Background/objectives: 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.
Subjects/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.