H2O, Hydration & Health

Water is a critical nutrient, the absence of which will be lethal within days. Water’s importance for preventing nutrition-related non-communicable diseases* has received more attention recently because of the shift toward consuming large quantities of caloric beverages, like soda, juices, and alcoholic beverages. Despite this focus, there are significant gaps in knowledge related to measuring total fluid intake and hydration at the population level. There are also few longer-term systematic interventions and no published randomized, longer-term controlled trials. Here we provide suggestions for ways to examine water requirements and encourages more dialogue on this vital topic.

  • Non-communicable diseases: Chronic conditions that do not result from an (acute) biological process and hence are “not communicable.” A disease with a prolonged course that does not resolve spontaneously, where a complete cure can be realized.
  • Examples: Cardiovascular disease (e.g., Coronary heart disease, stroke), cancer, chronic respiratory diseases, diabetes, chronic neurologic disorders (e.g., Alzheimer’s, dementias), arthritis/musculoskeletal diseases, unintentional injuries (e.g., from traffic crashes).
  • https://www.who.int/news-room/fact-sheets/detail/noncommunicable-diseases

Permission granted to modify and republish by Oxford University Press. License Number: 5121290946661 License Date: August 03, 2021


Barry M Popkin, Kristen E D’Anci, Irwin H Rosenberg, Water, hydration, and health, Nutrition Reviews, Volume 68, Issue 8, 1 August 2010, Pages 439–458, https://doi.org/10.1111/j.1753-4887.2010.00304.x


Water is essential for life. Since primeval species ventured from the oceans to live on land, the primary key to survival has been the prevention of dehydration. Without water, humans can survive only for days. Water comprises from 75% body weight in infants to 55% in the elderly and is essential for cellular homeostasis and life.[1] Nevertheless, there are many unanswered questions about this most critical component of our body and our diet.

This report will provide you with some basic knowledge of our current understanding of water. This will include the complex mechanisms behind water homeostasis, the effects of variation in water intake on health, diet, weight, human performance, and function.

Beyond these circumstances, it is not fully understood how hydration affects health and well-being, even the impact of water intake on chronic diseases.

Few countries have developed water requirements. Those that exist are based on weak population-level measures of water intake and urine concentration.[3,4]

The European Food Safety Authority (EFSA) was recently asked to revise existing recommended intakes of essential nutrients. Especially those with a physiological effect, including water, since this nutrient is vital for life and health.[5]

The US Dietary Recommendations for water are based on average water intake with no measurements of the dehydration status of the population to assist. One-time collection of blood samples for the analysis of serum concentration has been used by the National Health and Nutrition Examination Survey program. Unfortunately, there is no accepted method of assessing hydration status at the population level. Some scholars use muscle tightness, which is not even linked with hydration for all age groups.[6] 

Urine indexes are used often, but these reflect the recent volume of fluid consumed rather than an overall state of hydration.[7] Many scholars use urine concentration to measure recent and current hydration status.[8,–12] At this time, there are no adequate biomarkers to measure hydration at the population level.

In discussing water, the focus is first and foremost on all types of water, whether soft or hard, spring or well, caloric, carbonated, or distilled. Furthermore, water is not only consumed directly as a beverage. It is also obtained from food and, to a minimal extent, from water created inside humans and other living organisms through their metabolism by oxidizing energy-containing substances in their food.

The proportion of water from food and beverage varies according to the balance of fruits and vegetables in our diet. The ranges of water content in various foods are presented in Table 1. In the United States, it is estimated that about 22% of water intake comes from food. At the same time, the percentages are much higher in European countries, particularly a country like Greece with its higher intake of fruits and vegetables, or in South Korea.[3,–15] 

The only in-depth water use study performed in the United States found a 20.7% contribution from food water.[16,17] However, as shown below, this research depended on an inadequate overall water intake assessment.


Indeed, the subtle and essential regulation of hydration and water intake in individuals makes less severe the prevalent underhydration in populations and its effects on bodily function and disease state and progression.

