The Effect Of Morning Thirst On Afternoon Hydration Status

Thirst sensation is an important stimulus for drinking behavior; however, the effect of thirst sensation on later hydration status remains unclear. PURPOSE: To investigate the effects of morning (M) thirst on afternoon (A) hydration status. METHODS: Twelve men (mean ± standard deviation; age: 21 ± 2 years; mass: 81.0 ± 15.9 kg) and twelve women (age: 22 ± 3 years; mass: 68.8 ± 15.2 kg) visited the laboratory in the morning (first morning) and afternoon (2:00-4:00 pm) for three consecutive days under a free-living condition. At each visit, participants provided a urine sample where urine indices were analyzed (urine specific gravity [USG], urine color [UCOL], urine osmolality [UOSMO]), and nude body mass was collected to calculate body mass loss (BML). Then, thirst was assessed with a Likert scale, and a blood sample was collected to analyze plasma osmolality (POSMO), hemoglobin, and hematocrit to calculate % plasma volume change (PV). Participants recorded food and fluid intake between the morning and afternoon visits to determine total water intake (TWI). Linear regression was used to predict thirst from the M on hydration indices in the A. Also, a stepwise linear regression predicted thirst in the A from hydration indices in the M. Pearson’s product moment correlation was used to calculate the relationship between TWI and hydration markers. RESULTS: Higher M thirst significantly predicted lower UOSMO (r2=0.056, p=0.045), USG (r2=0.096, p=0.008), UCOL (r2=0.074, p=0.021), and higher thirst (r2=0.074, p=0.021) in the A. However, M thirst did not predict BML, PV, POSMO, and TWI in the A (p>0.05). Increased thirst and BML in the M together significantly predicted higher thirst (r2=0.125, p=0.010) in the A. Increased TWI is associated with lower UCOL (r=0.336, p=0.004) and BML (r=0.297, p=0.011) in the A. However, TWI is not associated with any variables in the M or the remaining variables in the A, including USG and UOSMO (p>0.05). CONCLUSION: Increased morning thirst impacts afternoon urine indices and thirst. However, morning thirst did not influence TWI between the morning and the afternoon, and TWI was not associated with morning hydration status. Therefore, afternoon hydration status might be impacted by morning thirst, although individuals might not consume fluid based on their morning thirst or hydration status.