24-h Void number as an indicator of hydration status

BACKGROUND/OBJECTIVES: Few user-friendly hydration assessment techniques exist for the general population to use on a daily basis. The present study evaluated void number over 24 h as a potential hydration assessment tool. SUBJECTS/METHODS: Male and female subjects collected urine for 24 h while adequately hydrated (n = 44; 22 +/- 4 years, 168 +/- 16 cm, 73 +/- 15 kg) or fluid restricted n = 43; 22 +/- 3 years, 175 +/- 10 cm, 81 +/- 24 kg). As a control, participants were asked to void when feeling the 'first urge to void' on a commonly used urge scale and noted the volume of each void. For each sample, 24-h urine volume, osmolality (UOSM), specific gravity (USG) and color were measured in the laboratory. RESULTS: As designed, the level of urge upon voiding was consistent throughout the study (2 +/- 0; 'first urge to void'). Samples were classified by USG as either euhydrated (USG < 1.020) or hypohydrated (USG < 1.020). Grouping by UOSM did not change results. Euhydrated versus hypohydrated individuals had greater 24-h urine volume (1933 +/- 864 versus 967 +/- 306 ml, respectively) and lower urine color (2 +/- 1 versus 5 +/- 1), USG (1.012 +/- 0.004 versus 1.025 +/- 0.004) and UOSM (457 +/- 180 versus 874 +/- 175 mOsm/kg H2O; all P < 0.001). Euhydrated individuals voided more than hypohydrated individuals over the 24-h period (5 +/- 2 versus 3 +/- 1 voids; P < 0.001). Additionally, void number inversely correlated with hydration status as identified by USG (r = -0.50; P < 0.05) and UOSM (r = -0.56; P < 0.05). CONCLUSIONS: In conclusion, over 24 h, individuals with a higher void number were euhydrated (that is, had less concentrated hydration biomarkers) than those with a lower void number. Based on these data, void number might be utilized as a simple and feasible hydration assessment for the general public, as it utilizes no equipment or technical expertise.