Regulation of Fluid Intake

To prevent underhydration, reptiles, birds, vertebrates, and all land animals have evolved a network of controls to maintain sufficient hydration and fluid intake by thirst. Humans may drink for various reasons, sometimes even for pleasure. Still, drinking is most often due to underhydraton that triggers a physical craving. The mechanism of thirst is well understood today. Unregulated drinking is frequently encountered due to the capacity of the kidneys to rapidly eliminate excesses of water or reduce urine secretion to temporarily economize on water.[1] But this excretory process can only postpone the necessity of drinking or ceasing to drink an excess of water. Unregulated drinking is often confusing, particularly in wealthy societies with flavorful drinks containing other substances the drinker seeks. The most common of these are sweeteners or alcohol, for which water is used as a vehicle. Drinking these beverages is not due to diminished thirst.[1]

Fluid balance of the two compartments

Maintaining steady water and mineral balance is a coordinated effort by various sensory organs throughout our body that are linked to our brain, where the information is processed. The self-governing decision-making centers of our brain then sends instructions to the kidneys, sweat glands, and salivary glands, along with the part of the brain responsible for corrective actions.[1]

Most of the components of fluid balance are controlled by mechanisms responding to the state of body water. These sensitive and precise mechanisms are activated with deficits or excesses of water amounting to only a few hundred milliliters or ten ounces. A water deficit produces an increase in the ionic concentration of the spaces between our cells, which takes water from these spaces, causing cells to shrink. This shrinkage is detected by two types of brain sensors. One controls thirst, and the other controls removal by sending a message to the kidneys by the antidiuretic hormone vasopressin*. This produces a smaller volume of more concentrated urine.[18] The reverse processes occur when the body contains an excess of water: the lower ionic concentration of body fluids allows more water to reach the spaces between our cells. The cells absorb, drinking is inhibited, and the kidneys release more water.

  • ADH is also called arginine vasopressin. It is a hormone made by the hypothalamus in the brain and stored in the posterior pituitary gland. It tells your kidneys how much water to conserve. (Healthline: Antidiuretic Hormone (ADH) Test -https://www.healthline.com/health/adh)

As you can see, the kidneys play a crucial role in regulating fluid balance. Our kidneys function more efficiently in the presence of abundant hydration.

Now, suppose the kidneys economize on water and produce more concentrated urine. In that case, they expend a greater amount of energy and incur more wear and tear. This happens when the kidneys are under stress, e.g., when the diet contains excessive amounts of salt or toxins that need to be eliminated. Consequently, drinking a sufficient amount of water helps protect this vital organ.

Regulatory Drinking

Apart from urinary excretion, the other primary fluid regulatory process is fluid consumption, mediated through the sensation of thirst. There are two distinct mechanisms of functional thirst: the intracellular and the extracellular mechanisms. 

Most drinking occurs in response to signals of water deficit or underhydration. When water alone is lost, ionic concentration increases. As a result, the space inside cells releases some of its water to the space outside of the cells. Once again, the brain detects the resulting shrinkage of cells, which sends messages to induce drinking. This is accompanied by an enhanced appetite for salt. Thus, people who have been sweating a lot prefer drinks relatively rich in salts rather than pure water. When excessive sweating is experienced, it is also important to supplement drinks with additional salt.

Our brain’s decision to start or stop drinking and choose the appropriate drink is made before that hydration reaches the areas of our body that need it. Our taste buds send messages to our brain about the nature, and especially the salt content, of the ingested fluid, along with responses that are triggered as if the incoming hydration had already reached our bloodstream. These are the anticipatory reflexes.[1]

Our brain is equipped with sensory receptors related to drinking. Neurons in these areas show enhanced firing when we become sufficiently hydrated. Their firing decreases when water is loaded in the carotid artery(neck) that soaks the neurons. Reduced firing in the same neurons occurs when the water load is applied on the tongue instead of being injected into the carotid artery. 

This anticipatory drop in firing is due to communication from neural pathways that depart from the mouth and converge onto neurons that simultaneously sense the blood’s state.

Nonregulatory drinking

Although everyone experiences thirst from time to time, it plays a minor role in the day-to-day water intake in healthy people living in temperate climates. What we refer to as a Mediterranean climate. In these regions, people generally consume fluids not to quench thirst but as components of everyday foods like soup and milk or stimulating beverages like tea, coffee, or energy drinks.

A typical example is alcohol consumption, which can increase individual pleasure and stimulate social interaction. Some drinks are also consumed for their energy content, as in energy drinks, soft drinks, or milk. While some drinks like coffee and tea are used in cold weather for warming. This kind of drinking also seems to be mediated through the taste buds, which communicate with the brain in a kind of “reward system.” These mechanisms are just beginning to be understood. This tendency to rehydrate may be advantageous because water losses can be replaced before thirst-producing dehydration occurs. Unfortunately, this tendency also carries some disadvantages. Drinking fluids other than water can contribute to excess caloric intake. In some cases, like with caffeinated beverages and alcohol, addiction may occur.

