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Purpose: Human daily total water intake (TWI) has a large inter-individual range. Recently, water supplementation has been suggested as a potential preventative and therapeutic modality. Thus, we aimed to measure hydration biomarkers in women with high (HIGH) versus low (LOW) daily TWI to determine baseline differences, and the efficacy of these markers during a systematic alteration in TWI. Methods: This cohort study identified 14 HIGH [3.34 (0.56) L day] and 14 LOW [1.62 (0.48) L day] from 120 women. Next, fluid intake was decreased in HIGH [2.00 (0.21) L day] while LOW increased [3.50 (0.13) L day] across 4 days. Body mass, fluid intake, serum osmolality ( S), total plasma protein (TPP), 24 h urine osmolality, and 24 h urine volume, were measured on each day of modified TWI. Estimated plasma volume ( E) was calculated using measured body mass and hematocrit values. Results: At baseline, urinary markers and TPP differentiated HIGH from LOW [7.0 (0.3) versus 7.3 (0.3) mg dL, respectively]. Upon TWI intervention, (1) body mass decreased in HIGH [−0.7 (1.1) kg, p = 0.010)] but did not increase in LOW [+0.0 (0.6) kg, p = 0.110], (2) E decreased 2.1 (2.4) %, p = 0.004, (3) urine osmolality increased in HIGH [397 (144)-605 (230) mOsm kg, p < 0.001] and decreased in LOW [726 (248)-265 (97) mOsm kg p < 0.001], and (4) no changes of serum osmolality occurred in either HIGH or LOW (all p > 0.05). Conclusions: Urinary markers and TPP are sensitive measures to habitual high and low TWI and to changes in TWI. Both groups through urinary and some hematological responses following TWI manipulation achieved regulation of hemoconcentration. [ABSTRACT FROM AUTHOR]

While associations exist between water, hydration, and disease risk, research quantifying the dose-response effect of water on health is limited. Thus, the water intake necessary to maintain optimal hydration from a physiological and health standpoint remains unclear. The aim of this analysis was to derive a 24 h urine osmolality (UOsm) threshold that would provide an index of “optimal hydration,” sufficient to compensate water losses and also be biologically significant relative to the risk of disease. Ninety-five adults (31.5 ± 4.3 years, 23.2 ± 2.7 kg·m−2) collected 24 h urine, provided morning blood samples, and completed food and fluid intake diaries over 3 consecutive weekdays. A UOsm threshold was derived using 3 approaches, taking into account European dietary reference values for water; total fluid intake, and urine volumes associated with reduced risk for lithiasis and chronic kidney disease and plasma vasopressin concentration. The aggregate of these approaches suggest that a 24 h urine osmolality ≤500 mOsm·kg−1 may be a simple indicator of optimal hydration, representing a total daily fluid intake adequate to compensate for daily losses, ensure urinary output sufficient to reduce the risk of urolithiasis and renal function decline, and avoid elevated plasma vasopressin concentrations mediating the increased antidiuretic effort. [ABSTRACT FROM AUTHOR]