Fetching brackish water from a well in Kenya
Water is necessary for our bodies' proper hydration, our health, and fully active life.
Water content
Water balance
Water requirements
Water roles
Hydration
Drinking water
'Water is the driving force of all nature'
Leonardo da Vinci (1452 - 1519)
Adequate hydration is an absolute requirement for our health [1637] and all active life. Liquid water is an essential nutrient throughout the living world [1628, 2011]. Although commonly treated rather trivially, no other nutrient is as essential or needed in as great amounts. In particular, we cannot live without it for more than about 100 hours, whereas other nutrients may be neglected for weeks or months. Our requirements are individual and depend on our physical activity, dietary pattern, alcohol intake, health problems, environment, and age.
The water content of our bodies (methodology reviewed [961]) varies and is variable between individuals, generally dropping, throughout our lives from above about 90% of total weight as a fetus to 74% as an infant, 60% as a child, 59% as a teenager (male; female 56%) 59% as an adult (male; female 50%) to 56% (male; female 47%) in the over-50’s. b The gender differences, from the teenage years onwards, are due to their differing fat levels, as is the drop in the elderly who replace muscle mass with fat. If an allowance is made for this fat content, there is little difference with gender or age from childhood onwards. Body water is distributed between the cell interiors (intracellular fluid, ICF, ≈ 59%; ≈ 26 L in a 75 kg man, ICF, ≈ 61%; ≈ 19 L in a 60 kg woman [1499]) and the extracellular fluid (ECF, ≈ 41%; ≈ 18 L in a 75 kg man including the ≈ 3 L of plasma, ≈ 12 L in a 60 kg woman [1499]) (for reference values for extracellular water see [1204]). Water is free to move between the ICF and the ECF with any net movement controlled by the effective osmotic and hydrostatic pressures. The water molecules have a biological half-life in our bodies of about 9-10 days with an average residence time of about two weeks. These estimates depend on age, gender, build, and water consumption, with higher intakes giving shorter half-lives. The majority of the ions in the ICF are K+ and protein anions, whereas in the ECF, they are Na+, Cl− and bicarbonate.
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Water balance in humans has been modeled [583]. Water intake and output are highly variable but closely matched to less than 0.1% over an extended period. Electrolyte intake and output are also closely linked, both to each other and the hydration status. Typical values for an adult in a temperate climate are given below:
Water input,
mL/day |
Water output,
mL/day |
||
|---|---|---|---|
| Drinks a | 1500 |
Urine d | 1500 |
| Food b | 700 |
From skin, sweat e | 500 |
| Metabolic water c | 300 |
Respiration f | 400 |
| Feces a | 100 |
||
a Water, fizzy drinks, tea, coffee,
alcoholic beverages, and so on. All water intake counts equally,
including coffee and alcoholic drinks, as any diuretic effect
is minimal or non-existent; once accustomed to caffeinated
beverages, these count and act as any other water intake [615].
b Water contained in foodstuffs,
varying from ≈ 6% in peanuts, ≈ 35% in bread, to ≈ 85% in fruits
and vegetables.
c Water produced on metabolizing
the foodstuffs and drinks (for example, 1 g fat gives ≈ 1.1
g H2O).
d A significant fraction of this
is required for the removal of urea and other solutes. The
rest is variable to equalize water input and output
e Used for temperature control,
varying with energy intake and expenditure, ambient temperature,
and humidity
f Varies with energy intake and
expenditure, ambient temperature, and humidity. Together with
losses from the skin, this typically amounts to about 50 mL/100
kcal food energy intake.
g Varies with diet, particularly
increasing with increased dietary
fiber
All values will vary with diet, activity, and climate. The water ingested is determined by social, practical, and psychological factors with need indicated by thirst when the body becomes dehydrated. Water output is regulated by hormonal action and the production of urine by the kidneys, which usually can adapt to the body‘s hydration status.
