adults a minimum of 3 months apart were included. Data Sources Studies were identified using first- and second-order search terms in the databases MEDLINE, SPORTDiscus, CINAHL Plus with Full Text, Scopus and Cochrane, with the last search taking place on 15 July 2021. Risk of Bias Studies were independently assessed by two authors for the risk of bias using a modified version of the McMaster critical review form. Data Extraction Data for the following outcome variables were extracted: participant age, sex, body mass, height, body fat percentage, maximal oxygen uptake, time spent exercising outdoors (i.e. intensity, duration, environmental conditions), heat response test (i.e. protocol, time between tests), core temperature, skin temperature, heart rate, whole-body sweat loss, whole-body and local sweat rate, sweat sodium concentration, skin blood flow and plasma volume changes. Results Twenty-nine studies were included in this systematic review, including 561 participants across eight countries with a mean summer daytime wet-bulb globe temperature (WBGT) of 24.9 °C (range: 19.5–29.8 °C). Two studies reported a reduction in resting core temperature (0.16 °C; p < 0.05), 11 reported an increased sweat rate (range: 0.03–0.53 L·h−1; p < 0.05), two observed a reduced heart rate during a heat response test (range: 3–8 beats·min−1; p < 0.05), and six noted a reduced sweat sodium concentration (range: − 22 to − 59%; p < 0.05) following summer. The adaptations were associated with a mean summer WBGT of 25.2 °C (range: 19.6–28.7 °C). Limitations The available studies primarily focussed on healthy male adults and demonstrated large differences in the reporting of factors that influence the development of seasonal heat acclimatisation, namely, exposure time and duration, exercise task and environmental conditions. Conclusions Seasonal heat acclimatisation is induced across various climates in healthy adults. The magnitude of adaptation is dependent on a combination of environmental and physical activity characteristics. Providing environmental conditions are conducive to adaptation, the duration and intensity of outdoor physical activity, along with the timing of exposures, can influence seasonal heat acclimatisation. Future research should ensure the documentation of these factors to allow for a better characterisation of seasonal heat acclimatisation. PROSPERO Registration CRD42020201883. Extended author information available on the last page of the article Vol.:(0123456789) H. A. Brown et al. Key Points Seasonal heat acclimatisation is induced across different climates, from hot and dry to warm and humid. The adaptations stemming from seasonal heat acclimatisation include reductions in resting core temperature and heart rate, as well as an attenuated rise in core temperature and an increased sweat rate during active and passive heat exposures. The magnitude of adaptation is dependent on several factors alongside the environmental characteristics, including the timing of environmental exposures during the day, and the duration and intensity of outdoor physical activity. 1 Introduction Environmental heat stress is known to impair aerobic exercise performance [1–3] in response to an increase in whole-body temperature and the consequent adjustments in cardiovascular, central nervous system and skeletal muscle function [4]. The rise in whole-body temperature is also associated with a greater risk of exertional heat illness in uncompensable conditions [5, 6]. However, frequent exposures to hot environments, alongside physical activity, can induce adaptations that attenuate the detrimental effects of environmental heat stress [7–9]. These physiological adaptations occur when thermal stress is sufficient to maintain a disruption of homeostasis to one or more of the biological systems that ensure physiological function and stability during heat exposures [10]. As adaptations develop, decrements in exercise performance are progressively restored [11, 12] and the risk of exertional heat illness reduced [13, 14]. Adaptations to heat stress are referred to as heat acclimation when induced in an artificial setting (e.g. climate chamber) [15–17] and heat acclimatisation when achieved through exposure to a natural environment [18–20]. Much like acclimation, acclimatisation is used to prepare athletes [21, 22] and military personnel [23, 24] for work in hot environments. Both interventions may be purposely implemented to improve thermoregulatory capacity, cardiovascular stability and thermal tolerance during heat exposure [10, 24–26]. However, seasonal heat acclimatisation is largely a background process wherein seasonal changes in ambient conditions can, at least theoretically, induce these same heat adaptations [27]. However, with an increasingly sedentary population [28] and the avoidance of physical activity in warmer parts of the day [29], the magnitude of adaptation induced via seasonal heat acclimatisation in healthy contemporary populations remains unclear. There is evidence to support the influence of the natural environment inducing heat adaptations during the summer months (i.e. seasonal heat acclimatisation) [30, 31]; however, the reported adaptations differ widely among studies. For example, a greater sweat rate and lower core temperature (Tc) during passive heating were attained following summer (mean maximum ambient temperature 28 °C) in SouthCentral Japan [30], reinforcing previous results in Japanese athletes using a similar heat response test (HRT) (i.e. 60 and 90 min of lower leg hot-water immersion, respectively) [31]. In contrast, these adaptations were not observed during an incremental running protocol in high-level distance runners following a summer (mean maximum ambient temperature 25 °C) of outdoor running training in the North-Eastern United States [18]. Seasonal heat acclimatisation was (...truncated)