title: Equivalence of afternoon spot and 24-h urinary hydration biomarkers in free-living healthy adults.
abstract: Urinary biomarkers of hydration (urine osmolality, UOsm; urine specific gravity, USG) follow circadian variations. For individuals, researchers and health-care professionals, there is value in identifying time frames during which spot values of UOsm and USG are representative of 24-h values in healthy young adults.
  
  
  
  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. UOsm and USG 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 (UOsm) and within 0.003 units (USG) of 24-h values.
  
  
  
  For both UOsm and USG, voids between 1400 and 2000 hours produced values that were equivalent to the 24-h sample, whereas earlier voids tended to overestimate 24-h UOsm and USG. For windows 1401-1600 hours, 1601-1800 hours and 1801-2000 hours, the mean difference (95% confidence interval) between spot and 24-h UOsm (mOsm/kg) was -25 (-72; 22), 28 (-35; 92) and 12 (-41; 66), respectively, whereas for USG the difference was 0.0014 (-0.0028; -0.0001), 0.0001 (-0.0017; 0.0019) and 0.0005 (-0.0018; 0.0009), respectively.
  
  
  
  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.
authors.affiliation: Danone Research, Palaiseau, France.
authors.forename: J H
authors.initials: JH
authors.lastname: Bottin
authors.affiliation: Danone Research, Palaiseau, France.
authors.forename: G
authors.initials: G
authors.lastname: Lemetais
authors.affiliation: Danone Research, Palaiseau, France.
authors.forename: M
authors.initials: M
authors.lastname: Poupin
authors.affiliation: Danone Research, Palaiseau, France.
authors.forename: L
authors.initials: L
authors.lastname: Jimenez
authors.affiliation: Danone Research, Palaiseau, France.
authors.forename: E T
authors.initials: ET
authors.lastname: Perrier
mesh_terms: D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D000074267:Organism Hydration Status / Q000502:physiology*; D009994:Osmolar Concentration; D013046:Specific Gravity; D013997:Time Factors; D014556:Urine / Q000502:physiology*; D059349:Urine Specimen Collection / Q000379:methods / Q000706:statistics & numerical data*; D055815:Young Adult