Total fluid intake increased from 79 fluid ounces in 1989 to 100 fluid ounces in 2002 among US adults, with caloric beverages making up that difference.[19]

Effects of Aging on Fluid Intake Regulation

Fluid ingestion by older people has been compared to those in their youth.[20] Following water deprivation, older individuals are less thirsty and drink less fluid compared to younger persons.[21,22] The decrease in fluid consumption is overwhelmingly due to a reduction of thirst, as the relationship between thirst and fluid intake is the same in young and old persons. Some older people drink insufficient amounts of water following fluid deprivation to replenish their body water deficit.[23] And when underhydrated older people are offered a highly palatable selection of drinks, these also fail to increase fluid intake.[23]

Because the elderly have low water reserves, it may be prudent for them to learn to drink regularly when not thirsty and to moderately increase their salt intake when they sweat. Better education on these principles may help prevent sudden hypotension and stroke or abnormal fatigue, leading to a vicious circle and eventually hospitalization.

  • Osmoreceptor – An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. -https://en.wikipedia.org/wiki/Osmoreceptor
  • Baroreceptor – They sense the blood pressure and relay the information to the brain to maintain proper blood pressure. – https://en.wikipedia.org/wiki/Baroreceptor


Our level of hydration is critical to our body’s process of temperature control. Body water loss through sweat is an essential cooling mechanism in hot climates and in times of physical activity. Sweat production depends on temperature and humidity, activity levels, and the type of clothing we wear. Water losses via the skin (both insensible perspiration and sweating) can range from 0.3 Liters per hour in sedentary conditions to 2 Liters per hour in high activity in the heat. Water intake requirements range from 2.5 to just over 3 Liters per day in adults under normal conditions. They can reach 6 Liters per day in extreme heat and activity.[27,28]

Evaporation of sweat from the body results in a cooling of the skin. However, suppose sweat loss is not balanced with fluid intake, especially during high physical activity levels. In that case, a state of underhydration can occur accompanied by an increase in core body temperature. Underhydration from sweating results in a loss of electrolytes and a reduction in plasma volume, leading to increased plasma concentration. During this state of reduced plasma volume and increased plasma concentration, sweat output becomes insufficient to offset increases in core temperature. When fluids are given to maintain euhydration, sweating remains an effective compensation for increased core temperatures. With repeated exposure to hot environments, the body adapts to heat stress. Cardiac output, blood flow, and pressure return to normal, sodium losses are conserved, and the risk for heat-stress-related illness is reduced.[29] Increasing water intake during this heat acclimatization process will not shorten the time needed to adapt to the heat. Still, even mild dehydration during this time is associated with elevations in stress hormones, increased sweating, and electrolyte imbalances.[29]

Our children and the elderly have differing responses to ambient temperature(room temperature) and different temperature regulating concerns than healthy adults. Children in warm climates may be more susceptible to heat illness than adults. This is due to their greater surface area to body mass ratio, lower rate of sweating, and a slower rate of acclimatization to heat.[30,31]

Children may respond to underhydration during activity with a more significant increase in core temperature than adults, and with a lower tendency to sweat, children lose some of the benefits of evaporative cooling.[32] However, it has been argued that children can dissipate a more significant proportion of body heat via dry heat loss. Their associated lack of sweating, however, provides a beneficial means of conserving water under heat stress.[30]

In response to cold stress, seniors show a reduced internal body temperature control function, resulting in the diversion of hydration from plasma into and throughout our skin.[33,34] Concerning heat stress, water lost through sweating decreases the water content of plasma, and the elderly are less able to compensate for increased blood viscosity and blood pressure.[33] Not only do they have a deficiency of the thirst mechanism, but this can be exaggerated by central nervous system disease[35] and dementia.[36] In addition, illness and limitations in daily living and physical activities can further limit fluid intake. When reduced fluid intake is coupled with advancing age, there is a decrease in total body hydration. Older individuals have impaired renal(kidney) fluid conservation mechanisms and, as noted above, have impaired responses to heat and cold stress.[33,34] All of these factors contribute to an increased risk of underhydration in the elderly.


What is Physiology?

Phys·​i·​ol·​o·​gy | \ ˌfi-zē-ˈä-lə-jē \: Noun

1a branch of biology that deals with the functions and activities of life or of living matter (such as organs, tissues, or cells) and the physical and chemical phenomena involved.

2the organic processes and phenomena of an organism or any of its parts or of a particular bodily function. 


