Variability Of Morning Urine Specific Gravity (Usg) Measurements And Effects Of Fluid Intake On Midafternoon Usg

The measurement of urine specific gravity (USG) has been shown to be a good indicator of hydration status. USG measurements are often used to assess the hydration status of athletes prior to training and competitions and the effectiveness of pre-event fluid intake. PURPOSE: This study assessed the day to day variability of morning USG measures and the USG responses to fluid ingestion in the afternoon. METHODS: Ten recreational athletes (3 males, 7 females, 79.2 + 4.4 kg, 26.9 + 0.9 yrs) volunteered to monitor their morning body mass (kg) and hydration status (USG) for five consecutive weekdays. On two occasions, separated by 48 hrs, participants arrived at the laboratory in mid-afternoon following ad libitum drinking during the day. They drank 300 ml of water at time (t) = 0 min and an additional 300 ml at t = 15 min. Body mass, urine volume and USG were recorded before fluid intake at t = 0 and following fluid intake at t = 30, 45 and 60 min RESULTS: Mean daily USG over the 5 consecutive days was 1.017 + 0.002. The variability of morning Usg (CV = 0.17 %) and body mass (CV = 0.27 %). Subjects consumed an average of 600 + 121 ml of fluid ad libitum in the 3 hrs prior to each trial. Subjects arrived at the laboratory mid-afternoon and were well hydrated (mean USG 1.012 + 0.002) and after drinking 600 ml of fluid, USG decreased to 1.008 + 0.003, 1.004 + 0.002, and 1.003 + 0.001 at 30, 45, and 60 min. The results from the second afternoon trial produced mean Usg data that was identical to the first afternoon trial. Repeatability was consistent between trials. Pre-trial fluid intake was negatively correlated with pre-trial Usg (r = 0.54). CONCLUSIONS: This study demonstrated that morning Usg and body mass measurements are consistent between days. The ingestion of 600 ml of fluid midafternoon, following a day of ad libitum drinking, significantly decreases urine Usg.