Water balance during sporting and similar activity can be derived from the weight loss less liquid intake, so long as metabolic activity (metabolic water gain and substrate oxidation mass loss) is also accounted for [1523]. As glycogen use, during high activity, is associated with the loss of the water stored alongside it (about 2.5-gram water per gram glycogen), not all the weight loss on exercise needs to be replaced by water immediately. Moving from a high carbohydrate diet to a high-fat diet can see a three-kilogram weight loss from glycogen depletion (see practical hydration).
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The actual amount of liquid water (from drinks) that an individual requires depends on their age, gender, physical activity, physiological condition or illness, and the temperature and humidity [2464] of their physical environment. A healthy individual may have slightly lower or somewhat higher water intake without harm by varying their urine output. The recommended amounts are somewhat higher (1.0-1.5 mL ˣ kcal −1 ) than the average daily intake, being about 2.9 L for men and 2.2 L for women (rising to 4.8 L if pregnant or 3.3 L if lactating) [962]l. These higher levels of water intake seem to reduce the occurrences of kidney stones, gallstones, and some cancers and may be otherwise beneficial [963]. However, there appears to be no scientific source for the argument in favor of an important nutrient. There is insufficient evidence for either the benefit or the lack of any benefit, from drinking increased amounts of water [1446]. However, low water intake levels do not seem to show any health benefits and may be harmful.
Men require more water than women due to their higher (on average) fat-free mass and energy expenditure. Infants a and young children need more water in proportion to their body weight as they cannot concentrate their urine as efficiently as adults. Their surface area relative to their weight is more extensive, giving rise to greater water loss from the skin. Often children are under-hydrated [1907]. The elderly should ensure adequate hydration, as aging diminishes the sensation of thirst and the ability to concentrate the urine [2393]. However, it is difficult to accurately screen older people for dehydration by non-invasive tests [2600].
The water requirements, regulation, and balance (hydroregulation) of cold-blooded organisms (ectotherms) are closely connected to their thermoregulation [3664].
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Water plays many roles within the body [3289];
Water in the atmosphere (relative humidity) controls how hot we feel. In hot weather. The heat is more unbearable when combined with higher relative humidity. Thus, where the air temperature is 32 °C, this feels like 32 °C, 38 °C, 45 °C, and 56 °C, at 40%, 60%, 80%, and 100% relative humidity respectively. This is due to the increased difficulty that sweating has in cooling us down at high humidities due to the slower rate of cooling evaporation at high relative humidities.
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Hydration status is difficult to define or determine precisely or accurately. An indicator of hydration status is the osmolality of the blood. However, it is usually closely controlled around about 284 mOsmol/kg (increasing slightly (1-2 %) in the elderly and decreasing ≈ 3% during pregnancy) and is, therefore, a relatively poor indicator of hydration status. Short-term hydration status may be determined easiy and accurately by weight. Only water content affects weight over short periods when food intake, fecal output, and other possibly confounding factors (such as sweaty or changed clothing) are controlled.
Although the problem of dehydration in elite athletes appears to be exaggerated [1910], dehydration (starting at about 2-3% loss of body weight) otherwise causes a range of symptoms from tiredness, headaches, and decreased alertness to collapse and death (at more than 10% loss of body weight). Mild symptoms may be seen in the lack of concentration of schoolchildren towards the end of their school day, with an improved concentration in those less thirsty [1558]. Severe symptoms of dehydration are sometimes evident in the elderly due to restricted water intake for medical, psychological, or social reasons [3767]. Hypohydration may give rise to impaired vascular function and greater cardiovascular disease risk. Increased water intake is usually easily controlled due to the effective functioning of the kidneys to produce more urine. Suppose this does not occur due to greatly excessive water intake (for example, > 1.0 L/hr) or kidney disorder, then the extra water (hyperhydration) may produce low blood sodium levels and cause the brain to swell, resulting in death.