Concerning physiology, the role of water in health is generally characterized in terms of underhydration, whether acute or chronic. To most people, the concept of dehydration encompasses both the process of losing body water and the final state of being. However, it would be better to understand dehydration as the process or action, with underhydration being the final state of the action.

Research regarding water on physical and mental function compares a euhydrated state to an underhydrated state, which is reached via withholding of fluids over time or during periods of heat stress and high activity. In general, supplemental water is beneficial in individuals with a water deficit. Still, little research supports the idea that additional water in adequately hydrated individuals confers any benefit.

Physical performance

The role of water and hydration in physical activity, particularly in athletes and in the military, has been of considerable interest and is well-described in medical and scientific literature.[37,–39] During challenging athletic events, it is not uncommon for athletes to lose 6–10% body weight through sweat, leading to underhydration if fluids are not replenished. However, reductions in the physical performance of athletes have been observed under much lower levels of underhydration. As little as 2%.[38] Under relatively mild levels of dehydration, individuals engaging in rigorous physical activity will experience a reduction in performance related to reduced endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort.[40,41]

Drinking more water can reverse these deficits and reduce oxidative stress caused by exercise and dehydration.[42] Hypohydration appears to have a more significant impact on high-intensity and endurance activity, such as tennis[43] and long-distance running,[44] than on anaerobic exercises,[45] such as weight lifting, or on shorter-duration activities, such as rowing.[46]

During exercise, individuals may not hydrate adequately when allowed to drink according to thirst.[32] After periods of physical exertion, voluntary fluid intake may be inadequate to offset the effects of working out.[1] Thus, mild-to-moderate underhydration can persist for some hours after the conclusion of physical activity. Research performed on athletes suggests that they are at particular risk for dehydration at the beginning of their training season due to changes in weather conditions and increased activity levels.[47,48] Several studies show that performance in temperate and hot climates is affected to a greater degree than performance in cold temperatures.[41,–50] Exercise in hot conditions with inadequate fluid replacement is associated with overheating, reduced stroke volume and cardiac output, a decrease in blood pressure, and reduced blood flow to muscle tissues.[51]

During exercise, children may be at greater risk for voluntary underhydration. Children may not recognize the need to replace lost fluids. Both children and coaches need specific guidelines for fluid intake.[52] Additionally, children may require more time to adjust to increases in environmental temperature than adults.[30,31] Recommendations are for child athletes or children in hot climates to begin athletic activities in a well-hydrated state, drinking fluid over and above their thirst.

Cognitive performance

Water(hydration), or its lack(underhydration), influences cognition. Mild levels of underhydration can produce disruptions in mood and cognitive functioning. This may be of particular concern in the very young, very old, those in hot climates, and those engaging in vigorous exercise. Mild underhydration produces alterations in several essential aspects of cognitive function such as concentration, alertness, and short-term memory in all age categories.[32,53-56,57]

Like physical functioning, mild-to-moderate levels of underhydration can impair performance on tasks such as short-term memory, perceptual discrimination(the brain’s ability to accurately perceive information in a complex, fluid, and confusing situation), mathematic ability, visuomotor tracking(vision and eye muscles working together), and psychomotor skills(the relationship between movement, learning, and related decisions).[53,-56] However, mild underhydration does not appear to alter cognitive functioning consistently.[53,56,–58]

In a series of studies using exercise in conjunction with water restriction as a means of producing underhydration, the only consistent effect of mild dehydration was significant elevations of subjective mood score, including fatigue, confusion, anger, and vigor. Heat stress may play a critical role in the effects of dehydration on cognitive performance. Reintroduction of fluids under conditions of mild dehydration can reasonably be expected to reverse dehydration-induced cognitive deficits. 

Extra fluids may alleviate the harmful effects of underhydration on cognitive performance and mood. One study[59] examined how water ingestion affected arousal and cognitive performance in young people following 12 hours of water restriction. While cognitive performance was not affected by either water restriction or water consumption, extra fluids did result in self-reported arousal. Participants reported increased alertness as a function of water intake. Water ingestion, however, had an opposite effect on cognitive performance as a function of thirst. High-thirst participants’ performance on a cognitively demanding task improved following water ingestion, but low-thirst participants’ performance declined. In summary, hydration status consistently affected self-reported alertness, but effects on cognition were less consistent.

