Thermoregulatory and Renal Responses to Simulated Heatwaves in Younger and Older Adults
Authors: McKenna Zachary , Foster Josh , Atkins Whitley , Belval Luke , Watso Joseph , Orth Bonnie , Benning Mayah , Johnson Elias , Jarrard Caitlin , Crandall Craig 
Published: May 2023

The frequency, intensity, and duration of heat waves are projected to increase in the coming decades. Older adults have impaired heat-dissipating capabilities and are at a greater risk for heat-related morbidity and mortality. Many hospitalizations and deaths in this population during heat waves are due to renal complications. Indeed, age-related reductions in kidney function, altered fluid regulation, and impaired thermoregulation, may place the elderly at an increased risk for heat-related renal complications. Purpose: Therefore, we tested the hypothesis that aging augments heatwave-induced renal stress. Methods: We assessed core temperature, mean skin temperature, whole-body sweat loss, and estimated glomerular filtration rate (eGFR) via serum creatinine in healthy older (8 Female/7 Male; 70±4 years) and younger (6 Female/7 Male; 29±4 years) adults exposed to three hours of 1) very hot and dry (DRY: 47ºC, 10% relative humidity) and 2) hot and humid (HUMID: 41ºC, 40% relative humidity) heatwave conditions on different days (random order). To simulate activities of daily living, participants performed seven 5-minute periods of light exercise (~3 METS; metabolic heat production: Old: 2.98±0.42; Young: 2.96±0.34 W/kg) dispersed across the heat exposure. Additionally, participants consumed 3 mL/kg/hr of tap-temperature water to maintain euhydration. We analyzed data from each heatwave condition independently using linear mixed effect models with main effects of time (pre and post) and group (Older and Younger) or unpaired t-tests, as appropriate. Repeated measures correlation was used to determine any relation between the change (Δ) in core temperature and Δ eGFR across both heatwave conditions. Results: Following exposure to DRY, older adults had greater increases in core temperature (Older: Δ 1.46±0.42, Younger: Δ 0.74±0.28ºC, interaction p<0.01), but no difference in mean skin temperature increases (Older: Δ 2.09±1.31, Younger: Δ 1.73±0.48ºC, interaction p=0.404) or whole body sweat loss (Older: 1479±391, Younger: 1556±542 mL, p=0.661). In addition, following DRY, eGFR decreased in older but increased in younger adults (Older: Δ -4.3±5.9; Younger: Δ 4.12±4.75 mL/min/1.73 m2, interaction p<0.01). Following exposure to HUMID, older adults had greater increases in core temperature (Older: Δ 1.09±0.29, Younger: Δ 0.55±0.28ºC, interaction p<0.01) and mean skin temperature (Older: Δ 2.43±1.05, Younger: Δ1.37±1.13ºC, interaction p=0.018), but no difference in whole body sweat loss (Older: 979±304, Younger: 1011 ± 269 mL, p=0.774). eGFR increased in younger, but not older, adults (Older: Δ 0.30±4.01, Younger: Δ 6.16±3.37 mL/min/1.73 m2, interaction p<0.01) following HUMID. There was a significant negative relation between Δ core temperature and Δ eGFR (r=-0.48, p<0.01). Conclusion: These preliminary data suggest that older individuals exhibit greater increases in core temperature during heatwave exposures that may contribute to reducedrenal function. Supported by NIH R01AG069005 (CGC), F32HL154559 & K01HL160772 (JCW), and F32HL154565 (LNB). This is the full abstract presented at the American Physiology Summit 2023 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.