Seven days of warm water immersion modulate baseline fluid-regulatory hormones without altering acute multi-system physiological stress responses in older males

Older adults exhibit heightened vulnerability to heat-related morbidity, yet it remains unclear whether short-term heat acclimation modifies integrative physiological stress responses in aging. We tested whether seven days of passive heat acclimation via warm water immersion alters basal concentrations and acute exercise-heat stress responses of biomarkers reflecting fluid regulation, gastrointestinal epithelial integrity, renal stress, and inflammation in older males. Twelve habitually active older males (median [IQR] age: 68 [64-73] years; V̇O 2 peak: 34.1 [29.4-36.1] mL O 2 ·kg -1 ·min -1 ) completed an exercise-heat stress test before and after seven consecutive days of warm water immersion (~40°C), with rectal temperature maintained at ~38.5°C for the final 60 min. Blood samples were collected before and after each exercise-heat stress test and analysed using linear mixed-effects models (Day x Time). Repeated warm water immersion increased resting renin by 68% [95% CI: 29, 120; P<0.001], while neutrophil gelatinase-associated lipocalin and lipopolysaccharide binding protein decreased by 24% [-40, -4; P=0.023] and 22% [-37, -3; P=0.026] respectively. Exercise heat stress increased aldosterone by 45% [12, 87%, P=0.004], renin by 41% [8, 84%, P=0.01], intestinal fatty acid-binding protein by 52% [26, 85%, P<0.001], and plasma osmolality by 5.6% [2.5, 8.7%, P<0.001]. However, there was no day x time interaction for any biomarker (all P>0.05), indicating that seven days of warm water immersion did not alter the magnitude of the exercise heat stress response. Short-term passive heat acclimation in older males induces selective fluid-regulatory endocrine adaptations consistent with preserved renin-angiotensin-aldosterone system plasticity, while gastrointestinal and renal stress responses to exercise-heat exposure remain unchanged.