Several recent studies have examined the utility of providing water to school children on attentiveness and cognitive functioning in children.[61,–63] In these experiments, children were not fluid restricted before cognitive testing but were allowed to drink as usual. Children were then provided with a drink or no drink 20–45 min before the cognitive test sessions. In these studies, as in the studies in adults, the findings were conflicting and relatively modest. Children in the groups given water showed improvements in visual attention.[61,62] However, effects on visual memory were less consistent. One study showed no impact of drinking water[61], while another showed a significant improvement in a similar task in 7–9-year-old children.[62] Yet another study demonstrated memory performance had improved by the provision of water.[62,63] Still, sustained attention was not altered with the provision of water in the same children. Clearly, there are a lot of factors at play.

Taken together, these studies indicate that low-to-moderate underhydration may alter cognitive performance. Rather than showing that the effects of hydration or water ingestion on cognition are contradictory, many studies differ significantly in procedure and the measurement of cognitive behaviors. These variances in process underscore the importance of consistency when examining relatively subtle changes in overall cognitive performance. 

However, in those studies in which dehydration was induced, combined heat and exercise make separating the effects of underhydration from cognitive performance challenging. Additionally, little is known about the mechanism of mild dehydration’s impact on mental performance. It has been proposed that mild dehydration acts as a physiological stressor that competes with and draws attention from cognitive processes.[64] Research on this hypothesis is limited and merits further exploration.

Dehydration and delirium

Dehydration is a risk factor for delirium and for delirium presenting as dementia in the elderly and in the very ill.[65,–67]

Recent studies have shown that dehydration is one of several historical factors for confusion observed in long-term-care residents.[67] Older people have been reported as having reduced thirst and hypodipsia relative to younger people. As well, fluid intake and maintenance of water balance can be complicated by factors such as disease, dementia, incontinence, renal insufficiency, restricted mobility, and drug side effects. In response to primary dehydration, older people have less thirst sensation and reduced fluid intakes compared to younger people. However, in response to heat stress, while older people still display a reduced thirst threshold, they do ingest comparable amounts of fluid to younger people.[20]

Gastrointestinal function

Fluids in the diet are generally absorbed in the proximal small intestine or the first part of the small intestine(the duodenum), which is the first loop that attaches to the distal(far) end of the stomach at the pyloric sphincter.

The absorption rate is determined by the speed of gastric emptying to the small intestine. Therefore, the total volume of fluid consumed will eventually be reflected in the water balance. Still, the rate at which rehydration occurs is dependent upon factors affecting the speed of delivery of fluids to the intestinal mucosa. The gastric emptying rate is generally accelerated by the consumed volume and slowed by higher energy density and concentration.[68] In addition to water consumed in food and beverages, digestive secretions account for a far greater portion of water that passes through and is absorbed by the gastrointestinal tract.[69] These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. The majority of this water is absorbed by the small intestine, compared to the colon. About three to one.[69]

Constipation, characterized by slow gastrointestinal transit, small, hard stools, and difficulty in passing stool, has several causes, including medication use, inadequate fiber intake, poor diet, and illness.[70] Bowel transit time will vary from person to person. An average time for those that are not constipated is 30-40 hours. Even up to 72 hours is considered normal. Transit time in women may take as long as 100 hours.

Underhydration is touted as a common culprit in constipation, and increasing fluid intake is a frequently recommended treatment. Evidence suggests, however, that increasing fluids is only helpful to individuals in an underhydrated state and is of little use in those that are sufficiently hydrated.[70] Increasing daily water intake by 50% in young children with chronic constipation did not affect their constipation.71 For Japanese women with low fiber intake, concomitant low water intake in the diet is associated with increased prevalence of constipation.72 

Inadequate fluid intake in older individuals is a predictor of increased levels of acute constipation.[73,74] Those consuming the least amount of fluid have more than twice the frequency of constipation as those drinking the most. In one trial, researchers compared the utility of carbonated mineral water in reducing functional indigestion and constipation scores to tap water in individuals with functional dyspepsia.[75] When comparing carbonated mineral water to tap water, participants reported improvements in subjective gastric symptoms. Still, there were no significant improvements in gastric or intestinal function. The authors indicate it is impossible to determine to what degree the mineral content of the two water’s contributed to perceived symptom relief. The mineral water contained more significant levels of magnesium and calcium than the tap water. The available evidence suggests that increased fluid intake should only be indicated in individuals in an underhydrated state.[69,71]

Significant water loss can occur through the gastrointestinal tract, which can be of great concern in the very young. In developing countries, diarrheal diseases are a leading cause of death in children, resulting in approximately 1.5–2.5 million deaths per year.[76] Diarrheal illness results in a reduction in body water and potentially lethal electrolyte imbalances. Mortality in such cases can often be prevented with appropriate oral rehydration therapy. Simple dilute solutions of salt and sugar in water can replace fluid lost by diarrhea. Many consider the application of oral rehydration therapy to be one of the significant public health developments of the last century.[77]

