Blood and urinary measures of hydration status during progressive acute dehydration

Purpose: To determine whether: a) plasma osmolarity (P_osm) is sensitive to small incremental changes in hydration status, b) urine specific gravity (U_sg) can accurately identify a state of euhydration, c) U_sg is a sensitive indicator of a change in hydration status, and d) U_sg correlates with P_osm Methods: Euhydrated (P _osm = 288 ± 4 mOsm·L^-1) subjects (N = 12) were dehydrated by 5% of their body weight via exercise in the heat (40°C, 20% RH). P_osm, urine osmolarity (U_osm), and U_sg 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. Results: P_osm increased incrementally with each successive increase in percent body weight loss (%BWL). U_sg was not significantly different from baseline until 3% BML. U_osm was not significantly different from baseline until 5% BWL. _sg correlated moderately (r = 0.46, P> 0.10) with P_osm but reasonably well (r = 0.68, P < 0.02) with U_osm. Conclusions: P _osm accurately identifies a state of euhydration and is sensitive to changes in hydration status during acute dehydration and rehydration. U_sg and U_osm are also sensitive to changes in hydration status but lag behind during periods of rapid body fluid turnover and therefore correlate only moderately with P_osm during acute dehydration.