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First morning urine (FMU) assessment would be a practical and convenient solution for clinically acceptable detection of underhydration prior to competition/training, and for the general public. Thus, we thus sought to determine the diagnostic accuracy of FMU as a valid indicator of recent (previous 24 hr, 5 days average) hydration practices. For 5 consecutive days and one final morning, 67 healthy women (n = 38) and men (n = 29; age: 20 [1] years, body mass index: 25.9 [5.5]) completed 24-hr diet logs for total water intake (from beverages and foods, absolute and relative to body mass), 24-hr urine and FMU collection (last morning only) for osmolality (Osm), specific gravity (SG), and color (Col), and morning blood sampling for plasma osmolality and copeptin. Correlations determined significance and relationship strength among FMU and all other variables. Area under the receiver operating characteristic curves, sensitivity, specificity, and positive likelihood ratios were employed using previously reported values to indicate underhydration (total water intake < 30 ml/kg, osmolality > 500, and >800 mOsm/kg, specific gravity > 1.017, and copeptin > 6.93 pmol/L). FMU_Osm and FMU_SG were significantly correlated (p <.05) to all variables except the previous 5-day plasma osmolality. FMU_Col was only significantly correlated with other color time intervals and total water intake per gram. FMU_Osm held greatest utility (area under the receiver operating characteristic curve, sensitivity, and specificity >80%) overall, with the best outcome being FMU_Osm indicating a previous 24-hr osmolality threshold of 500 mOsm/kg (FMU_Osm criterion >710 mOsm/kg and positive likelihood ratio = 5.9). With less effort and cost restriction, FMU is a viable metric to assess underhydration. [ABSTRACT FROM AUTHOR]

Few previous review articles have focused on the associations between inadequate daily water intake (LOW) or urinary biomarkers of dehydration (UD; low urine volume or high urine osmolality) and multiple diseases. Accordingly, we conducted manual online searches (47 key words) of the PubMed, Embase, and Google Scholar databases with these inclusion criteria: English language, full-text, peer reviewed, no restriction on research design, and three publications minimum. Initially, 3,903 articles were identified based on their titles and abstracts. Evaluations of full length.pdf versions identified 96 studies that were acceptable for inclusion. We concluded that the evidence is insufficient or conflicting for seven disorders or diseases (i.e. suggesting the need for additional clarifying research) and it is lacking for all-cause mortality. Differential characterizations among women and men have been reported in the results of nine studies involving five diseases. Finally, the evidence for associations of LOW or UD is strong for both kidney stones and type 2 diabetes with hyperglycemia. This suggests that great public health value (i.e. reduced disease risk) may result from increased daily water intake—a simple and cost-effective dietary modification. [ABSTRACT FROM AUTHOR]

Urine color (Ucol) as a hydration assessment tool provides practicality, ease of use, and correlates moderately to strongly with urine specific gravity (Usg) and urine osmolality (Uosm). Indicative of daily fluid turnover, along with solute and urochrome excretion in 24-hr samples, Ucol may also reflect dietary composition. Thus, the purpose of this investigation was to determine the efficacy of Ucol as a hydration status biomarker after nutritional supplementation with beetroot (880 mg), vitamin C (1000 mg), and riboflavin (200 mg). Twenty males (Mean ± SD\ age, 21 ± 2 y; body mass, 82.12 ± 15.58 kg; height, 1.77 ± 0.06 m) consumed a standardized breakfast and collected all urine voids on one control day (CON) and 1 day after consuming a standardized breakfast and a randomized and double-blinded supplement (SUP) over 3 weeks. Participants replicated exercise and diet for one day before CON, and throughout CON and SUP. Ucol, Usg, Uosm, and urine volume were measured in all 24-hr samples, and Ucol and Usg were measured in all single samples. Ucol was a significant predictor of single sample Usg after all supplements (p < .05). Interestingly, 24-hr Ucol was not a significant predictor of 24-h Usg and Uosm after riboflavin supplementation (p = .20, p = .21). Further, there was a significant difference between CON and SUP 24-h Ucol only after riboflavin supplementation (p < .05). In conclusion, this investigation suggests that users of the UCC (urine color chart) should consider riboflavin supplementation when classifying hydration status and use a combination of urinary biomarkers (e.g., Usg and Ucol), both acutely and over 24 hr. [ABSTRACT FROM AUTHOR]