Kidney function

As noted above, the kidneys are crucial in regulating plasma, water balance, blood pressure, and removing waste(excess) from the body. Water metabolism by the kidney can be classified into regulated and obligate. Water regulation is hormonally mediated to maintain a tight plasma concentration range (between 275 and 290 mOsm/kg). Increases in plasma concentration and activation of osmoreceptors (intracellular) and baroreceptors (extracellular) stimulate the hypothalamic release of arginine vasopressin (AVP). AVP acts at the kidney to decrease urine volume and promote water retention, and the urine becomes hypertonic*. With decreased plasma concentration, vasopressin release is inhibited, and the kidney increases hypotonic urinary output.

hy·per·ton·ic – /ˌhīpərˈtänik/ – adjective

  1. [BIOLOGY] having a higher osmotic pressure than a particular fluid, typically a body fluid or intracellular fluid.

In addition to regulating fluid balance, the kidneys require water to filter excesses from the bloodstream to be excreted via urine. Water excretion via the kidney removes solutes from the blood. A minimum obligate urine volume is required to remove the solute load with a maximum output volume of 1 L/h.[78] This required volume is not fixed but is dependent upon the amount of metabolic solutes(components) to be excreted and levels of AVP.

The ability to both concentrate and dilute urine decreases with age.[79,-81] Under typical conditions, a urine volume of 1.5 to 2.0 L/day would be sufficient to clear a solute load of 900 to 1,200 mOsm/day in an average adult. During water conservation and the presence of AVP, this obligate volume can decrease to 0.75–1.0 L/day, and during maximal diuresis, up to 20 L/day can be required to remove the same solute load.[78,–81] In cases of water loading, if the volume of water ingested cannot be compensated for with urine output, having overloaded the kidney’s maximal output rate, an individual can enter a hyponatremic state. A condition where sodium levels in the blood are lower than usual.

Heart function and hemodynamic response

Blood volume, blood pressure, and heart rate are closely linked. Blood volume is normally tightly regulated by matching water intake and water output, as described in the section on kidney function. In healthy individuals, slight changes in heart rate and vasoconstriction balance normal fluctuations in blood volume on blood pressure.[82] Decreases in blood volume can occur, through blood loss (or blood donation), or loss of hydration through sweat, as seen with exercise. Blood volume is distributed differently relative to the position of the heart, whether the body is supine or upright, and moving from one position to the other can lead to increased heart rate, a fall in blood pressure, and, in some cases, a temporary loss of consciousness. Low blood pressure can be remedied by drinking 30-500 mL or 10-16 ounces of water. This water intake will reduce heart rate and increase blood pressure in those with normal and high blood pressure.[85] These effects occur within 15–20 minutes of drinking water. They can last for up to 60 minutes. Water ingestion is also beneficial in preventing loss of consciousness in blood donors.[86] The effect of water intake in these situations is thought to be due to effects on the sympathetic nervous system rather than changes in blood volume.[83,84] Interestingly, and in rare cases, individuals may experience a slow heart rate and fainting after swallowing cold liquids.[87,–89] 


Water deprivation and dehydration can lead to the development of headaches.[90] This observation is largely unexplored in the medical literature. However, some observations indicate that water deprivation, in addition to impairing concentration and increasing irritability, can serve as a trigger for migraine along with prolonged migraine.[91,92]

In those with water deprivation-induced headaches, water ingestion provided relief from headaches in most individuals within 30 min to 3 h.[92] It is proposed that water deprivation-induced headaches result from intracranial dehydration and a deficit of total plasma volume. Although the provision of water may help relieve dehydration-related headaches, the utility of increasing water intake for the prevention of headaches is less well documented.

The folk wisdom that drinking water can stave off headaches has been relatively unchallenged and has more traction in the popular press than in the medical literature. Recently, one study examined increased water intake and headache symptoms in headache patients.[93] Water intake did not affect the number of headache episodes. Still, it was modestly associated with a reduction in headache intensity and reduced duration of the headache. The data from this study suggest that the utility of water as a treatment is limited in headache sufferers, and the ability of water to reduce or prevent headaches in the broader population remains unknown.


