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

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/m(2)) 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.