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.

Background/Objectives:Urinary biomarkers of hydration (urine osmolality, UOsm; urine specific gravity, USG) follow circadian variations. For individuals, researchers and health-care professionals, there is value in identifying time frames during which spot values of UOsm and USG are representative of 24-h values in healthy young adults.Subjects/Methods:Eighty-two free-living adults (22.3±2.9 years, 22.2±1.5 kg/m2) collected individual urine voids over a 24-h period. UOsm and USG were measured on each void and on the pooled 24-h sample. To determine the time of day when a spot sample was likely to be equivalent to the 24-h value, daytime voids were binned by time and equivalence was tested for each 2-h window. Equivalence was a priori defined as being within 100 mOsm/kg (UOsm) and within 0.003 units (USG) of 24-h values.Results:For both UOsm and USG, voids between 1400 and 2000 hours produced values that were equivalent to the 24-h sample, whereas earlier voids tended to overestimate 24-h UOsm and USG. For windows 1401–1600 hours, 1601–1800 hours and 1801–2000 hours, the mean difference (95% confidence interval) between spot and 24-h UOsm (mOsm/kg) was −25 (−72; 22), 28 (−35; 92) and 12 (−41; 66), respectively, whereas for USG the difference was 0.0014 (−0.0028; −0.0001), 0.0001 (−0.0017; 0.0019) and 0.0005 (−0.0018; 0.0009), respectively.Conclusions:In free-living healthy French adults, 24-h urine concentration can be approximated from a mid- to late-afternoon spot urine sample. This finding suggests that an afternoon sample may be an accurate and practical tool for hydration monitoring, useful to individuals and health-care practitioners.

ObjectivePrevious research has highlighted links between fluid intake (FI), hydration parameters, and disease risk. Specifically, hydration markers in 24h urine samples respond quickly to a change in FI and have been linked to risk of CKD and lithiasis. However, 24h urine collection is burdensome, inconvenient, and time consuming. Spot urine collections would simplify daily hydration monitoring for researchers, clinicians, and individuals. Therefore, we assessed the equivalence of 24h and spot urine osmolality (24h Uosm; Spot Uosm) to determine whether a single spot may replace 24h collection.Methods82 healthy adults with low to high habitual FI (50%M; age 23.6±2.9 y; BMI 22.2±1.5 kg/m2) recorded all fluid consumed and collected individual voids over 24h. Uosm was measured on each void and on the pooled 24h sample. Equivalence between Spot and 24h Uosm was evaluated using the Two One‐Sided Test method (α=0.025).ResultsSpot Uosm from afternoon samples (14:00‐16:00; 16:00‐18:00; 18:00‐20:00) was equivalent to 24h Uosm whereas morning (08:00‐10:00; 10:00‐12:00) and midday (12:00‐14:00) spots tended to overestimate 24h Uosm. The mean [95% CI] difference between 24h and Spot Uosm was 287 [209;365] between 08:00‐10:00; 83 [12;154] between 10:00‐12:00; and 107 [58;157] between 12:00‐14:00. From 14:00‐16:00; 16:00‐18:00; and 18:00‐20:00, the mean difference was ‐25 [‐72;22]; 29 [‐35;92] and 12 [‐41;66].ConclusionAfternoon Spot Uosm is equivalent to 24h Uosm. Thus, short, well‐timed urine collections may represent a convenient alternative to 24h samples, providing an easy way to monitor daily hydration in real‐life conditions. Afternoon Spot could also serve as an early warning signal to increase FI for maintaining 24h euhydration.

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