Self-Reported Changes in Thirst and Alertness during Variable Prescribed Fluid Intake

PURPOSE: To evaluate the relationship between self-reported thirst and alertness in people drinking variable amounts of prescribed water. METHODS: Subjects (n = 115, 59 males, 32 ± 10 y; 24.6 ± 4.4 kg·m-2) visited the lab 3 times over 10 days: V1, a baseline visit that prior to participants were drinking ad libitum; V2, following 3 days of fluid restriction (1 L·d-1, of which 250 mL was consumed in the morning prior to the visit); and V3, the morning following a prescribed increase in water intake. The increase in water intake at V3 varied by group assignment: a control group (CON) maintained 250 mL of morning water consumption, while LOW and HIGH intake groups (n = 45 each) consumed 496 ± 82 mL and 878 ± 125 mL, respectively. At each visit, which occurred after the morning water consumption period, subjects indicated on an open-ended visual analog scale (VAS) how thirsty and alert they felt. Two-way ANOVA for thirst and alertness between groups from V1 to V2 and V2 to V3 were completed. Repeated measures correlation procedure was completed for change in alertness and thirst from V1 to V2 and V2 to V3. RESULTS: Groups were similar at baseline (V1) for fluid intake, thirst and alertness (all p ≥ 0.17). Fluid restriction (V2) resulted in a main effect of time for both thirst and alertness (both p < 0.01), with no main effect of group. On average, thirst increased (35 ± 35 mm) and alertness decreased (-19 ± 31 mm) from V1 to V2. The prescribed increase in water intake (V3) revealed a significant interaction of time and group for both thirst and alertness (both p < 0.01). Independent-samples t-tests revealed that HIGH reduced thirst (-38 ± 37 mm) and increased alertness (18 ± 25 mm) significantly more than both LOW (thirst, -7 ± 37 mm; alertness -1 ± 24 mm) and CON (thirst, -6 ± 23 mm; alertness 0 ± 23 mm; all p < .01). There was no difference between LOW and CON (both p > 0.92). Repeated measures correlation analysis showed a negative relationship between change in alertness and thirst (R2 = 0.29, p < 0.01). CONCLUSION: An inverse relationship was observed between self-reported alertness and thirst. Following fluid restriction, drinking a larger volume of water (750-1000 mL) in the morning decreased thirst and increased alertness. Investigation funded by Danone Research