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Objective: To recognize the causality of environmental factors (i.e. temperature, humidity and sun exposure) on nutritional variables, specifically body water balance and water-seeking behaviour. Design: Author perspective. Setting: Global. Participants: Not applicable. Results: A free-standing code supplement is provided to facilitate investigators in accessing meteorological data for incorporation into analyses related to nutrition and hydration. Conclusions: Analytical models related to human hydration should account for the environment de rigueur. [ABSTRACT FROM AUTHOR]

Background: Surprisingly little is known about the physiological and perceptual differences of women who consume different volumes of water each day. The purposes of this investigation were to (a) analyze blood osmolality, arginine vasopressin (AVP), and aldosterone; (b) assess the responses of physiological, thirst, and hydration indices; and (c) compare the responses of individuals with high and low total water intake (TWI; HIGH and LOW, respectively) when consuming similar volumes of water each day and when their habitual total water intake was modified. Methods: In a single-blind controlled experiment, we measured the 24 h total water intake (TWI; water + beverages + food moisture) of 120 young women. Those who consumed the highest (HIGH, 3.2 ± 0.6 L·day−1, mean ± SD) and the lowest (LOW, 1.6 ± 0.5 L·day−1) mean habitual TWI were identified and compared. Outcome variables were measured during two ad libitum baseline days, a four-day intervention of either decreased TWI (HIGH) or increased TWI (LOW), and one ad libitum recovery day. Results: During the four-day intervention, HIGH and LOW experienced differences in thirst (p = 0.002); also, a statistically significant change of AVP occurred (main effect of TWI and day, p < 0.001), with no effect (TWI or day) on aldosterone and serum osmolality. Urine osmolality and volume distinguished HIGH from LOW (p = 0.002) when they consumed similar 24 h TWI. [ABSTRACT FROM AUTHOR]

Establishing suitable evidence aligning hydration practices and health outcomes demands simplicity, low cost, and accurate hydration monitoring. First morning urine (FMU) assessment may be a practical solution. PURPOSE: Determine the strength of FMU as a valid indicator of recent (previous 24h and 5d average) hydration practices that would otherwise be denoted by 5d dietary intake, 24h urine, and morning circulating copeptin. METHODS: For 5 consecutive days, 67 healthy men (n = 29) and women (n = 38) (age: 20 ± 1y, BMI: 25.9 ± 5.5) completed 24h diet logs, 24h urine collection, FMU collection (last day only), and morning blood sampling. Diet logs were analyzed for total water intake (TWI; from beverages and foods) relative to body mass, urine was measured for osmolality (Uosm) and specific gravity (Usg), and blood was measured for plasma copeptin (Pcop). Pearson correlations were evaluated for significance and relationship strength amongst FMU and all other variables. Area under the Receiver Operating Characteristic curves (AUC) and positive likelihood ratios (+LR) were employed for significantly correlated variables using previously reported values to indicate underhydration (TWI <30 ml/kg, Uosm >800 mOsm/kg, Usg >1.017, and Pcop >6.93 pmol/L). RESULTS: FMUosm and FMUsg (statistics presented in respective order; p < 0.05 unless noted) were significantly correlated with 24h TWI (r = -0.44 and -0.42), 5d TWI (r = -0.52 and -0.50), 24h Uosm (r = 0.76 and 0.68), 5d Uosm (r = 0.71 and 0.67), 24h Usg (r = 0.49 and 0.42), 5d Usg (r = 0.67 and 0.63), and concurrent morning's Pcop (r = 0.26 and 0.26; FMUosm p = 0.054) but not 5d Pcop. Of those variables, FMUosm had the greatest utility (AUC > 70%; sensitivity and specificity > 80%) in indicating 5d Uosm (FMUosm criterion >885.8 and +LR = 4.4). Diagnostic accuracy using FMU was generally greater for previous 5d than 24h hydration indicators, except for Pcop. CONCLUSION: These data support previous claims that FMU sampling better represents chronic than acute hydration practices. Further, FMUosm has slightly greater utility than FMUsg in recent hydration history assessment. With less effort and cost restriction, FMU is a viable metric to assess hydration practices for mitigating related risk of chronic illness. Funding by the Drinking Water Research Foundation and University of Hartford. [ABSTRACT FROM AUTHOR]

