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      Water Intake, Water Balance, and the Elusive Daily Water Requirement

      review-article
      1 , 2 , *
      Nutrients
      MDPI
      water-electrolyte balance, drinking water, body water, water restriction

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          Abstract

          Water is essential for metabolism, substrate transport across membranes, cellular homeostasis, temperature regulation, and circulatory function. Although nutritional and physiological research teams and professional organizations have described the daily total water intakes (TWI, L/24h) and Adequate Intakes (AI) of children, women, and men, there is no widespread consensus regarding the human water requirements of different demographic groups. These requirements remain undefined because of the dynamic complexity inherent in the human water regulatory network, which involves the central nervous system and several organ systems, as well as large inter-individual differences. The present review analyzes published evidence that is relevant to these issues and presents a novel approach to assessing the daily water requirements of individuals in all sex and life-stage groups, as an alternative to AI values based on survey data. This empirical method focuses on the intensity of a specific neuroendocrine response (e.g., plasma arginine vasopressin (AVP) concentration) employed by the brain to regulate total body water volume and concentration. We consider this autonomically-controlled neuroendocrine response to be an inherent hydration biomarker and one means by which the brain maintains good health and optimal function. We also propose that this individualized method defines the elusive state of euhydration (i.e., water balance) and distinguishes it from hypohydration. Using plasma AVP concentration to analyze multiple published data sets that included both men and women, we determined that a mild neuroendocrine defense of body water commences when TWI is ˂1.8 L/24h, that 19–71% of adults in various countries consume less than this TWI each day, and consuming less than the 24-h water AI may influence the risk of dysfunctional metabolism and chronic diseases.

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          Most cited references113

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          Urinary indices of hydration status.

          Athletes and researchers could benefit from a simple and universally accepted technique to determine whether humans are well-hydrated, euhydrated, or hypohydrated. Two laboratory studies (A, B) and one field study (C) were conducted to determine if urine color (Ucol) indicates hydration status accurately and to clarify the interchangeability of Ucol, urine osmolality (Uosm), and urine specific gravity (Usg) in research. Ucol, Uosm, and Usg were not significantly correlated with plasma osmolality, plasma sodium, or hematocrit. This suggested that these hematologic measurements are not as sensitive to mild hypohydration (between days) as the selected urinary indices are. When the data from A, B, and C were combined, Ucol was strongly correlated with Usg and Uosm. It was concluded that (a) Ucol may be used in athletic/industrial settings or field studies, where close estimates of Usg or Uosm are acceptable, but should not be utilized in laboratories where greater precision and accuracy are required, and (b) Uosm and Usg may be used interchangeably to determine hydration status.
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            Case records of the Massachusetts General Hospital. Weekly clinicopathological exercises. Laboratory reference values.

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              Hydration biomarkers in free-living adults with different levels of habitual fluid consumption

              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.
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                Author and article information

                Journal
                Nutrients
                Nutrients
                nutrients
                Nutrients
                MDPI
                2072-6643
                05 December 2018
                December 2018
                : 10
                : 12
                : 1928
                Affiliations
                [1 ]University of Connecticut, Human Performance Laboratory and Department of Nutritional Sciences, Storrs CT 06269-1110, USA; lawrence.armstrong@ 123456uconn.edu
                [2 ]University of Wyoming, Human Integrated Physiology Laboratory, Division of Kinesiology and Health, Laramie, WY 82071, USA
                Author notes
                [* ]Correspondence: evan.johnson@ 123456uwyo.edu ; Tel: +307-766-5282
                Article
                nutrients-10-01928
                10.3390/nu10121928
                6315424
                30563134
                20715aaf-bd7a-4429-8b7d-28ebf0187012
                © 2018 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 13 October 2018
                : 27 November 2018
                Categories
                Review

                Nutrition & Dietetics
                water-electrolyte balance,drinking water,body water,water restriction
                Nutrition & Dietetics
                water-electrolyte balance, drinking water, body water, water restriction

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