One of the more pervasive myths regarding water intake is its relation to improvements of the skin or complexion. By improvement, it is generally understood that individuals are seeking to have a more “moisturized” look to the surface skin or to minimize acne or other skin conditions. Numerous lay sources such as beauty and health magazines and postings on the Internet suggest that drinking 8–10 glasses of water a day will “flush toxins from the skin” and “give a glowing complexion” despite a general lack of evidence to support these proposals.[94,95] The skin is vital for maintaining hydration levels and preventing water loss into the environment.

The skin contains approximately 30% water, which contributes to plumpness, elasticity, and resiliency. The overlapping cellular structure of the stratum corneum and lipid content of the skin serves as “waterproofing” for the body.[96] Loss of hydration through sweat is not indiscriminate across the total surface of the skin but is carried out by eccrine sweat glands, which are evenly distributed over most of the body surface.[97] Dry skin is typically associated with exposure to dry air, prolonged contact with hot water, scrubbing with soap, medical conditions, and medications. While more severe levels of dehydration can be reflected in a reduction of skin elasticity,[98,99] with tenting of the skin acting as a flag for dehydration, overt skin turgor in individuals with adequate hydration is not altered. Water intake, particularly in individuals with low initial water intake, can improve skin thickness and density as measured by a sonogram.[100,101] Adequate skin hydration, however, is not sufficient to prevent wrinkles or other signs of aging, which are related to genetics, sun exposure, and environmental damage. Of more utility to individuals already consuming adequate fluids is topical emollients; these will improve skin barrier function and improve the look and feel of dry skin.[102,103]


Many chronic diseases have multifactorial origins. In particular, differences in lifestyle and the impact of our immediate environment are involved and constitute risk factors. Water is quantitatively the most critical and essential nutrient to human existence and well-being. In the past, scientific interest regarding water metabolism was mainly directed toward the extremes of severe dehydration and water intoxication. [4,104] There is currently no consensus on a “gold standard” for hydration markers, particularly for mild dehydration. There is evidence, however, that mild dehydration may also account for some morbidities. Consequently, the effects of mild dehydration on the development of several disorders and diseases have not been well documented.

There is strong evidence that good hydration reduces the risk of kidney stones(urolithiasis) (see Table 2 for evidence categories). Evidence links good hydration with reduced constipation, exercise asthma, hypertonic dehydration in the infant, and hyperglycemia in diabetic ketoacidosis. Good hydration reduces urinary tract infections, hypertension, fatal coronary heart disease, blood clotting, and stroke. For other conditions such as bladder or colon cancer, evidence of a preventive effect of maintaining good hydration is not consistent (see Table 3).


Water consumption, water requirements, and energy intake are linked in fairly complex ways, partially because physical activity and energy expenditures affect the need for hydration. A significant shift in beverage consumption over the past century or more has led to a considerable amount of our energy intake from caloric beverages. Unregulated beverage intake, as noted earlier, has assumed a much more significant role for individuals.[19]

Patterns and trends of water consumption

Measurement of total fluid water consumption in individuals is relatively new. As a result, the state of the science is poorly developed, data are most likely somewhat incomplete, and adequate validation of the measurement techniques used is not available. Presented here are varying patterns and trends of water intake for the United States over the past three decades, followed by a brief review of the work on water intake in Europe.

There is really no existing information to support an assumption that consumption of water alone or beverages containing water affects hydration differentially. However, the information is at best suggestive of an issue deserving further exploration. [3,105] Epidemiological* data suggests water might have different metabolic effects when consumed alone rather than as a component of caffeinated, flavored, or sweetened beverages. [106,107] Beverages not consisting solely of water do contain less than 100% water.

*Epidemiology: The study of how often diseases occur in different groups of people and why.

One study in the United States has attempted to examine all the dietary sources of water.[16,17] A great deal of time was spent working out ways to convert USDA dietary data into water intake, including water absorbed during the cooking process, water in food, and all drinking water sources.

Researchers created several categories and used a range of factors measured in other studies to estimate the water categories. A key point illustrated by nationally representative US data is the enormous variability between survey waves in the amount of water consumed. Although water intake by adults and children increased and decreased simultaneously, the variation was more significant among children than adults for reasons that cannot be explained. This is partly because the questions the surveys posed varied over time. There was no detailed probing for water intake because the focus was on obtaining macro- and micronutrients. Dietary survey methods used in the past have focused on acquiring data on foods and beverages containing nutrient and non-nutritive sweeteners but not on water. 