Because body mass change (ΔMb) does not represent all water losses and gains, the present field investigation determined if (a) ΔMbequalled the net effective body water change during ultra-endurance exercise and (b) ground speed and exercise duration influenced these variables. Thirty-two male cyclists (age range, 35–52 years) completed a 164-km event in a hot environment, were retrospectively triplet matched and placed into one of three groups based on exercise duration (4.8, 6.3, 9.6 h). Net effective body water loss was computed from measurements (body mass, total fluid intake and urine excreted) and calculations (water evolved and mass loss due to substrate oxidation, solid food mass and sweat loss), including (ΔEBWgly) and excluding (ΔEBW) water bound to glycogen. With all cyclists combined, the mean ΔMb(i.e. loss) was greater than that of ΔEBWglyby 1200 ± 200 g (P= 1.4 × 10–18), was similar to ΔEBW (difference, 0 ± 200 g;P= .21) and was strongly correlated with both (R2= .98). Analysis of equivalence indicated that ΔMbwas not equivalent to ΔEBWgly, but was equivalent to ΔEBW. Due to measurement complexity, we concluded that (a) athletes will not calculate the effective body water calculations routinely and (b) body mass change remains a useful field-expedient estimate of net effective body water change. [ABSTRACT FROM AUTHOR]

A long-standing body of clinical observations associates low 24-h total water intake (TWI = water + beverages + food moisture) with acute renal disorders such as kidney stones and urinary tract infections. These findings prompted observational studies and experimental interventions comparing habitual low volume (LOW) and high volume (HIGH) drinkers. Investigators have learned that the TWI of LOW and HIGH differ by 1–2 L·d−1, their hematological values (e.g., plasma osmolality, plasma sodium) are similar and lie within the laboratory reference ranges of healthy adults and both groups appear to successfully maintain water-electrolyte homeostasis. However, LOW differs from HIGH in urinary biomarkers (e.g., reduced urine volume and increased osmolality or specific gravity), as well as higher plasma concentrations of arginine vasopressin (AVP) and cortisol. Further, evidence suggests that both a low daily TWI and/or elevated plasma AVP influence the development and progression of metabolic syndrome, diabetes, obesity, chronic kidney disease, hypertension and cardiovascular disease. Based on these studies, we propose a theory of increased disease risk in LOW that involves chronic release of fluid-electrolyte (i.e., AVP) and stress (i.e., cortisol) hormones. This narrative review describes small but important differences between LOW and HIGH, advises future investigations and provides practical dietary recommendations for LOW that are intended to decrease their risk of chronic diseases. [ABSTRACT FROM AUTHOR]

With the collection of water-intake data, the National Health and Nutrition Examination Survey (NHANES) is becoming an increasingly popular resource for large-scale inquiry into human hydration. However, are we leveraging this resource properly? We sought to identify the opportunities and limitations inherent in hydration-related inquiry within a commonly studied database of hydration and nutrition. We also sought to critically review models published from this dataset. We reproduced two models published from the NHANES dataset, assessing the goodness of fit through conventional means (proportion of variance, R2). We also assessed model sensitivity to parameter configuration. Models published from the NHANES dataset typically yielded a very low goodness of fit R2 < 0.15. A reconfiguration of variables did not substantially improve model fit, and the goodness of fit of models published from the NHANES dataset may be low. Database-driven inquiry into human hydration requires the complete reporting of model diagnostics in order to fully contextualize findings. There are several emergent opportunities to potentially increase the proportion of explained variance in the NHANES dataset, including novel biomarkers, capturing situational variables (meteorology, for example), and consensus practices for adjustment of co-variates. [ABSTRACT FROM AUTHOR]