Blood and urinary measures of hydration status during progressive acute dehydration.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Popowski
L A
LA
Department of Exercise Science, University of Iowa, Iowa City, IA 52242, USA.
Oppliger
R A
RA
Patrick Lambert
G
G
Johnson
R F
RF
Kim Johnson
A
A
Gisolf
C V
CV
D000328:Adult; D003681:Dehydration / Q000503:physiopathology*; D015444:Exercise; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D011934:Reagent Strips; D012680:Sensitivity and Specificity; D016482:Urinalysis; D014882:Water-Electrolyte Balance
D011934:Reagent Strips