Effect of Age on Fluid Replacement during Heat Exposure – The Human Heat Stress Project

Introduction: Periods of extreme heat are associated with health risks, particularly among older adults. Electrolyte imbalances and acute kidney injuries are the main causes of heat-related hospitalizations in this population. It is often suggested that this is due to insufficient fluid replacement. However, limited research has explored differences in hydration behavior between older and younger adults during heat exposure. Objective/Hypothesis: This study examines the effect of age on hydration behavior during exposure to a hot and humid environment that includes light physical exercise to simulate activities of daily living. We hypothesize that advancing age is associated with less effective sweat loss replacement. Methodology: Eleven young adults (YA: 18-39 years old, 9 females), 13 middle-aged adults (MA: 45-63 years old, 5 females), 16 older adults (OA: 65-70 years old, 5 females), and 15 oldest older adults (OOA: 71-88 years, 6 females) were exposed to an environmental chamber maintained at 38°C with 60% humidity for 4 hours. Participants performed light exercise (~3.5 METs, walking or cycling) for 10 minutes each hour. Participants could drink water served at 15 ± 1°C ad libitum, without knowing their hydration behavior was being measured. They were asked to rate their thirst before each water intake using a visual analogue scale (180 mm, ranging from “not thirsty” to “extremely thirsty”). Sweat production was estimated from the change in body mass, corrected for urinary losses and fluid intake. Dehydration was estimated via changes in nude body mass. Participants completed a 24-hour urine collection before arriving to the laboratory and a blood sample was drawn upon arrival. Data are presented as mean ± standard deviation. Results: Pre-exposure urine volume (YA: 1.6 ± 0.6 L, MA: 2.3 ± 1.3 L, OA: 1.9 ± 0.8 L, OOA: 1.8 ± 0.7 L) and osmolality (560 ± 143 mOsm/kg, 535 ± 271 mOsm/kg, 596 ± 253 mOsm/kg, 504 ± 178 mOsm/kg) were not different across groups, suggesting comparable hydration status at baseline. However, baseline plasma osmolality was higher in OOA (297 ± 4 mOsm/kg), followed by OA (294 ± 7 mOsm/kg), MA (292 ± 4 mOsm/kg), and YA (289 ± 2 mOsm/kg). YA drank sooner (12 ± 11 min) than MA (20 ± 31 min) and OA (30 ± 58 min), but not OOA (19 ± 16 min). Thirst perception was higher in MA (90 ± 33 mm) followed by YA (85 ± 29 mm), OA (81 ± 44 mm), and OOA (76 ± 39 mm). MA consumed more water (1.3 ± 0.5 L) than OA (1.0 ± 0.7 L), YA (0.9 ± 0.3 L) and OOA (0.9 ± 0.5 L). Sweat losses were higher in MA (1.4 ± 0.3 L) followed by OA (1.3 ± 0.4 L), OOA (1.3 ± 0.3 L), and YA (1.0 ± 0.2 L). Consequently, MA (93 ± 44%) and YA (90 ± 28%) more effectively replaced their sweat losses compared to OA (80 ± 63%) and OOA (72 ± 39%), leading to greater dehydration in OOA (0.8 ± 0.7%) compared with OA (0.6 ± 0.8%), MA (0.6 ± 0.7%), and YA (0.5 ± 0.4%). Conclusion: These preliminary data suggest that older adults, particularly the oldest old, are less effective at replacing sweat losses during heat exposure combined with light physical exercise, leading to greater dehydration. Funding: Canadian Institutes of Health Research This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.