Thermoregulatory Responses to Very Hot and Dry Heat Exposure: Interactions Between Age and Sex

Extreme heat events can cause mass morbidity and mortality. Older adults are more susceptible to the adverse effects of heat. Additionally, sex is known to independently influence the physiological responses to heat stress. However, less is known regarding the interaction between age and sex on thermoregulation. Purpose: This study investigated differences in thermoregulatory responses between males and females in older and younger individuals exposed to a hot and dry environment with accompanying activities of daily living. Methods: Twenty older (70 ± 4 yrs) and twenty younger (29 ± 5 yrs) adults (10 males and 10 females per group) were exposed to 3-hours of ambient heating, replicating peak environmental conditions during the 2018 Los Angeles heat wave (47 °C, 15% relative humidity). Participants performed seven 5-minute periods of light exercise (≍3 METS; metabolic heat production: 2.99 ± 0.33 W/kg) dispersed throughout the heat exposure to simulate heat generation associated with activities of daily living. Participants ingested 3 mL/kg/hr of tap-temperature water throughout the trial. Core temperature, skin temperature, and heart rate changes were assessed from pre-post heat exposure. Whole-body sweat rate and forearm blood flow changes were indexed to changes in core temperature. Data were analyzed using a two-way ANOVA (Age x Sex) Results: Delta core temperature was greater in older individuals (males: 1.43 ± 0.47°C, females: 1.31 ± 0.39°C) than in younger (males: 0.83 ± 0.22°C, females: 0.53 ± 0.24°C, p<0.001) but there was no difference between sex (p = 0.063) nor was there an interaction (p = 0.423). There was an interaction of age and sex on delta heart rate (p = 0.019). Older females (32 ± 9 bpm) had greater increases in heart rate than older males (18 ± 8 bpm), younger males (18 ± 6 bpm), and younger females (18 ± 12 bpm, p ≤ 0.009 for all comparisons). There was no effect of age (p = 0.529), sex (p = 0.670), or interaction (p = 0.422) on delta skin temperature. There was an interaction between age and sex for whole-body sweat loss (WBSL) indexed to changes in core temperature and body surface area (p = 0.009). Younger females (1688 ± 685 mL/m2/°C) had greater WBSL than older females (612 ± 211 mL/m2/°C, p < 0.0001), older males (694 ± 327 mL/m2/°C, p < 0.0001), and younger males (1038 ± 282 mL/m2/°C, p < 0.0001). There was a main effect of sex (p = 0.003) and age (p < 0.0001), but no interaction (p = 0.152) on delta forearm blood flow indexed to changes in core temperature (older males: 116 ± 67 mL/min/°C, older females: 194 ± 73 mL/min/°C, younger males: 263 ± 72 mL/min/°C, younger females: 468 ± 243 mL/min/°C). Conclusion: These data suggest that many thermoregulatory responses to heat stress are influenced by the interaction between age and sex. Supported by NIH R01AG069005 (CGC). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.