Age alters the thermoregulatory responses to extreme heat exposure with accompanying activities of daily living

Abstract

Older adults are at greater risk for heat-related morbidity and mortality, due in part to age-related reductions in heat dissipating capabilities. Previous studies investigating the impact of age on responses to heat stress used approaches that lack activities of daily living and therefore may not accurately depict the thermal/physiological strain that would occur during actual heatwaves. We sought to compare the responses of young (18-39 yr) and older (≥65 yr) adults exposed to two extreme heat simulations. Healthy young (n = 20) and older (n = 20) participants underwent two 3-h extreme heat exposures on different days: 1) DRY (47°C and 15% humidity) and 2) HUMID (41°C and 40% humidity). To mimic heat generation comparable with activities of daily living, participants performed 5-min bouts of light physical activity dispersed throughout the heat exposure. Measurements included core and skin temperatures, heart rate, blood pressure, local and whole body sweat rate, forearm blood flow, and perceptual responses. Δ core temperature (Young: 0.68 ± 0.27°C vs. Older: 1.37 ± 0.42°C; P < 0.001) and ending core temperature (Young: 37.81 ± 0.26°C vs. Older: 38.15 ± 0.43°C; P = 0.005) were greater in the older cohort during the DRY condition. Δ core temperature (Young: 0.58 ± 0.25°C vs. Older: 1.02 ± 0.32°C; P < 0.001), but not ending core temperature (Young: 37.67 ± 0.34°C vs. Older: 37.83 ± 0.35°C; P = 0.151), was higher in the older cohort during the HUMID condition. We demonstrated that older adults have diminished thermoregulatory responses to heat stress with accompanying activities of daily living. These findings corroborate previous reports and confirm epidemiological data showing that older adults are at a greater risk for hyperthermia.NEW & NOTEWORTHY Using an experimental model of extreme heat exposure that incorporates brief periods of light physical activity to simulate activities of daily living, the extent of thermal strain reported herein more accurately represents what would occur during actual heatwave conditions. Despite matching metabolic heat generation and environmental conditions, we show that older adults have augmented core temperature responses, likely due to age-related reductions in heat dissipating mechanisms.

Keywords: aging; core temperature; sweating.

Graphical abstract

Figure 1.

Overview of experimental protocol. Healthy young (n = 20) and older (n = 20) participants were exposed to two 3-h extreme heat exposures on different days: 1) DRY (47°C and 15% humidity) and 2) HUMID (41°C and 40% humidity). To mimic heat generation comparable with activities of daily living, participants performed seven 5 min bouts of light physical activity (∼3 METs) dispersed throughout the heat exposure. Participants consumed 3 mL/kg body mass of tap temperature water (14°C–16°C) every hour during the heat exposures. Measurements included core and skin temperatures, heart rate, blood pressure, local and whole body sweat rate, forearm blood flow, plasma osmolality, change in plasma volume, and perceptual responses.

Figure 2.

Core temperature responses during the 3-h extreme heat exposures in young (n = 20) and older (n = 20) adults. Data are shown as means ± standard deviation with the circles/squares in B, C, E, and F representing individual data points. Older participants had greater increases in (Δ) core temperature and higher ending core temperatures in the DRY condition (B and C). Similarly, older participants had greater increases in (Δ) core temperature, but there was no difference in ending core temperature in the HUMID condition (E and F). Data in A and D were analyzed using linear mixed effects models with main effects of time (within: baseline and end of EHE) and group (between: young and older). Data in B, C, E, and F were analyzed using unpaired t tests (two-tailed). EHE, extreme heat exposure; RH, relative humidity.

Figure 3.

Whole body sweat loss (WBSL) and forearm blood flow (FBF) responses indexed to changes in mean body temperature in young and older adults. Data are shown as means ± standard deviation with the circles/squares representing individual data points. Whole body sweat loss and the changes in (Δ) forearm blood flow were lower in the older cohort when indexed to changes in (Δ) mean body temperature (MBT) during the DRY (A and B) and HUMID (C and D) heat exposures. Data were compared using unpaired t tests (two-tailed). RH, relative humidity.