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.
Highlights
Individuals with a normal plasma osmolality (POSM) (e.g., 285–295 mOsm/kg) may be considered to be normally hydrated without regard to daily total water intake (TWI; [1]) or urinary biomarkers [2] because the brain actively regulates both total body water volume and blood concentration across a wide range of TWI
In contrast to the methods used in the National Academy of Medicine (NAM) and European Food Safety Authority (EFSA) reports, we propose that minimal/baseline fluid-electrolyte regulatory responses by the brain signal body water balance, and that increased neuroendocrine responses represent the threshold at which the brain begins to defend body water volume and concentration
At the end of these water restriction periods, group mean plasma Arginine vasopressin (AVP) concentrations ranged from 2.9–3.5 pg/ml, representing a mild-to-moderate neuroendocrine response (Figures 3 and 4). This range of concentrations is similar to the plasma AVP levels reported for individuals who habitually consume a low daily TWI of 0.7 L/24h (2.4 pg/ml; [16]), 1.0 L/24h (2.5–3.6 pg/ml; [54]), and 1.6 L/24h (2.5–2.9 pg/ml; Figure 5; [13]). This suggests that women who consume a TWI of 0.7–1.6 L/24h (i.e., ~20–30% of young healthy women; Figure 2)—well below the Adequate Intakes (AI) recommended by EFSA and NAM (Table 1)—experience a chronic mild-to-moderate neuroendocrine defense of total body water
Summary
Individuals with a normal POSM (e.g., 285–295 mOsm/kg) may be considered to be normally hydrated without regard to daily total water intake (TWI; [1]) or urinary biomarkers [2] because the brain actively regulates both total body water volume (within 0.5% day-to-day; [3]) and blood concentration (within a normal POSM range of 285–295 mOsm/kg; [4]) across a wide range of TWI (women, 1.3–6.1; men, 1.7–7.9 L/24h; [5,6]). In contrast to the methods used in the NAM and EFSA reports, we propose that minimal/baseline fluid-electrolyte regulatory responses by the brain signal body water balance (i.e., euhydration), and that increased neuroendocrine responses (e.g., plasma AVP levels) represent the threshold at which the brain begins to defend body water volume and concentration (i.e., hypohydration). This is important because no measurement or biomarker has previously been proposed to define a state of euhydration (i.e., often defined loosely as normal total body water or water balance). Distinct from the term dehydration (i.e., the process of losing water), the term hypohydration is presently defined as a steady-state condition of reduced total body water
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