Fluid replacement and acute systemic inflammatory response syndrome during eight-hour extreme heat exposure in young men

Prolonged exposure to extreme heat poses significant risks, including systemic inflammatory response syndrome (SIRS), organ damage and hormonal imbalance. While fluid replacement is commonly recommended to mitigate these effects, its efficacy under uncompensable heat stress remains unclear. This study investigated the impacts of fluid replacement on thermoregulation, systemic inflammation, organ stress, cortisol levels and plasma electrolyte balance during eight-hour of extreme heat exposure in healthy young men. Twelve participants (age: 24.7{+/-}1.6 years; body surface area: 1.9{+/-}0.1 m{superscript 2}) underwent two randomized trials (dehydration: 125 mL/hour; euhydration: 375 mL/hour) in a heat chamber (40 {degrees}C, 55% RH). Biomarkers of inflammation (e.g., IL-6, IL-1{beta}), oxidative stress (e.g., MDA, SOD), organ function (ALT, BUN), cortisol, and electrolytes (sodium, potassium, chloride) were measured before and after exposure. Core temperature (Tcore) was continuously monitored. Results showed that fluid replacement significantly reduced Tcore at the end of the exposure (38.0{+/-}0.12 {degrees}C vs. 38.2{+/-}0.10 {degrees}C, p=0.046). However, it exacerbated systemic inflammation (IL-6: euhydration 19.8{+/-}4.3 pg/mL vs. dehydration 12.5{+/-}2.8 pg/mL, p<0.01) and liver stress (ALT: euhydration 45.3{+/-}6.7 U/L vs. dehydration 34.1{+/-}5.5 U/L, p=0.03). Cortisol levels decreased significantly in the euhydration group (p=0.041), potentially indicating attenuated stress resilience. Electrolyte imbalances (reduced sodium and potassium concentrations) were observed in the euhydration condition. Taken together, while fluid replacement reduced Tcore, it did not mitigate SIRS and instead exacerbated systemic inflammation, liver stress, and electrolyte imbalances, potentially through hypotonic osmotic stress. These findings underscore the need for personalized hydration strategies that balance fluid and electrolyte intake during extreme heat exposure to minimize health risks. Key pointsO_LIThis study investigated the effects of fluid replacement on acute inflammatory responses, thermoregulation, and organ stress markers in healthy young males exposed to eight hours of uncompensable heat stress. C_LIO_LIFluid replacement did not mitigate acute systemic inflammatory response syndrome (SIRS) but exacerbated inflammatory and organ stress markers despite reducing core temperature. C_LIO_LICore temperatures remained significantly lower in the euhydration group, but systemic and hepatic inflammatory markers worsened, highlighting the complexity of hydration strategies under extreme heat. C_LIO_LIWe emphasize the need for individualized hydration strategies incorporating electrolyte balance and sweat rate monitoring to minimize SIRS risk. C_LIO_LIWe provide actionable insights into heat stress physiology, with implications for occupational health guidelines, public health policies, and climate change adaptation. C_LI