Evaluation of Uosm:Posm ratio as a hydration biomarker in free-living, healthy young women

Background/objectives: Urinary and plasma indices are utilized to assess whole-body water balance in healthy adults, whereas the urine-to-plasma osmolality ratio ( U osm : P osm ) rarely is. To explore the efficacy of U osm : P osm 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 U osm (24 h collection) and P osm (morning) samples were analyzed on all days via freezing point depression osmometry. Results: In the evaluation of relative water excess ( U osm : P osm