Twenty-four-hour urine osmolality as a physiological index of adequate water intake

Abstract

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 (U(Osm)) 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 U(Osm) 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.

Figure 1

Distributions of 24 h total fluid intake, urine volume, and urine osmolality.

Figure 2

ROC analysis curve for urine osmolality as an indicator of fluid intake meeting EFSA fluid intake guidelines. AUC = 0.895; optimal cutoff 544 mOsm·kg⁻¹.

Figure 3

(a) Mean (95% CI) 24 h urine osmolality and (b) proportion of urine osmolality measures above (light grey) and below (dark grey) 500 mOsm·kg⁻¹ according to urine volume categories reported by Clark et al. [7].

Figure 4

From left to right: mean (standard deviation) plasma AVP concentration grouped by 24 h U_Osm (<500; 500–800; >800 mOsm·kg⁻¹); plasma AVP reported before and after 12 h of total fluid deprivation [25]; plasma AVP reported before and after 24 hours of total fluid deprivation [26].