Harry A. Brown

Advancing age is associated with progressive increases in the rise in core temperature during extreme heat exposure, and this trajectory is evident across the entire adult lifespan rather than emerging only in older adults. Thermoregulatory capacity declines steadily with each decade, leading to greater thermal strain even in mid-adulthood. Although higher cardiorespiratory fitness enhances sweating, it does not eliminate the age-related increase in core temperature, underscoring that heat vulnerability may develop gradually throughout adulthood.

The integrative influence of heat acclimation (HA) protocol characteristics and approach on adaptation kinetics and exercise capacity/performance in the heat remains unclear. Bayesian multilevel regression models were used to estimate adaptations with the number of exposures, exposure duration, ambient temperature, water vapor pressure, and HA approach (e.g., constant workrate) as predictors. Data from 211 papers were included in meta‐analyses with results presented as posterior means and 90% credible intervals. Mean protocol characteristics were as follows: 8 ± 4 exposures, 90 ± 36 min/exposure, 39.1°C ± 4.8°C, and 2.78 ± 0.83 kPa. HA decreased resting (−5 beats·min⁻¹ [−7, −3]) and end‐exercise heart rate (−17 beats·min⁻¹ [−19, −14]), resting (−0.19°C [−0.23, −0.14]) and end‐exercise core temperature (−0.43°C [−0.48, −0.36]), and expanded plasma volume (5.6% [3.8, 7.0]). HA also lowered exercise metabolic rate (−87 mL·min⁻¹ [−126, −49]), increased whole‐body sweat rate (WBSR) (163 mL·h⁻¹ [94, 226]), time to exhaustion (49% [35, 61]) and incremental exercise time (14% [7, 24]), and improved time trial performance (3.1% [1.8, 4.5]). An additional HA exposure increased hemoglobin mass (1.9 g [0.6, 3.2]) and WBSR (9 mL·h⁻¹ [1, 17]), and an additional 15 min/exposure further lowered end‐exercise core temperature (−0.04°C [−0.05, −0.03]) and expanded plasma volume (0.4% [0.1, 0.7]). A 5°C increase in ambient temperature further lowered end‐exercise HR (−2 beats·min⁻¹ [−3, −1]) and a 1 kPa increase enhanced WBSR (37 mL·h⁻¹ [4, 72]). End‐exercise heart rate and core temperature decreased similarly following controlled hyperthermia (−16 beats·min⁻¹ [−18, −14]; −0.43°C [−0.48, −0.36]) and constant workrate HA (−17 beats·min⁻¹ [−18, −16]; −0.45°C [−0.49, −0.42]). HA protocol characteristics influence the adaptive response and may be manipulated to optimize adaptations. A predictor for estimating HA adaptations based on protocol characteristics is available at: https://www.canberra.edu.au/research/centres/uc‐rise/research/environmental‐physiology/exercise‐heat‐acclimation‐predictor.

Purpose: The aim of this study was to confirm the impact of heat acclimation on aerobic performance in hot conditions and elucidate the transfer of heat adaptations to cool and hypoxic environments. Methods: Ten males (VO_2peak: 4.50 ± 0.50 L/min) completed two three-week interventions consisting of heat acclimation (HA: 36°C and 59% RH) and temperate training (TEMP: 18°C and 60% RH) in a counter-balanced crossover design. Training weeks consisted of four work-matched controlled heart rate sessions interspersed with one intermittent sprint session, and two rest days. Before and after the interventions VO_2peak and 20-min time trial performance were evaluated in COOL (18°C), HOT (35°C) and hypoxic (HYP: 18°C and FiO₂: 15.4%) conditions. Results: Following HA, VO_2peak increased significantly in HOT (0.24 L/min [0.01, 0.47], P = 0.040) but not COOL (P = 0.431) or HYP (P = 0.411), whereas TEMP had no influence on VO_2peak (P ≥ 0.424). Mean time trial power output increased significantly in HOT (20 W [11, 28], P < 0.001) and COOL (12 W [4, 21], P = 0.004), but not HYP (7 W [−1, 16], P = 0.075) after HA, whereas TEMP had no influence on mean power output (P ≥ 0.110). Rectal (−0.13°C [−0.23, −0.03], P = 0.009) and skin (−0.7°C [−1.2, −0.3], P < 0.001) temperature were lower during the time trial in HOT after HA, whereas mean heart rate did not differ (P = 0.339). Conclusions: HA improved aerobic performance in HOT in conjunction with lower thermal strain and enhanced cardiovascular stability (similar heart rate for higher workload), whereas the mechanistic pathways improving performance in COOL and HYP remain unclear.