Water should be drunk little but often throughout the day such that we are never thirsty [474], but with care certainly that we are not over-hydrated as a 2% over-hydration producing generalized edema that can impair athletic and mental performance [1910]. It is particularly important to hydrate as the last activity at night to prepare for the significant loss of water during sleeping and rehydrate first thing in the morning. This is a time when the blood is most viscous and strokes particularly prevalent. We should also drink before, during, and after exercise to maintain our level of hydration. The thirst-quenching ability of soft drinks has been assessed [964]. Acidity was the most closely related taste attribute, with thirst-quenching, with sweetness and ‘thickness’ (viscosity) being the most contra-indicated.
The hydration rate is best measured by the use of D2O uptake into saliva [1231]. Water has to be rapidly emptied from the stomach and absorbed by the small intestine for rapid absorption, Typically this takes as little as 20-30 minutes, with half of a 300 mL drink being absorbed within about 15 (at rest) to 20 (after exercise) minutes. The uptake rate is faster when at rest than when exercising with 'sports drinks', containing sugar and salt, showing a marginally faster rate at rest, but a similar time for complete absorption. In this study [1231], there was no difference in the absorption rate, or time for complete absorption, between 'sports drinks' and water when taken after exercise.
In the light of the increased promotion of 'special' water preparations, it is essential to note that there are definite and proven health benefits from simply drinking more water and from changing fluid intakes from coffee, tea, alcohol, and hypertonic soft drinks to mineral or tap water [413]. That cup of coffee first thing in the morning is best, perhaps, replaced by a glass of water in order to reduce the higher risk of heart attacks at this time of day.
A beverage hydration index, involving the volume of urine produced after drinking relative to a standard treatment (still water), has been developed to identify the short-term hydration potential of different beverages [4354].
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There is no such thing as naturally pure water; all waters we drink contain dissolved solutes, and many contain some microorganisms [965]. Indeed, drinking 'pure' water, even if obtainable, when it would be costly and prone to unwanted materials being introduced during its production and storage, is not a healthy option as essential minerals are absent [1145]. There are several forms that the water we drink may take [2652], which vary subtly from each other; drinking water, spring water, tap water, natural mineral water, and water preparations are promoted with various health claims. Our perception of the water's drinkability is governed by its 'descriptors' rather than its contents [2092]. Bottled waters are subject to international regulations but are not necessarily safer than tap water. All such water must be drinkable, contain solutes (including those classed as contaminants) below the legally allowed limits, be bacteriologically safe, and be subject to continued monitoring. The presence of minerals (Ca2+, Mg2+) is noteworthy. Hard drinking water (>≈ 1.2 mM Ca2+, Mg2+) seems protective against cardiovascular disease compared to soft water [3246]. The absence of such minerals may be harmful if the water is habitually drunk or drunk in large amounts.
Natural and recycled water usually undergoes several processes before it is considered drinkable tap water. These include, in order,
Despite the high quality and rigorous control of tap water in developed countries, bottled water consumption is highly significant and has substantially increased over recent years. This is despite its high cost and environmental concerns over plastic bottles. The main reason given for this is taste, but blind taste tests have not supported this, with taste differences being illusionary rather than actual [3338].
| Tap water | Water, from any source, is treated to meet legal and quality standards. It may contain low or moderate amounts of minerals depending on the source of the water (for example, hard or soft water areas). It has usually been clarified to remove most particulates and treated to ensure no agricultural chemicals or harmful microorganisms are present. This is the major water product with over a billion glasses a day being consumed in the US alone, although most domestic tap water is used for washing, flushing the toilet, and through wastage. Often it is chlorinated, which ensures microbiological safety for long periods of storage and eliminates all risks from otherwise devastating diseases such as cholera and dysentery. Although chlorination has been shown to possibly produce potentially hazardous byproducts, the association between exposure and demonstrable adverse health effects is still unproven. The protection chlorination offers far outweighs this risk. Some countries (e.g.the Netherlands, but not the UK or USA), with high-quality modern distribution systems and low leakage, can distribute tap water without such residual disinfectant, and they still enjoy a lower risk of water-borne disease [2506]. Low-level fluoridation of water (for example, by adding SiF62−) to reduce dental caries is generally regarded as safe [966a], succeeds in its purpose, and is certainly economically beneficial in saving dental treatment [1580]. The health claims for fluoridation remain controversial [1048], but its use is expanding [1580]. Groundwaters containing excessive amounts of fluoride (> 1 mg L−1) are widespread [966b], where the fluoride derives mainly from the dissolution of natural minerals in the rocks and soils. Defluoridation may be necessary [966c].
