Evaluation of [U.sub.osm]: [P.sub.osm] ratio as a hydration biomarker in free-living, healthy young women

• Published in: European journal of clinical nutrition Vol. 67; no. 9; p. 934
• Main Authors: Armstrong, L.E, Johnson, E.C, Munoz, C.X, Bellego, L. Le, Klein, A, McKenzie, A.L, Casa, D.J, Maresh, C.M
• Format: Journal Article
• Language: English
• Published: Nature Publishing Group 01.09.2013
• Subjects: Biological markers; Blood plasma; Drinking (Physiology); Physiological aspects; United States
• ISSN: 0954-3007; 1476-5640
• DOI: 10.1038/ejcn.2013.79

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.sub.osm]:[P.sub.osm]) rarely is. To explore the efficacy of [U.sub.osm]:[P.sub.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 24h; 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.sub.osm] (24 h collection) and [P.sub.osm] (morning) samples were analyzed on all days via freezing point depression osmometry. RESULTS: In the evaluation of relative water excess ([U.sub.osm]:[P.sub.osm]< 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 [U.sub.osm]:[P.sub.osm] were significantly associated ([[chi square].sub.1:0.00] = 23.5, P < 0.001). Statistical regression analyses detected a strong, significant relationship between renal free-water clearance (FWC) and [U.sub.osm]:[P.sub.osm] ([r.sup.2] = 0.86, P < 0.00001); this was not true for FWC and [P.sub.osm] ([r.sup.2] = 0.00, P = 0.40) because [P.sub.osm] values were stable across 7 days. CONCLUSIONS: These findings support the use of [U.sub.osm]:[P.sub.osm] as a hydration biomarker. European Journal of Clinical Nutrition (2013) 67, 934-938; doi: 10.1038/ejcn.2013.79; published online 17 April 2013 Keywords: fluid consumption; urine volume; plasma osmolality; urine osmolality; free-water clearance