Impact of age, cardiorespiratory fitness, and regular physical activity on physiological strain and cognitive performance during a 6-h extreme heat exposure

Abstract

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. Sixty-one participants (27 females; 20–79 yr) wore an accelerometer for 7 days, completed a graded exercise test to determine peak oxygen uptake (V_ O2peak), and underwent a 6-h heat exposure (43°C, 25% relative humidity) with episodic exercise to simulate daily living tasks. Rectal (Tre) and skin temperature, heart rate, whole body sweat rate, and cognitive performance (episodic memory, executive function, attention, and processing speed) were measured. Time, age, V_ O2peak, moderate-vigorous physical activity (MVPA) level, and sex were entered into linear mixed-effects models (mean [95% CI]). Advancing age exacerbated the rise of Tre (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 ¼ 0.01) and reduced end-exposure heart rate (per 10 mL·kg-1·min-1: -4 beats/min [-8, -1], P ¼ 0.03). Weekly MVPA and sex showed minimal association with outcome variables. Across adulthood, advancing age exacerbated the rise in Tre and lowered whole body sweating during a 6-h extreme heat exposure. Cardiorespiratory fitness improved whole body sweating but did not offset age-related increases in Tre. Therefore, physiological strain during extreme heat exposure develops progressively across the lifespan, with cardiorespiratory fitness moderating, but not preventing, age-related susceptibility. NEW & NOTEWORTHY 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.