Equivalence of afternoon spot and 24-h urinary hydration biomarkers in free-living healthy adults

Abstract

Background/Objectives:Urinary biomarkers of hydration (urine osmolality, U Osm; urine specific gravity, U SG) follow circadian variations. For individuals, researchers and health-care professionals, there is value in identifying time frames during which spot values of U Osm and U SG are representative of 24-h values in healthy young adults.Subjects/Methods:Eighty-two free-living adults (22.3±2.9 years, 22.2±1.5 kg/m2) collected individual urine voids over a 24-h period. U Osm and U SG were measured on each void and on the pooled 24-h sample. To determine the time of day when a spot sample was likely to be equivalent to the 24-h value, daytime voids were binned by time and equivalence was tested for each 2-h window. Equivalence was a priori defined as being within 100 mOsm/kg (U Osm) and within 0.003 units (U SG) of 24-h values.Results:For both U Osm and U SG, voids between 1400 and 2000 hours produced values that were equivalent to the 24-h sample, whereas earlier voids tended to overestimate 24-h U Osm and U SG. For windows 1401-1600 hours, 1601-1800 hours and 1801-2000 hours, the mean difference (95% confidence interval) between spot and 24-h U Osm (mOsm/kg) was-25 (-72; 22), 28 (-35; 92) and 12 (-41; 66), respectively, whereas for U SG the difference was 0.0014 (-0.0028;-0.0001), 0.0001 (-0.0017; 0.0019) and 0.0005 (-0.0018; 0.0009), respectively.Conclusions:In free-living healthy French adults, 24-h urine concentration can be approximated from a mid-to late-afternoon spot urine sample. This finding suggests that an afternoon sample may be an accurate and practical tool for hydration monitoring, useful to individuals and health-care practitioners.