Urine specific gravity as an indicator of dehydration in Olympic combat sport athletes; considerations for research and practice

This Position Stand provides guidance on fluid replacement to sustain appropriate hydration of individuals performing physical activity. The goal of prehydrating is to start the activity euhydrated and with normal plasma electrolyte levels. Prehydrating with beverages, in addition to normal meals and fluid intake, should be initiated when needed at least several hours before the activity to enable fluid absorption and allow urine output to return to normal levels. The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. Because there is considerable variability in sweating rates and sweat electrolyte content between individuals, customized fluid replacement programs are recommended. Individual sweat rates can be estimated by measuring body weight before and after exercise. During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water alone under certain circumstances. After exercise, the goal is to replace any fluid electrolyte deficit. The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.

For addressing the influence of muscle mass on serum and urinary creatinine and serum cystatin C, body composition was assessed by skinfold thickness measurement and bioelectrical impedance analyses. A total of 170 healthy individuals (92 women, 78 men) were classified as sedentary or with mild or moderate/intense physical activity. Blood, 24-h urine samples, and 24-h food recall were obtained from all individuals. Serum and urinary creatinine correlated significantly with body weight, but the level of correlation with lean mass was even greater. There was no significant correlation between body weight and lean mass with cystatin C. Individuals with moderate/intense physical activity presented significantly lower mean body mass index (23.1 +/- 2.5 versus 25.7 +/- 3.9 kg/m(2)) and higher lean mass (55.3 +/- 10.0 versus 48.5 +/- 10.4%), serum creatinine (1.04 +/- 0.12 versus 0.95 +/- 0.17 mg/dl), urinary creatinine (1437 +/- 471 versus 1231 +/- 430 mg/24 h), protein intake (1.4 +/- 0.6 versus 1.1 +/- 0.6 g/kg per d), and meat intake (0.7 +/- 0.3 versus 0.5 +/- 0.4 g/kg per d) than the sedentary individuals. Conversely, mean serum cystatin did not differ between these two groups. A multivariate analysis of covariance showed that lean mass was significantly related to serum and urinary creatinine but not with cystatin, even after adjustment for protein/meat intake and physical activity. Cystatin C may represent a more adequate alternative to assess renal function in individuals with higher muscle mass when mild kidney impairment is suspected.

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.