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 U(col). 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 U(col) changed from > 4 to < 4 (+ 1435 ± 812 mL) than those whose U(col) remained ≥ 4 (+ 667 ± 722 mL, p < 0.001). CONCLUSIONS: An additional 500 mL of water is not sufficient, while approximately 1500 mL of additional water (for a total intake between 2990 and 3515 mL day(−1)) is required to return to a urine color associated with adequate water intake, following 3 days of WR.