Related to this are the vast differences between the USDA surveys and the National Health and Nutrition Examination Survey (NHANES) performed in 1988–1994 and in 1999 and later. In addition, even the NHANES 1999–2002 and 2003–2006 surveys differ significantly. These differences reflect a shift in the mode of questioning, with questions on water intake being included as part of a standard 24-h recall rather than as stand-alone questions. A review of the data reveals apparent differences in how the questions have been asked and the limitations on questions on water intake. In the past, people were asked how much water they consumed in a day. Now they are asked for this information as part of a 24-h recall survey. However, unlike other caloric and diet beverages, there are limited probes for water alone. The results must thus be viewed as crude approximations of total water intake without any substantial research to show if they are over or underestimated.

Effects of water consumption on overall energy intake

An extensive body of literature focuses on the impact of sugar-sweetened beverages on weight and the risk of obesity, diabetes, and heart disease. However, the perspective of providing more water and its impact on health has not been examined. The literature on water does not address portion sizes; instead, it focuses mainly on water in general as needed or in selected portions compared with other caloric beverages. Aside from portion size, factors such as the timing of beverage and meal intake and types of caloric sweeteners remain to be considered. However, when beverages are consumed in normal conditions where five to eight daily eating occasions are the norm, the delay between beverage and meal consumption matters less.[112,–114]

The literature on the water intake of children is extremely limited. However, the excellent German school intervention with water suggests the effects of water on the overall energy intake of children might be comparable to that of adults.[115] In this German study, children were educated on the value of water and provided with special filtered drinking fountains and water bottles in school. Children are dependent on adults for access to water. Studies suggest that their larger surface area to volume ratio makes them susceptible to changes in skin temperatures linked with ambient temperature changes.[116]

Despite its critical importance in health and nutrition, the array of available research that serves as a basis for determining fluid intake, or even rational recommendations for populations, is limited compared to most other nutrients. While this deficit may be partly explained by complex adaptations and adjustments that occur to protect body hydration and osmolarity, this deficit remains a challenge for the nutrition and public health community.

As a graphic acknowledgment of the limited database upon which to express estimated average requirements for water for different population groups, the Committee and the Institute of Medicine stated: “While it might appear useful to estimate an average requirement (an EAR) for water, an EAR based on data is not possible.” Given the extreme variability in water needs that are not solely based on differences in metabolism but also on environmental conditions and activities, there is not a single level of water intake that would assure adequate hydration and optimum health for half of all apparently healthy persons in all environmental conditions. Thus, an adequate intake (AI) level was established in place of an EAR for water.

Suppose the AI for adults, as expressed in Table 5, is taken as a recommended intake. In that case, the wisdom of converting an AI into a recommended water or fluid intake seems questionable. The first problem is the almost certain inaccuracy of the fluid intake information from the national surveys, even though that problem may also exist for other nutrients. More importantly, from translating an adequate intake into a recommended fluid intake for individuals or populations, the decision was made when setting the adequate intake to add an additional 20%. This is derived from some foods in addition to water and beverages. 

EDITORS NOTE(Opinion): This, and the following, may have been a worthy effort to set a standard, but it missed the point that hydration should first be derived from foods and then supplemented by distilled water.

While this may have been a legitimate effort, the recommendations that derive from the report would be better directed at recommendations for water and other fluid intakes on the assumption that the water content of foods would be a “passive” addition to total water intake.

In this case, the observations of the dietary reference intake committee that water intake must meet needs imposed by metabolism and environmental conditions must be extended to consider three added factors, namely body size, gender, and physical activity. Those are the well-studied factors that allow for relatively precise measurement and determination of energy intake requirements. It is, therefore, logical that those same factors might underlie recommendations to meet water intake needs in the same populations and individuals. Consideration should also be given to the possibility that water intake needs would best be expressed relative to the calorie requirements, as is done regularly in the clinical setting. Data should be gathered to this end through experimental and population research.

It is important to note that only a few countries include water on their list of nutrients.[118] The European Food Safety Authority is developing a standard for all of Europe.[105] At present, only the United States and Germany provide AI values for water.[3,119]

Another approach to estimating water requirements, beyond the limited usefulness of the average intake method, is to express water intake requirements in relation to energy requirements. An argument for this approach includes observing that energy requirements for each age and gender group are strongly evidence-based and supported by extensive research accounting for body size and activity level. These are crucial determinants of energy expenditure that must be met by dietary energy intake.



This review has pointed out several issues related to water, hydration, and health. Undoubtedly, water is the most important nutrient and the only one for which an absence will prove lethal within days. Understanding water measurement and water requirements are very important. The effects of water on daily performance and short- and long-term health are quite clear. The existing literature indicates few adverse effects of water intake, while the evidence for positive effects is quite clear.


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