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Blood and urinary measures of hydration status during progressive acute dehydration
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2001
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The study aimed to assess whether plasma osmolarity and urine specific gravity are sensitive indicators of incremental hydration changes and whether Usg correlates with Posm in healthy adults undergoing controlled dehydration. Researchers measured plasma osmolarity, urine osmolarity, and urine specific gravity at 1%, 3%, and 5% body‑weight loss and at 30‑ and 60‑minute recovery, with participants rehydrating fully to match weight loss. Plasma osmolarity accurately identified euhydration and sensitively tracked dehydration and rehydration, whereas urine specific gravity and urine osmolarity lagged, showing significant changes only at 3% and 5% loss respectively and correlating only moderately with plasma osmolarity.
To determine whether: a) plasma osmolarity (Posm) is sensitive to small incremental changes in hydration status, b) urine specific gravity (Usg) can accurately identify a state of euhydration, c) Usg is a sensitive indicator of a change in hydration status, and d) Usg correlates with Posm.Euhydrated (Posm = 288 +/- 4 mOsm.L-1) subjects (N = 12) were dehydrated by 5% of their body weight via exercise in the heat (40 degrees C, 20% RH). Posm, urine osmolarity (Uosm), and Usg were measured at 1%, 3%, and 5% dehydration, and 30 and 60 min of recovery (rec). Subjects consumed water in recovery equal to their loss of body weight.Posm increased incrementally with each successive increase in percent body weight loss (%BWL). Usg was not significantly different from baseline until 3% BML. Uosm was not significantly different from baseline until 5% BWL. Usg correlated moderately (r = 0.46, P > 0.10) with Posm but reasonably well (r = 0.68, P < 0.02) with Uosm.Posm accurately identifies a state of euhydration and is sensitive to changes in hydration status during acute dehydration and rehydration. Usg and Uosm are also sensitive to changes in hydration status but lag behind during periods of rapid body fluid turnover and therefore correlate only moderately with Posm during acute dehydration.