title: Evaluation of Uosm:Posm ratio as a hydration biomarker in free-living, healthy young women.
abstract: OBJECTIVE
  Urinary and plasma indices are utilized to assess whole-body water balance in healthy adults, whereas the urine-to-plasma osmolality ratio (Uosm:Posm) rarely is. To explore the efficacy of Uosm:Posm as a hydration biomarker, diet records of 120 college women were analyzed (beverage water+food water=total fluid intake (TFI); 5 days) to identify habitual high-volume (HIGH) and low-volume (LOW) drinkers.
  
  
  METHODS
  The experimental protocol first involved two ad libitum baseline days for HIGH (TFI, 3.21 l per 24 h; n=14) and LOW (TFI, 1.64 l per 24 h; n=14). During a controlled intervention (days 3-6), mineral water was the only beverage; HIGH consumed less than baseline (TFI, 2.00 l per 24 h), and LOW consumed more than baseline (TFI, 3.50 l per 24 h). During ad libitum recovery (day 7), TFI were 3.17 and 1.71 l per 24 h for HIGH and LOW, respectively. Duplicate Uosm (24 h collection) and Posm (morning) samples were analyzed on all days via freezing point depression osmometry.
  
  
  RESULTS
  In the evaluation of relative water excess (Uosm:Posm<1.0), 11/13 values occurred for HIGH on days 1, 2 and 7; for LOW, 28/29 occurred on intervention days 3-6. Chi-squared analysis indicated that the treatment and Uosm:Posm were significantly associated (χ(2)1:0.001=23.5, P<0.001). Statistical regression analyses detected a strong, significant relationship between renal free-water clearance (FWC) and Uosm:Posm (r(2)=0.86, P<0.00001); this was not true for FWC and Posm (r(2)=0.00, P=0.40) because Posm values were stable across 7 days.
  
  
  CONCLUSIONS
  These findings support the use of Uosm:Posm as a hydration biomarker.
authors.affiliation: Department of Kinesiology, University of Connecticut, Storrs, CT, USA. lawrence.armstrong@uconn.edu
authors.forename: L E
authors.initials: LE
authors.lastname: Armstrong
authors.forename: E C
authors.initials: EC
authors.lastname: Johnson
authors.forename: C X
authors.initials: CX
authors.lastname: Munoz
authors.forename: L
authors.initials: L
authors.lastname: Le Bellego
authors.forename: A
authors.initials: A
authors.lastname: Klein
authors.forename: A L
authors.initials: AL
authors.lastname: McKenzie
authors.forename: D J
authors.initials: DJ
authors.lastname: Casa
authors.forename: C M
authors.initials: CM
authors.lastname: Maresh
mesh_terms: D001628:Beverages; D015415:Biomarkers / Q000097:blood* / Q000652:urine*; D004326:Drinking / Q000502:physiology*; D005260:Female; D006801:Humans; D007668:Kidney / Q000378:metabolism; D016014:Linear Models; D009994:Osmolar Concentration; D014867:Water; D014882:Water-Electrolyte Balance; D055815:Young Adult
chemical_list: D015415:Biomarkers; D014867:Water