Habitual fluid intake and hydration status influence cortisol reactivity to acute psychosocial stress

Shared pathways connect water regulation and cortisol release, and exaggerated cortisol reactivity to stress is associated with poor long-term health. This study investigated the unknown influence of habitual fluid intake and hydration status on saliva cortisol reactivity to psychosocial stress. After screening 62 healthy males and females and adopting low and high fluid intake quartiles from a national database, we identified 16 individuals with habitual low (LOW: 1.3 ± 0.4 L/day) and 16 with habitual high fluid intake (HIGH: 4.4 ± 1.2 L/day) who were comparable for factors likely to influence stress reactivity (e.g., trait anxiety). In pairs (one LOW and one HIGH), participants underwent 7 days of monitored habitual fluid intake. The following day, participants individually completed the Trier Social Stress Test (TSST). Hydration status was assessed in the days preceding and on the day of the TSST (urine osmolality and color; UOsm, UCol). Both UOsm and UCol were greater in LOW (<i>P</i> < 0.001). The TSST evoked similar increases in state anxiety and heart rate in LOW and HIGH, yet saliva cortisol increased significantly post-TSST only in LOW (group × time interaction, <i>P</i> = 0.02). Moreover, cortisol reactivity was greater in LOW (Δ, 6.2 ± 2.9 vs. HIGH: 4.0 ± 1.8 nmol/L; <i>P</i> = 0.03, Cohen's <i>d</i> = 0.9) and was associated with hydration status assessed as UOsm (Pearson <i>R</i> = 0.7, <i>P</i> < 0.001). These novel findings show greater cortisol reactivity to acute psychosocial stress in adults with habitual low fluid intake and suboptimal hydration, which may influence long-term health. Clinical Trial Registry Number: NCT05491122.<b>NEW & NOTEWORTHY</b> Cortisol reactivity to acute stress predicts long-term health. Our novel findings show greater cortisol reactivity to acute stress in adults with habitual low fluid intake. Suboptimal hydration (e.g., darker morning urine) was associated with greater cortisol reactivity to acute stress. These findings provide one possible explanation for why habitual low fluid intake and suboptimal hydration are related to poor long-term health. Furthermore, researchers should account for hydration when assessing cortisol reactivity to acute stressors.