Introduction Aging, cardiorespiratory fitness, and habitual physical activity influence thermoregulation and cognition, yet their combined impact during prolonged extreme heat exposure is not well understood. This study investigated how these factors influence thermal strain and cognitive performance across the adult lifespan under extreme heat stress. Methods Sixty-one participants (27 females; 20-79 years) wore an accelerometer for seven days, completed a graded exercise test to determine peak oxygen uptake (V̇O 2peak ), and underwent a six-hour heat exposure (43°C, 25% humidity) with episodic exercise to simulate daily living tasks. Rectal (T re ) and skin temperature, heart rate, whole-body sweat rate and cognitive performance (episodic memory, executive function, attention, processing speed) were measured. Time, age, V̇O 2peak , moderate-vigorous physical activity (MVPA) level and sex were entered into linear mixed effects models (mean 95% CI). Results Advancing age exacerbated the rise of T re (per decade: 0.08°C 0.04,0.13, P<0.01) and reduced whole-body sweating (per decade: -6 g·m -2 ·h -1 -10,-1, P<0.01). Cognitive performance reduced with advancing age, independent of heat exposure (P<0.01). Higher cardiorespiratory fitness improved whole-body sweat rate (per 10 mL·kg -1 ·min -1 : 11 g·m -2 ·h -1 2,19, P=0.01) and reduced end-exposure heart rate (per 10 mL·kg -1 ·min -1 : -4 beats·min -1 -8,-1, P=0.03). Weekly MVPA and sex showed minimal association with outcome variables. Conclusion Across adulthood, advancing age exacerbated the rise in T re and lowered whole-body sweating during a six-hour extreme heat exposure. Cardiorespiratory fitness improved whole-body sweating but did not offset age-related increases in T re . Therefore, physiological strain during extreme heat exposure develops progressively across the lifespan, with cardiorespiratory fitness moderating, but not preventing, age-related susceptibility.
Brown et al. (2026) studied this question.