Little is known about the impact of habitual fluid intake on physiology. Specifically, biomarkers of hydration status and body water regulation have not been adequately explored in adults who consume different fluid volumes in everyday conditions, without prolonged exercise or environmental exposure. The purpose of the present study was to compare adults with habitually different fluid intakes with respect to biomarkers implicated in the assessment of hydration status, the regulation of total body water and the risk of kidney pathologies. In the present cross-sectional study, seventy-one adults (thirty-two men, thirty-nine women, age 25–40 years) were classified according to daily fluid intake: thirty-nine low drinkers (LD; ≤ 1·2 litres/d) and thirty-two high drinkers (HD; 2–4 litres/d). During four consecutive days, urinary parameters (first morning urine (FMU) on day 1 and subsequent 24 h urine (24hU) collections), blood parameters, and food and beverage intake were assessed. ANOVA and non-parametric comparisons revealed significant differences between the LD and HD groups in 24hU volume (1·0 (se 0·1) v. 2·4 (se 0·1) litres), specific gravity (median 1·023 v. 1·010), osmolality (767 (se 27) v. 371 (se 33) mOsm/kg) and colour (3·1 (se 0·2) v. 1·8 (se 0·2)). Similarly, in the FMU, the LD group produced a smaller amount of more concentrated urine. Plasma cortisol, creatinine and arginine vasopressin concentrations were significantly higher among the LD. Plasma osmolality was similar between the groups, suggesting physiological adaptations to preserve plasma osmolality despite low fluid intake. The long-term impact of adaptations to preserve plasma osmolality must be examined, particularly in the context of renal health.

BACKGROUND/OBJECTIVES:In sedentary adults, hydration is mostly influenced by total fluid intake and not by sweat losses; moreover, low daily fluid intake is associated with adverse health outcomes. This study aimed to model the relation between total fluid intake and urinary hydration biomarkers. SUBJECTS/METHODS: During 4 consecutive weekdays, 82 adults (age, 31.6±4.3 years; body mass index, 23.2±2.7 kg/m 2 ; 52% female) recorded food and fluid consumed, collected one first morning urine (FMU) void and three 24-h (24hU) samples. The strength of linear association between urinary hydration biomarkers and fluid intake volume was evaluated using simple linear regression and Pearson's correlation. Multivariate partial least squares (PLS) modeled the association between fluid intake and 24hU hydration biomarkers. RESULTS: Strong associations (|r|X0.6; Po0.001) were found between total fluid intake volume and 24hU osmolality, color, specific gravity (USG), volume and solute concentrations. Many 24hU biomarkers were collinear (osmolality versus color: r ¼ 0.49-0.76; USG versus color: r ¼ 0.46-0.78; osmolality versus USG: 0.86-0.97; Po0.001). Measures in FMU were not strongly correlated to intake. Multivariate PLS and simple linear regression using urine volume explained 450% of the variance in fluid intake volume (r 2 ¼ 0.59 and 0.52, respectively); however the error in both models was high and the limits of agreement very large. CONCLUSIONS: Hydration biomarkers in 24hU are strongly correlated with daily total fluid intake volume in sedentary adults in free-living conditions; however, the margin of error in the present models limits the applicability of estimating fluid intake from urinary biomarkers.

The kinetic parameters of absorption and distribution of ingested water (300 ml labeled with D2O; osmolality <20 mOsm kg−1) in the body water pool (BWP) and of its disappearance from this pool were estimated in 36 subjects from changes in plasma or urine deuterium to protium ratio (D/H) over 10 days using one- and two-compartment and a non-compartmental pharmacokinetic models (1-CM, 2-CM and N-CM which applied well to 58, 42 and 100% of the subjects, respectively). Compared with the volume and turnover of the BWP computed with the slope-intercept method (60.7 ± 4.1% body mass or 72.7 ± 3.2% lean body mass; turnover 4.58 ± 0.80 l day−1: i.e., complete renewal in ~50 days; n = 36), the values were accurately estimated with the N-CM and 1-CM and were slightly overestimated and underestimated, respectively, with the 2-CM (~7–8% difference, significant for water clearance only). Ingested water appeared in plasma and blood cells within 5 min and the half-life of absorption (~11–13 min) indicates a complete absorption within ~75–120 min. The 2-CM showed that in 42% of the subjects, ingested water quickly distributed within a central compartment before diffusing with a very short half-life (12.5 ± 4.3 min) to a peripheral compartment (18.5 ± 4.3 and 31.6 ± 6.4 L, respectively), which were in complete equilibrium within ~90 min. Pharmacokinetic analyses of water labeled with D2O can help describe water absorption and distribution, for which there is no well defined reference method and value; depending on the characteristics of the subjects and the drinks, and of environmental conditions.

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