Water: how much to drink each day for adequate hydration?

Water. Advice varies on how much a person should drink per day and indeed, until now, there has been little objective reference for consensus. Now thanks to a scientific review published in the excellent journal Nutrients, a helpful guideline for how much water to drink each day based on the brain's response to hydration through measurements of plasma arginine vasopressin (AVP) concentration has been established. 

Water and AVP

The authors explain why AVP, produced by the brain, is a an especially good biomarker as a metric for adequate hydration:

"[AVP is]...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."

Until now a minimum daily water requirement could only be defined in general terms due to variations in physical activity, climate, caloric consumption, the concentrating-diluting capacities of the kidneys, water losses via excretion and secretion and overall dietary intake. It was not possible to define daily water requirement for an individual, and there was not even general consensus on euhydration (water balance). As a result, definitions of adequate intake were asserted based on median values (USA) or life-stage groups (Europe). So until now, there has never been a method to assess the water requirement of individuals or neuroendocrine data to indicate daily adequate intake (AI) values for water.

"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)."

A Novel Approach to Dynamic Complexity

The main reason why daily water requirements have never been determined before is due to dynamic complexity. 

"To maintain normal physiological functions (e.g.., blood pressure, pH, internal body temperature) and optimal health, and to deliver essential substances (e.g., oxygen, water, glucose, sodium, potassium) to cells, the CNS and neuroendocrine hormones act constantly to preserve internal homeostasis via a complex network of many organ and neural systems."

All of these variables are simultaneously maintained at a specific set point beyond which the brain responds; and this is constantly changing throughout the life in response to water and food intake, urine production, and other water losses. 

By applying a metric to the thresholds and intensity of responses within the brain and neuroendocrine system with measurements of AVP, they were able to discriminate between euhydration and dehydration.

Should people be drinking more water?

Until now there have been too many confounders for researchers to isolate the effects of water intake on health outcomes, yet...

"...a surprising number of adults in developed countries do not meet water AI recommendations (Table 1). This fact is significant in terms of long-term health outcomes because a growing body of epidemiological evidence shows that chronically elevated plasma AVP ( likely due to an insufficient daily TWI) is related to cardiovascular, obesity and cancer morbidity and mortality, as well as the regulation of glucose metabolism."

And, interestingly...

"...chronically low water intake is associated with elevated plasma AVP [16], both of which predict the development of impaired glucose regulation."

Water intake affects glucose and insulin regulation

Importantly, water intake influences glucose metabolism by modulating AVP expression, which stimulates V1a and V1b receptors in the liver...

"In the liver, V1a stimulation by AVP initiates calcium signaling reactions which increase glycogenolysis and blood glucose, directly in hepatocytes and indirectly via vasoconstriction-mediated ischemia [139]. Indeed, multiple animal investigations and human genetic studies have described the role of V1a receptors in glucose regulation and dysfunction [140,141,142]."

And by stimulation of V1b receptors in the brain it influences adrenal cortisol production and insulin sensitivity.

 "Independently, V1b receptors are concentrated within the hypothalamic-pituitary-adrenocortical (HPA) axis, where they enhance ACTH and cortisol release, and the physiological effects of CRH, when stimulated by AVP. The downstream effect of increased plasma glucocorticoids is increased hepatic gluconeogenesis and decreased insulin sensitivity [143]."

Moreover...

 "...men with type 2 diabetes exhibited deteriorated glucose control and increased plasma cortisol following three days of water restriction [144]. Thus, mounting evidence suggests that a link exists between AVP and glucose regulation."

 

Bottom line: how much water to drink each day

The authors summarize their results of assessing neuroendocrine responses through AVP during euhydration and following experimental alterations of key variables.

"A plasma AVP concentration < 2.0 pg/ml represents a baseline euhydrated state (i.e., the brain is not attempting to conserve water), whereas plasma AVP ≥2.0 pg/ml indicates dehydration or hypohydration because the brain is acting to conserve water...Our examination of data from multiple research studies demonstrates that a plasma AVP concentration of 2.0 pg/ml is equivalent to a TWI [total water intake] of 1.8 L/24h."

Given variations for constitution, climate and activity, try to consume 1.8 liters of water every 24 hours.

Although the percentage of people who hydrate adequately varies widely, many adults fail to do so with adverse consequences.

"Observational studies demonstrate that 19–71% of adults in various countries consume this volume of water or less each day [13,109,110]. This is significant because increasing evidence shows that chronically elevated plasma AVP (likely due to an insufficient daily TWI) could contribute to a number of negative health outcomes...Fortunately, the chronic reduction of plasma AVP by consuming a daily TWI that maintains plasma AVP <2.0 pg/ml suggests a safe, cost-effective, and easy-to-implement primary preventive intervention that can be evaluated in future long-term clinical trials."