Limited studies have investigated the thermoregulatory responses of boys and girls exercising under uncompensable heat stress. Boys and girls often differ in physiological characteristics other than biological sex, such as aerobic fitness and body fat percentage, which may confound interpretations. We investigated the influence of biological sex on exercise thermoregulation in children, independent of differences in aerobic fitness and body fatness.

The aim of this audit was to quantify female representation in research on heat adaptation. Using a standardized audit tool, the PubMed database was searched for heat adaptation literature from inception to February 2023. Studies were included if they investigated heat adaptation among female and male adults (≥18–50 years) who were free from noncommunicable diseases, with heat adaptation the primary or secondary outcome of interest. The number and sex of participants, athletic caliber, menstrual status, research theme, journal impact factor, Altmetric score, Field-Weighted Citation Impact, and type of heat exposure were extracted. A total of 477 studies were identified in this audit, including 7,707 participants with ∼13% of these being female. Most studies investigated male-only cohorts (∼74%, n = 5,672 males), with ∼5% (n = 360 females) including female-only cohorts. Of the 126 studies that included females, only 10% provided some evidence of appropriate methodological control to account for ovarian hormone status, with no study meeting best-practice recommendations. Of the included female participants, 40% were able to be classified to an athletic caliber, with 67% of these being allocated to Tier 2 (i.e., trained/developmental) or below. Exercise heat acclimation was the dominant method of heat exposure (437 interventions), with 21 studies investigating sex differences in exercise heat acclimation interventions. We recommend that future research on heat adaptation in female participants use methodological approaches that consider the potential impact of sexual dimorphism on study outcomes to provide evidence-based guidelines for female athletes preparing for exercise or competition in hot conditions.

Background Athletes and military personnel are often expected to compete and work in hot and/or humid environments, where decrements in performance and an increased risk of exertional heat illness are prevalent. A physiological strategy for reducing the adverse effects of heat stress is to acclimatise to the heat. Objective The aim of this systematic review was to quantify the effects of relocating to a hotter climate to undergo heat acclimatisation in athletes and military personnel. Eligibility Criteria Studies investigating the effects of heat acclimatisation in non-acclimatised athletes and military personnel via relocation to a hot climate for < 6 weeks were included. Data Sources MEDLINE, SPORTDiscus, CINAHL Plus with Full Text and Scopus were searched from inception to June 2022. Risk of Bias A modified version of the McMaster critical review form was utilised independently by two authors to assess the risk of bias. Data Synthesis A Bayesian multi-level meta-analysis was conducted on five outcome measures, including resting core temperature and heart rate, the change in core temperature and heart rate during a heat response test and sweat rate. Wet-bulb globe temperature (WBGT), daily training duration and protocol length were used as predictor variables. Along with posterior means and 90% credible intervals (CrI), the probability of direction (Pd) was calculated. Results Eighteen articles from twelve independent studies were included. Fourteen articles (nine studies) provided data for the meta-analyses. Whilst accounting for WBGT, daily training duration and protocol length, population estimates indicated a reduction in resting core temperature and heart rate of − 0.19 °C [90% CrI: − 0.41 to 0.05, Pd = 91%] and − 6 beats·min⁻¹ [90% CrI: − 16 to 5, Pd = 83%], respectively. Furthermore, the rise in core temperature and heart rate during a heat response test were attenuated by − 0.24 °C [90% CrI: − 0.67 to 0.20, Pd = 85%] and − 7 beats·min⁻¹ [90% CrI: − 18 to 4, Pd = 87%]. Changes in sweat rate were conflicting (0.01 L·h⁻¹ [90% CrI: − 0.38 to 0.40, Pd = 53%]), primarily due to two studies demonstrating a reduction in sweat rate following heat acclimatisation. Conclusions Data from athletes and military personnel relocating to a hotter climate were consistent with a reduction in resting core temperature and heart rate, in addition to an attenuated rise in core temperature and heart rate during an exercise-based heat response test. An increase in sweat rate is also attainable, with the extent of these adaptations dependent on WBGT, daily training duration and protocol length. PROSPERO Registration CRD42022337761.