Hydration, water and health
Water for drinking
Adequate hydration is an absolute requirement for our health  and all active life.
Liquid water is the most important nutrient throughout the living world .
In particular, we cannot live without it for more than about
100 hours, whereas other nutrients may be neglected for weeks
or months. Although commonly it is treated rather trivially,
no other nutrient is more essential or needed in as great
The water content of our bodies (methodology reviewed )
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. The gender
differences, from the teenager years onwards, are due to their
differing fat levels, as is the drop in the elderly who replace
muscle mass with fat. There is little difference with gender
or age from childhood onwards, if allowance is made for this
fat content. Body water is distributed between the cells (intracellular
fluid, ICF, ~59%; ~26 L in a 75 kg man, ICF, ~61%; ~19 L in a 60 kg woman )
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 ) (for reference values for extracellular water see ). Water
is free to move between the ICF and the ECF with any net movement
controlled by the effective osmotic and hydrostatic pressures.
The molecules of water have a biological half-life in our bodies of about 9-10 days with an average residence time of about 2 weeks (these estimates depend on the person's age, gender, build and water consumption with higher intakes giving shorter half life). 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 intake and output are highly variable but closely matched
to less than 0.1% over an extended period. Water balance in
humans has been modeled .
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:
Human water balance
|From skin, sweate
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
drinks, these count and act as any other water intake .
b Water contained in foodstuffs,
varying from ~6% in peanuts, ~35% in bread to ~85% in fruits
c Water produced on metabolizing
the foodstuffs and drinks (for example, 1 g fat gives ~1.1
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
f Varies with energy intake and
expenditure, ambient temperature and humidity. Together with
losses from skin this typically amounts to about 50 mL/100
kcal food energy intake.
g Varies with diet, particularly
increasing with increased dietary
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 is becoming
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 weight loss less liquid intake, so long as metabolic activity (metabolic water gain and substrate oxidation mass loss) is also accounted for .
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The actual amount of liquid water (from drinks) that an individual
needs depends on their age, gender, physical activity, physiological
condition or illness and the temperature and humidity of their
physical environment. A healthy individual may have slightly
lower or somewhat higher water intakes without harm by varying
their urine output. The recommended amounts are somewhat higher
(1.0-1.5 mL/kcal) than the average intakes, being about 3.0
L for men and 2.2 L for women (rising to 2.3 L if pregnant
or 3.1 L if lactating) l.
These higher levels of water intake seem to reduce the occurrences
of kidney stones, gall stones and some cancers and may be
otherwise beneficial .
However, there seems to be no scientific source for the argument
in favor of much increased water intake (for example, for
the statement "Drink at least eight glasses of water
a day" or similar) 
with both benefits and potential hazards of extra water intake
being documented . Perhaps surprisingly for such an important nutrient, there is insufficient evidence for either the benefit, or the lack of any benefit, from drinking increased amounts of water . However,
low levels of water intake 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. Infantsa and
young children have need for more water in proportion to their
body weight as they cannot concentrate their urine as efficiently
as adults and their surface area relative to their weight
is more extensive, giving rise to greater water loss from
the skin. Often children are under-hydrated . The elderly should take care to ensure adequate
hydration, as ageing diminishes the sensation of thirst as
well as the ability to concentrate the urine.
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Water plays many roles within the body; as a media for, and
contributor to, molecular interactions; as a solvent and separating
medium, to carry and distribute nutrients, metabolites, hormones
and other materials around the body and within cells; to remove
waste products, mainly via the urine and feces; as a reactant
in many metabolic reactions; as a thermoregulator due to its high specific heat and heat
of evaporation; as a lubricant between bodily structures
and in forming mucous as well as facilitating necessary structural
shifts in macromolecules such as proteins and nucleic acids;
as a structure-former , maintaining cellular shape; and as
a protective shock absorber, for example, for the brain.
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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 normally 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 simply and accurately by
weight as 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 an exagerated problem , 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 improved concentration in those less thirsty . Severe
symptoms of dehydration are sometimes evident in the elderly,
due to restricted water intake for medical, psychological
or social reasons. Increased water intake is normally easily
controlled due to the effective functioning of the kidneys
to produce more urine. If 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
Water should be drunk little but often throughout the day
such that we are never thirsty, but with care certainly that we are not over-hydrated as a 2% over-hydration producing a generalised oedema that can impair athletic and mental performance . It is particularly important
to hydrate last thing at night to prepare for the significant
loss of water during sleeping and rehydrate first thing in
the morning as 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
was found to be the taste attribute most closely related with
thirst-quenching with sweetness and ‘thickness’ (viscosity)
being the most contra-indicated.
The rate of hydration is best measured by the use of D2O uptake into saliva . For rapid uptake water has to be rapidly emptied from the stomach and absorbed by the small intestine. 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 rate of uptake is faster when at rest than when exercising with 'sports drinks', containing sugar and salt, showing a marginally faster rate, at rest, but similar time for complete absorption. In this study , 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 important to take notice 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
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.
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Water for drinking
There is no such thing as naturally pure water; all waters
we drink contain dissolved solutes and many contain some microorganisms
. Indeed, drinking 'pure' water even if obtainable, when it would be very expensive and prone to unwanted materials being introduced during its production and storage, is not a healthy option as important minerals are absent
are several forms that the water we drink may take, which
vary subtly from each other; drinking water, spring water,
tap water, natural mineral water and water preparations promoted
with various health claims. Bottled waters are subject to
international regulations but are not necessarily safer than
tap water. Clearly, all such water must be drinkable, contain
solutes (including those classed as contaminants) below the
legally-allowed limits, to be bacteriologically safe and be
subject to continued monitoring.
Types of water for drinking
||Water, from any source, 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). 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 and the protection chlorination offers far outweighs
this risk. Fluoridation of water (for example, by adding
SiF62-) for the purpose of reducing
dental caries, is generally regarded as safe [966a], succeeds in its purpose and is certainly economically beneficial in saving dental treatment . The health claims for fluoridation remain controversial  but its use is expanding .
Groundwaters containing excessive amounts of
fluoride (> 1 mg/liter) are widespread [966b].
Water intended for human consumption and may contain
disinfectants and/or other solutes within legal quality
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 reaction between the added
ozone and calcium chloride containing calcium bromide
during production (for background science see ).
|Natural mineral water
||Water from a spring, artesian well or well that naturally
contains dissolved salts  (above 250 ppm in the US).
It may be carbonated. It is characterized by its mineral
content, which may vary between far lower to much higher
than tap water, according to 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, but the water must be regularly tested  as bottled mineral water is recognised as a potential source of antibiotic resistant bacteria to humans . Higher silica content distinguishes mineral
water from surface (for example, reservoir) water. The
price of mineral water is over a thousand times that of
quality tap water.
||Water from an underground naturally flowing aquifer, collected as it flows
and bottled at source.
||Water from a well tapping an underground aquifer, with a water level above that of the top of the aquifer.
||Water from a hole tapping an underground aquifer.
||Water produced by distillation, deionization, reverse osmosis or similar process.
|Processed 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 ,
although showing a transient moderate increase in oxygen
radicals (using 6 mM O2) and signs of activation
of the immune response, was not conclusive.
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 actually
health-promoting. A recent paper has argued that this
may be a determining factor in the initiation of cancer
It has been found that cancer cells and older cells  do have greater
water with increased fluidity 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 damage.
||Sports drinks 
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/L) 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 as well as replacing
material lost by sweat. Hypotonic drinks give more rapid
hydration but clearly 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 benefit. 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 ; breast milk having a higher water content for its energy content than the adult diet. Compared with adults (and in spite of 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 . [Back]