Liquid water is an absolute requirement for all active life. It 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 amounts.
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 foetus 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&rsquos.
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, ~65%; ~30 L in a 75 kg man, ~20 L in a
60 kg woman) and the extracellular fluid (ECF, ~35%; ~15 L in a 75
kg man including the ~3 L of plasma, ~10 L in a 60 kg woman). Water
is free to move between the ICF and the ECF with any net movement
controlled by the effective osmotic and hydrostatic pressures. The
majority of the ions in the ICF are K+ and protein
anions whereas in the ECF they are Na+, Cl-
and bicarbonate.
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 [583]. 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 balance
|
Water input, ml/day
|
Water output, ml/day
|
| Drinksa
|
1500
|
Urined
|
1500
|
| Foodb
|
700
|
From skin, sweate
|
500
|
| Metabolic waterc
|
300
|
Respirationf
|
> 400
|
| |
Fecesg
|
100
|
a Water, fizzy drinks, tea,
coffee, alcoholic beverages, etc. 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 [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 (e.g. 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 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 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&lsquos hydration status.  >
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) [962]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 [963]. However, there seems to be no scientific
source for the argument in favor of much increased water intake
(e.g. for the statement "Drink at least eight glasses of
water a day" or similar) [474] with both benefits and potential hazards
of extra water intake being documented [474].
Generally, low levels of water intake do not seem to show any
health benefits.
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. 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.
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 facilitaing necessary structural shifts
in macromolecules such as proteins and nucleic acids; as a
structure-former , maintaining cellular shape; and as a protective
shock absorber, e.g. for the brain.
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.
Dehydration (starting at about 2-3% loss of body weight) 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. 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
(e.g. > 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. 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 [964]. Acidity was found to be the taste
attribute most closely related with thirst-quenching with sweetness
and &lsquothickness&rsquo (viscosity) being the most
contra-indicated.
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 very expensive and prone to unwanted
materials being introduced during its production and storage, is
not a healthy option as important minerals are absent [1145]. There 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. >
| Tap water |
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 (e.g. 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 (e.g. by adding
SiF62-) for the purpose of reducing dental
caries, is generally regarded as safe [966a]. However, groundwaters containing
excessive amounts of fluoride (> 1 mg/liter) are widespead [966b]. The
health claims for fluoridation remain contraversial [1048]. |
| Drinking water |
Water intended for human consumption and may contain
disinfectants and/or other solutes within legal quality standards.
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 [1000]). |
| Natural mineral water |
Water from a spring, artesian well or well that naturally
contains dissolved salts [967]. 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. Higher silica content distinguishes mineral water from
surface (e.g. reservoir) water. The price of mineral water
is over a thousand times that of quality tap water. |
| Spring water |
Water from an underground aquifer, collected as it flows and
bottled at source. |
| 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 [968], 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
&lsquohealth&rsquo 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 [969]. It has been found that cancer cells do
have greater water with increased fluidity but the cause and effect
relationship (i.e. whether increased cellular hydration
initiates cancer or cancer initiates high cellular hydration) has
not yet been established.
|
| &lsquoSports&rsquo 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 (i.e. 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 &lsquopower&rsquo 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. |
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 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 Fully breastfed babies do
not require extra water [1111].
Source: http://www.lsbu.ac.uk/water/health.html
|
