The cortisol awakening response (CAR), the sharp rise in cortisol level upon awakening, represents a unique aspect of hypothalamic pituitary adrenal (HPA) axis activity, combining features of circadian regulation and reactivity to awakening. Studies highlight that the CAR is influenced by state-like factors (e.g., subjective stress and poor sleep) and relates to health and ageing(1). Fluid regulation and the CAR share a common pathway, whereby hydration directly influences the secretion of arginine-vasopressin (AVP), which in-turn modulates HPA-axis activity and cortisol release. Studies have observed higher circulating and saliva cortisol in healthy, low drinking adults presenting with suboptimal hydration (e.g., elevated urine osmolality, UOsm); however, cortisol was only assessed at a single timepoint(2, 3). The study aim was to assess the influence of a change in daily fluid intake on the dynamic CAR.

From a sample of 71 eligible healthy adults (25 females, 46 males), 16 low drinkers (1315 ± 401 ml/day) and 16 high drinkers (4372 ± 1220 ml/day) were identified by adopting daily fluid intake thresholds from a matched UK population (4). In pairs, comprising a low and high drinker, participants underwent a 7-day habitual phase followed by a 7-day intervention phase. During the intervention phase low drinkers increased (+1953 ± 430 ml/day) and high drinkers reduced daily fluid intake (-3168 ± 1379 ml/day), adjusting only water intake. Saliva samples were collected at 0, 15, 30, and 45 minutes after awakening on the last two mornings of each phase, with compliance to sample timing (± 5 minutes) monitored as recommended (1). Saliva cortisol was assessed by ELISA and the CAR was reported as the area under the curve with respect to increase (AUCI) (1). Urine samples were collected the day before and the day of CAR sample collection (16:00-20:00) to assess UOsm.

Linear mixed model revealed a group (low drinkers, high drinkers)*phase (habitual drinking, intervention drinking) interaction for hydration (P < 0.01), whereby UOsm changed significantly in both low drinkers (mean ± SD: 582 ± 213 to 363 ± 200 mOsm/kg) and high drinkers (265 ± 153 to 612 ± 189 mOsm/kg), indicating the success of the intervention. However, no group*phase interaction was observed for the CAR (P = 0.9). The CAR was similar before and after the intervention in both low (AUCI: 15 ± 15 vs. 16 ± 8) and high drinkers (AUCI: 11 ± 14 vs. 12 ± 11), indicating that changing fluid intake did not affect the CAR. These findings remained when observations were restricted to participants who accurately complied to the CAR sampling protocol (N = 19).

These results suggest that the cortisol awakening response, which combines elements of circadian regulation and reactivity to awakening, is not influenced by daily fluid intake.