Relationships between Morning Thirst and Later Hydration Status and Total Water Intake

Purpose: To investigate the effects of thirst on later hydration status, total water intake (TWI-MA), and its potential sex differences. 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 thing in the morning) and afternoon (2:00-4:00 p.m.) for three consecutive days under a free-living condition. At each visit, urine osmolality (U_OSM), urine specific gravity (USG), urine color (U_COL), body mass loss (BML), thirst, and plasma osmolality (P_OSM) were collected and analyzed. The participants recorded their food and fluid intake between the visits to determine TWI-MA. Linear regression was used to predict the effect of morning thirst on the afternoon hydration indices for all the participants, as well as for males and females separately. Results: Higher morning thirst predicted lower U_OSM (r² = 0.056, p = 0.045), USG (r² = 0.096, p = 0.008), U_COL (r² = 0.074, p = 0.021), and higher thirst (r² = 0.074, p = 0.021) in the afternoon. However, morning thirst did not predict afternoon BML, P_OSM, or TWI-MA (p > 0.05). In males, higher morning thirst predicted lower afternoon U_OSM (r² = 0.130, p = 0.031) and USG (r² = 0.153, p = 0.018). Additionally, higher morning thirst predicted higher TWI-MA (r² = 0.154, p = 0.018) in females. Conclusions: Morning thirst had a negligible impact on later hydration status, specifically with afternoon urine indices. Furthermore, higher thirst sensation did not impact BML, P_OSM, or TWI-MA. However, thirst sensation minimally contributed to drinking behavior in females. Overall, individuals may not rely solely on thirst sensation to manipulate their drinking behavior to optimize their fluid balance during their daily lives due to the complexity of thirst mechanisms.