Post-Exercise Voluntary Drinking Cessation Is Associated with the Normalization of Plasma Osmolality and Thirst Perception, but Not of Urine Indicators or Net Fluid Balance

Post-exercise rehydration has been widely studied, with particular emphasis on retention of ingested fluid; comparatively little research has been conducted on why we drink more or less. To identify physiological values corresponding to voluntary drinking cessation (VDC), nine males exercised intermittently at 70-80% HRmax in the heat (WBGT = 28.1 +/- 0.7 degrees C) to achieve a dehydration of approximately 4.0% body mass (BM). After exercise, participants were instructed to drink water as long and as much as they needed. Urine color (U-color), specific gravity (USG), osmolality (U-osm), plasma osmolality (P-osm), fullness, BM, and thirst perception (TP) were measured pre- and post-exercise and at VDC. Each variable was compared for the three points in time with a one-way ANOVA. Participants reached dehydration of -3.6 +/- 0.3% BM. Pre-exercise USG (1.022 +/- 0.004) was lower than at VDC (1.029 +/- 0.004, p = 0.022), U-osm did not change over time (p = 0.217), and U-color was lower pre-exercise (3.4 +/- 0.7) vs. post-exercise (5.5 +/- 1.23, p = 0.0008) and vs. VDC (6.3 +/- 1.1, p < 0.0001). P-osm showed a difference between pre-exercise (289.5 +/- 2.3) and post-exercise (297.8 +/- 3.9, p = 0.0006) and between post-exercise and VDC (287.3 +/- 5.4, p < 0.0001). TP post-exercise (96.4 +/- 4.34) was significantly higher than pre-exercise (36.2 +/- 19.1) and VDC (25.0 +/- 18.2, p < 0.0001). At VDC, participants had recovered 58.7 +/- 12.1% of BM loss. At the point of voluntary drinking cessation, P-osm and thirst perception had returned to their pre-exercise values, while rehydration relative to initial BM was still incomplete.