1, 2 Recently, we proposed a 24-h U Osm of ⩽500 mOsm/kg as a reasonable target for urine concentration, reflecting sufficient total water intake to compensate daily losses, reduce circulating vasopressin and ensure sufficient urinary output to reduce the risk of some renal health outcomes. Specifically, urine osmolality ( U Osm) is the most precise, non-invasive biomarker available to evaluate the 24-h hydration process, as it represents the net sum of water gains, losses and neuroendocrine responses that act to maintain body water homeostasis, and responds rapidly to changes in daily water intake. Various biomarkers of urine concentration allow for individual-level daily hydration monitoring. Thus, a dietary reference value for the general population is unlikely to have much relevance for the individual.
However, adequate intakes are not linked to specific health outcomes, and daily water needs are highly individual and depend upon environment, activity, diet and other factors. Adequate intakes have been established based upon population median data. Water is essential to life, represents the largest single nutrient in terms of intake, and must be replenished daily through food and fluid consumption. A receiver operating characteristic curve analysis performed on 817 urine samples demonstrates that a U SG ⩾1.013 detects U Osm>500 mOsm/kg with very high accuracy (AUC 0.984), whereas a subject-assessed U Col ⩾4 offers high sensitivity and moderate specificity (AUC 0.831) for detecting U Osm >500 m Osm/kg. In this analysis, we calculate criterion values for urine-specific gravity ( U SG) and urine color ( U Col), two measures which have broad applicability in clinical and field settings. In clinical practice and field monitoring, the measurement of U Osm is not practical. Previously, we have proposed that maintaining a 24-h urine osmolality ( U Osm) of ⩽500 mOsm/kg is a desirable target for urine concentration to ensure sufficient urinary output to reduce renal health risk and circulating vasopressin. Growing evidence suggests a distinction between water intake necessary for maintaining a euhydrated state, and water intake considered to be adequate from a perspective of long-term health.
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