Urinary markers of hydration during 3-day water restriction and graded rehydration

Purpose This investigation had three purposes: (a) to evaluate changes in hydration biomarkers in response to a graded rehydration intervention (GRHI) following 3 days of water restriction (WR), (b) assess within-day variation in urine concentrations, and (c) quantify the volume of fluid needed to return to euhydration as demonstrated by change in Ucol. Methods 115 adult males and females were observed during 1 week of habitual fluid intake, 3 days of fluid restriction (1000 mL day−1), and a fourth day in which the sample was randomized into five different GRHI groups: no additional water, CON; additional 500 mL, G+0.50; additional 1000 mL, G+1.00; additional 1500 mL, G+1.50; additional 2250 mL, G+2.25. All urine was collected on 1 day of the baseline week, during the final 2 days of the WR, and during the day of GRHI, and evaluated for urine osmolality, color, and specific gravity. Results Following the GRHI, only G+1.50 and G+2.25 resulted in all urinary values being significantly different from CON. The mean volume of water increase was significantly greater for those whose Ucol changed from > 4 to