Short-term Heat Acclimation Improves Perceptions of Thirst and Thermal Comfort in Hot Conditions

Introduction: Warfighters are often deployed to hot environments at short notice, increasing the potential for heat related illness and reduced performance. Higher heart rates (HR) and core temperatures (T rec ) during heat stress are associated with negative outcomes, but perceptions of increased effort and decreased thermal comfort are also associated with performance decrements independent of physiological responses. A 10-14 day heat acclimation (HA) protocol prior to environmental heat exposure reduces HR and T rec , however, these physiological adaptations are possible with a shorter induction period (5 day; 5HA). It is unclear if short-term HA protocols also impact perceptual measures of heat strain. Therefore, this study aimed to determine whether completing a 5HA protocol resulted in changes to perceptual measures of thermal and physiological stress during a Time to Temperature trial. Hypothesis: Completing a 5HA protocol will be related to reduced heat strain evidenced by lower perceptual measures of Rate of Perceived Exertion (RPE), Thermal Sensation (TS), thirst, and fatigue, while physiological measures of HR and T rec are improved. METHODS: Forty physically active participants (mean±SD; females= 21, males= 19 age= 23±4 years, height= 171.2±9.9 cm, weight= 69.0±12.7 kg, VO 2max = 47.8±6.3 ml·kg -1 ·min -1 ) completed a time to temperature trial (65% vVO2max, 2% grade treadmill run, 40°C, 40% rh, until rectal temperature reached 39.5°C) before ( pre HA) and after ( post HA) completing a 5HA protocol (40°C, 40% rh, T rec = ≥38.5°C for 60 mins). Perceptual measures were recorded every 10 minutes using an analogue Likert type scale and included RPE, TS, thirst, and fatigue. HR and T rec were monitored throughout. Pre HA to post HA measures were compared using paired t-tests. Cohen’s d was used to determine effect sizes (ES). Results: Here we show that after a 5HA protocol, participants showed increased thermal tolerance with lower HR ( pre HA:185±14 bpm vs post HA 180±17 bpm, p=0.03, ES=0.3) and longer time to T rec of 39.5 ( pre HA: 30.6±7.5 min vs post HA: 35.9±11.2 min, p=0.003, ES=0.6) during the second trial. While no differences were found in post HA fatigue or RPE (p=0.08 and 0.11 respectively, ES=0.3 and 0.2), participants reported lower thermal strain ( pre HA: 7.1±0.7 vs post HA: 6.7±0.9, p=0.014, ES=0.4) and less thirst ( pre HA: 6.1±2.1 vs post HA:5.2±2.2, p=0.036, ES=0.4). Conclusion: Our results suggest that a short-term HA protocol shows improved TS and thirst, but did not affect fatigue or RPE. Short-term HA may impact physiological and perceptual stress in hot conditions, when preparation time is limited, lowering the risk of heat illness and poor performance. DISCLAIMER Funding Source: DoD BA200299 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.