Self-Reported Changes in Thirst and Alertness during Variable Prescribed Fluid Intake
Authors: Yoder Hillary A. , Huffman Ainsley E. , McCullough Shane , Johnson Evan C. 
Published: February 2022

<b><i>Background:</i></b> Maintaining euhydration is beneficial for health, safety, and physical performance [<xref ref-type="bibr" rid="ref1">1</xref>]; however, it may also improve subjective feelings [<xref ref-type="bibr" rid="ref2">2</xref>, <xref ref-type="bibr" rid="ref3">3</xref>]. <b><i>Objective:</i></b> The aim of this study was to evaluate the relationship between changes in self-reported thirst and alertness in people undergoing changes in drinking water volume. <b><i>Methods:</i></b> Subjects (mean ± SD) (<i>n</i> = 115, 59 males, 32 ± 10 years; 24.6 ± 4.4 kg·m<sup>−2</sup>) visited the lab 3 times over 10 days: V1, a baseline visit prior to participants were drinking ad libitum; V2, following 3 days of fluid restriction (1 L·d<sup>−1</sup>, 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: control group (CON) maintained 250 mL, while LOW and HIGH groups (<i>n</i> = 45 each) consumed 496 ± 82 mL and 878 ± 125 mL, respectively. At each visit, subjects indicated on an open-ended visual analog scale (VAS) how thirsty and alert they felt and were measured in millimeters (mm). Four, two-way ANOVAs (group × visit) for change in thirst and alertness between V1–V2 and V2–V3 were completed. A repeated-measures correlation (<i>r</i><sub>rm</sub>) procedure was completed for change in alertness and thirst from V1 to V2 and V2 to V3 [<xref ref-type="bibr" rid="ref4">4</xref>]. The study was approved by the University of Wyoming’s Institutional Review Board (protocol #20160524EJ01208), and all subjects provided written informed consent. <b><i>Results:</i></b> Groups were similar at baseline (V1) for fluid intake, thirst, and alertness (all <i>p</i> ≥ 0.17). Fluid restriction (V2) resulted in a main effect of visit for thirst and alertness (both <i>p</i> &#x3c; 0.01), with no main effect of group. 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 visit and group for thirst and alertness (both <i>p</i> &#x3c; 0.01) (Table <xref ref-type="table" rid="t01">1</xref>). Independent-samples <i>t</i> tests with a Bonferroni correction revealed that HIGH reduced thirst (−38 ± 37 mm) and increased alertness (18 ± 25 mm), while no change was observed for LOW (thirst, −7 ± 37 mm; alertness −1 ± 24 mm) and CON (thirst, −6 ± 23 mm; alertness 0 ± 23 mm; all <i>p</i> &#x3c; 0.01) (Fig. <xref ref-type="fig" rid="f01">1</xref>). There was no difference between LOW and CON (both <i>p</i> &#x3e; 0.92). Repeated-measures correlation analysis revealed an inverse relationship between change in alertness and thirst (<i>r</i><sub>rm</sub> [114] = −0.53, 95% CI [−0.65, −0.38], <i>p</i> &#x3c; 0.01). <b><i>Conclusion:</i></b> A reduction in water intake resulted in an increase in thirst and decrease in alertness. Following 3 days of fluid restriction, 750–1,000 mL of water intake was needed to decrease thirst and increase alertness. Overall, an inverse relationship was observed between self-reported thirst and alertness.