Circadian variation and responsiveness of hydration biomarkers to changes in daily water intake

Abstract

Biomarkers of hydration change in response to acute dehydration; however, their responsiveness to changes in fluid intake volume, without exercise or heat exposure, has not been adequately described. Moreover, patterns of circadian variation in hydration biomarkers have not been established. The study aims were to (1) assess the response of hydration biomarkers to changes in daily water intake; and (2) evaluate circadian variation in urinary and salivary biomarkers. Fifty-two adults (24.8 ± 3.1 years; 22.3 ± 1.6 kg/m2; 79 % female), grouped based on habitual fluid intake (low drinkers, n = 30, <1.2 L/day; high drinkers, n = 22, >2.0 L/day), completed a 5-day inpatient crossover trial. On days 1 and 2, low drinkers received 1.0 L/day of water while high drinkers received 2.5 L/day. On days 3 through 5, intake was reversed between groups. Plasma and saliva osmolality were assessed daily at predetermined times, and all urine produced over 24 h was collected in timed intervals. ANOVA with intake (1.0 vs. 2.5 L/day), day, and time revealed that (1) urine concentration (osmolality, specific gravity, color) and volume, but not plasma nor saliva osmolality, responded to changes in water intake; (2) urinary hydration biomarkers and saliva osmolality vary as a function of the time of day; and (3) urine osmolality measured in samples collected during the afternoon most closely reflects the corresponding 24 h value. Overall, urinary hydration biomarkers are responsive to changes in water intake, and stabilize within 24 h of modifying intake volume. Moreover, short afternoon urine collections may be able to replace 24 h collections for more convenience in hydration assessment.

Validation of Surrogates of Urine Osmolality in Population Studies

“…It is important to note that U osm varies greatly among different subjects, as shown in the 4 populations of the present study and in a few previous reports [31,42]. In usual conditions, some subjects produce hypo-osmotic urine while others show U osm up to 1,200 mosm/kg H 2 O.…”

“…Recent experimental and epidemiological findings have renewed the interest in the components of the water balance and in the parameters reflecting this integrative function [1,4,9,18,22,24,28,29,32,42,43]. It is indeed quite different for the kidney to excrete a daily osmolar load of 900 mosm in 1 L of urine at 900 mosm/L or in 3 L of urine at 300 mosm/L.…”

“…In usual conditions, some subjects produce hypo-osmotic urine while others show U osm up to 1,200 mosm/kg H 2 O. This wide range of spontaneous U osm is possibly due to large inter-individual variations in the daily solute load [48], fluid intake [42], and thresholds for AVP secretion and/or thirst that are, in part, genetically determined [49]. Both AVP concentration and U osm are known to differ between sexes.…”

Prevalence of Inadequate Hydration Among US Children and Disparities by Gender and Race/Ethnicity: National Health and Nutrition Examination Survey, 2009–2012

“…We estimated linear regression models, taking into account the complex survey design, using PROC SURVEYREG (SAS version 9.3; SAS Institute, Cary, NC) to determine whether mean urine osmolality differed by age, race/ ethnicity, gender, and income, as well as a multivariable adjusted model that also included time of examination (morning vs afternoon or evening), given circadian variations in urine osmolality. 20 We estimated logistic regression models using PROC SURVEYLOGISTIC to evaluate demographic differences in risk of inadequate hydration, also controlling for all variables above. Lastly, we estimated the relationship between beverage intake for the day prior to the examination and urine osmolality using linear regression models, first evaluating the crude relationship between each beverage individually and urine osmolality and then simultaneously adjusting for other beverage and moisture from food intake as well as age, race/ethnicity, gender, income, time of examination, and BMI.…”

Effects of maternal hydration status on the osmolality of maternal milk

“…Given the importance of this observation, subsequent literature on hydration status in lactating women was scarce in our literature search. A range of techniques can be used to estimate hydration status, from the color of the urine and its specifi c gravity, to daily liquid intake or urine output, to more refi ned laboratory techniques involving freezing-point depression osmometry of plasma or urine (7,8). The urine specimen can be derived from a causal spot sample or from a 24-h quantitative collection.…”