If tap water is stored for extended periods (for example, within 'green' water conservation strategies [2490]), it will lose its disinfectant safety and be prone to microbiological growth such as Legionella. |
|---|---|
| Drinking water |
Dasani bottled water as sold in the UK
Water intended for human consumption [2433] and may contain disinfectants or other solutes within legal quality standards.
Many dissolved ions may be present in drinking water. Some of the ions are essential and beneficial (e.g., Na+, K+, Ca2+, Fe2+, Mg2+, Mn2+, F−, Cl−, I−, PO43−, MoO42− , and B(OH)4−), others are essential and beneficial but toxic in excess (e.g., Cu2+, Cr3+, Mn2+, and Zn2+), others are toxic in high amounts (e.g., Al3+). In contrast, others are toxic in trace amounts (e.g., Pb2+, Hg2+, Cd2+) and have no known nutritional value.
Such bottled water is not necessarily better for health than tap water, as shown in 2004 when Coca-Cola was awarded an Ig® Nobel prize for producing Dasani in the UK. Dasani was a bottled 'pure' water prepared from London tap water. It was found that it contained high levels of the carcinogen bromate, which is (and was) not present in the tap water. The bromate was introduced by the reaction between the added ozone and bromide [3332], naturally found within the calcium chloride, during production (for background science, see [1000]). |
| Natural mineral water |
Drinking water from a spring, artesian well, or well, naturally contains dissolved salts [967] (above 250 ppm in the US). It may be carbonated. It is characterized by its mineral content, which may vary from far lower to much higher than tap water, according to the water's source. Mineral waters must be naturally safe with no parasitic or pathogenic organisms as they are not subject to disinfection. The presence of safe microorganisms is used as proof that no disinfection has taken place,. However, the water must be regularly tested [1386] as bottled mineral water is recognized as a potential source of antibiotic-resistant bacteria to humans [1799]. Higher silica content distinguishes mineral water from surface water (for example, reservoir water). The price of mineral water is over a thousand times that of quality tap water. |
| Spring water |
Water from an underground naturally flowing aquifer, collected as it flows and bottled at the source. |
| Artesian water |
Water from a well tapping an underground aquifer, with a water level above the top of the aquifer. |
| Well water |
Water from a hole tapping an underground aquifer. |
| Purified water |
Purified water is water produced by distillation, deionization, reverse osmosis, or similar processes. Such water contains little or no mineral content. Many of these are micronutrients necessary for good health (Ca, P, Mg, F for bones and teeth; Zn, Cu, Se, Mg, Mn, Mo for enzymes; Fe, I, Cr for blood and hormones). Generally, the amounts involved are far less than obtained by the rest of the diet. However, where the pure water is used for extended periods (e.g., where the sole source of water is obtained by reverse osmosis) and this water is used for cooking (and hence leach ions from) vegetables, there may be mineral shortages in the diet and thought must be given to the provision of additives [2082]. Also, 'pure' water is unprotected (e.g., by chlorination) and will be prone to microbial spoilage [2082]. The ISO 3696:1995 and ASTM D1193-06 standards for pure water state the conductivity should be no greater than 10 μS cm−1 or 0.056 μS cm−1respectively at 25 °C. |
| Processed water with health claims |
Bottled water with health claims
There is an increasing market in bottled water and domestic water processing equipment claiming that the water has considerable health benefits varying from more rapid hydration to cures for AIDS and cancer. Generally, there are no proper scientific trials to prove these claims, only isolated testimonial evidence. Oxygenated drinks have been proposed to improve the immune status. However, a randomized, blinded clinical study [968a] was not conclusive, although showing a transient moderate increase in oxygen radicals (using 6 mM O2) and signs of immune response activation. Claims that oxygenated water improves athletic performance have not been proven [968b]. However, an increase in superoxide radical anion may increase seed germination.
One factor often used to promote these ‘health’ waters is supposed greater cellular hydration or ease of hydration. It is unclear whether increased cell hydration is health-promoting. It has been argued that this may be a determining factor in cancer initiation [969]. It has been found that cancer cells and older cells [1745] do have greater water with increased fluidity [1998] and free reactive hydroxyl groups [2280] but the cause and effect relationship (that is, whether increased cellular hydration initiates cancer or cell aging or cancer or cell aging initiates high cellular hydration) has not yet been established. In both cases, the increased fluidity (greater high-density water) leads to the generation of reactive oxygen species, and the cell becomes less able to repair any damage. More recently (2022) a higher plasticity has been found in some cancer tissues [4470].
Hydrogen gas therapy has increasingly been studied for a wide range of clinical purposes as it can cross cellular membranes, diffusing throughout the body and neutralizing the detrimental effects of reactive oxygen species [3563]. Care must be taken at concentrations above 4% because of its combustible properties. Water containing molecular hydrogen (H2) has been presented with several health claims, including type 2 diabetes, neonatal cerebral hypoxia and Parkinson's disease [3550, 3563]. This is despite intestinal bacteria producing a large amount of rapidly dissipating hydrogen gas that might be supposed to swamp out any such effect. The antioxidant capacity of hydrogen nanobubble water has been shown to remove superoxide radical ions, hydroxyl radicals, nitrate radicals, and chlorine monoxide radicals [3541]. Certainly, hydrogen water has therapeutic effects for this reason [3542]. Ingesting alkaline electrolyzed hydrogen water daily mproves health, intestinal normalization, and exercise capacity [3543]. As oxidative stress is one of the causative factors in the pathogenesis of major neurodegenerative diseases, including Alzheimer's dementia, mild cognitive impairment, and Parkinson's disease, it is probable that hydrogen water will be helpful in their treatment. It is also beneficial to cardiovascular function in diet-induced obesity mice [4287]. Hydrogen water is reported as having the potential for suppressing dementia in apolipoprotein E4 carriers with mild cognitive impairment [3545]. Most molecular hydrogen introduced into the body by drinking hydrogen water will be expelled in the breath with significant amounts reaching the colon and aiding methanogenic, sulfate-reducing, or acetogenic fermentation processes. Electrolytically produced hydrogen water may be more effective if it contains platinum nanoparticles from the electrode [3544]. The reduction in superoxide radical anion may reduce seed germination. |
| ‘Sports’ drinks |
Sports drinks [973] are intended to reduce fluid, mineral (e.g., particularly Na+), and energy imbalance due to exercise. The carbohydrate content and osmolality must both be low to encourage efficient hydration (that is, the drink must be hypotonic (<280 mOsmol ˣ L−1) or isotonic (≈ 280 mOsmol/L)). Na+ ions (usually as NaCl) are a necessary ingredient as they stimulate both sugar and water uptake in the small intestine and replace material lost by sweat. Hypotonic drinks give more rapid hydration but contain less sugar and minerals. Chilling improves palatability, so encouraging consumption. Some sports drinks contain ‘power’ ingredients such as caffeine or taurine, where there is patchy evidence of some sports benefits. These products are usually promoted with testimonials from athletes or sports teams but without double-blinded trial evidence. |
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a Fully breastfed babies do not require extra water [1111]; breast milk having a higher water content for its energy content than the adult diet. Compared with adults (despite appearances to the contrary), they do not produce relatively more urine than adults as they tend to retain more water for growth and have a high loss through their relatively larger surface area [1346]. [Back]
b The water content of our bodies is sometimes misreported as for fat-free bodies without making this clear (i.e., the fat-free status is not mentioned). Such reports give the impression that our bodies contain higher proportions of water, as we contain substantial amounts of fat. The figures presented here are for 'average' total weights, inclusive of fat content. [Back]
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This page was established in 2006 and last updated by Martin Chaplin on 17 July, 2022
