Download Water intake recommendations

Water
Water in the body contains numerous dissolved minerals, called electrolytes that
are kept in constant balance. To live, each cell must have just the right mix of
water and electrolytes. Despite a continuous flow of molecules between
intracellular fluid, extracellular fluid, and the outside environment, the body
maintains its electrolyte balances through the intake and excretion of water and
the movement of ions.
Humans can survive for weeks without food, but can live only a few days
without water. The body has no capacity to store “spare” water, so it must
quickly replace lost fluid.
Overall, water makes up between 50 and 75 percent of body weight. The main
constituent of the body is water. Adult males have more lean tissue and less fat
than adult females.
An adult male is approximately 62% water, 17% protein, 15% fat, and 6%
minerals and glycogen.
Leaner people have proportionately more water, because muscle tissue is nearly
three-fourths water by weight, while adipose tissue is only about 10 percent
water.
Water has a strong surface tension, high heat capacity and ability to dissolve
many substances result from hydrogen bonds between a hydrogen atom of one
water molecule and the oxygen atom of another water molecule.
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Functions of Water
Water moves nutrients and wastes between cells and organs. In the intestines,
water solubilizes and moves nutrients to your cells and tissues, and it also
carries waste out of your body in urine.
Heat Capacity
The heat capacity of a substance is the amount of energy required to raise its
temperature 10C. Raising the temperature of a substance with a high heat
capacity requires more energy than raising the temperature of a substance with a
low one. Water, for instance, has about three times the heat capacity of iron.
Warming or cooling a substance with a high heat capacity requires a relatively
large amount of energy. Because of water’s high heat capacity, it takes a lot of
heat to change the temperature of the body.
Cooling Ability
A rise in body temperature, whether due to exercise, environmental conditions,
or illness, triggers the body’s cooling system. If you get too warm, blood vessels
dilate and you begin to sweat. The perspiration evaporates on the skin, thereby
cooling your body.
Participation in Metabolism
Nearly all the chemical reactions of metabolism involve water. Water is the
solvent for many biologically essential molecules (e.g., glucose, vitamins,
minerals, and amino acids), and it is a product or reactant in many biochemical
reactions.
pH Balance
Water is also an essential component of the body’s mechanisms to maintain pH
(acid-base) balance in the narrow range necessary for life. One of the major
buffer systems involves carbonic acid and bicarbonate. Carbonic acid forms
when dissolved carbon dioxide reacts with water (CO2 + H2O
H2CO3).
Carbonic acid can then dissociate to form H+ and HCO3; (bicarbonate). The
resulting H+ helps increase acidity, lowering pH.
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CO2 + H2O
H2CO3
H2CO3
H+ + HCO3
Body Fluids
Water is the major component of all body fluids. Water also has a protective,
mechanical and cleansing functions, for example; amniotic fluid provides a
gentle cushion that protects the fetus, synovial fluid allows joints to move
smoothly, tears cleanse the eyes, and saliva moistens food and makes
swallowing possible.
Electrolytes and Water
The body precisely controls and balances the concentration of electrolytes
dissolved in its watery fluids. When salts, such as sodium chloride, dissolve in
water they come apart and form free ions, which are positively (e.g., Na +) and
negatively (e.g., Cl-) charged particles. In an electrolyte solution the number of
positive charges always equals the number of negative charges. The main
positively charged ions (cations) in the body are sodium and potassium, and the
main negatively charged ions (anions) are chloride and phosphate.
There are two major fluid compartments in the body. About two-thirds of body
water is in intracellular fluid and one-third is in extracellular fluid. The major
components of extracellular fluid are interstitial fluid (the fluid between cells)
and blood plasma (the fluid portion of blood). Sodium is the main cation in
extracellular fluid, whereas potassium is the predominant cation of intracellular
fluid. To maintain the balance of sodium and potassium, all cell membranes
incorporate sodium-potassium pumps that actively pump sodium out of the cell
while allowing potassium back in. If solutes are more concentrated on one side
of a semipermeable membrane (through which water, but not solutes, can pass
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easily), water flows to the side of higher concentration until the concentrations
on both sides are the same. This movement is called osmosis; osmotic pressure
is the force that causes water to flow across a membrane to the side with a
higher concentration of ions.
Electrolyte: Substances that dissociate into charged particles (ions) when
dissolved in water or other solvents and thus become capable of conducting an
electrical current.
Hydrogen bond: Non-covalent bond between hydrogen and an atom, usually
oxygen, in another molecule.
Heat capacity: The amount of energy required to raise the temperature of a
substance 1OC.
salts Compounds that result from the replacement of the hydrogen of an acid
with a metal or a group that acts like a metal.
ion An atom or group of atoms with an electrical charge resulting from the loss
or gain of one or more electrons.
cation Ion that carries a positive charge
anion Ion that carries a negative charge.
osmosis The movement of a solvent, such as water, through a semipermeable
membrane from the low-solute to the high-solute solution until the concentrations on both sides of the membrane are equal.
osmotic pressure The pressure exerted on a semipermeable membrane by a
solvent, usually water, moving from the side of low-solute to the side of highsolute concentration.
Plasma The fluid portion of the blood that contains blood cells and other
components.
sodium-potassium pump Mechanism that pumps sodium ions out of a cell,
allowing potassium ions to enter the cell.
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Conclusion: Water is the most essential nutrient. Water’s functions in the body
include temperature regulation, metabolism, acid-base regulation and
protection. The balance of body fluids and the amount of electrolytes dissolved
in the body water are controlled precisely. Potassium is the main intracellular
cation, and sodium is the main extracellular cation.
Water intake recommendations
Each individual needs a different amount of water depending on body size and
composition, activity level, and the temperature and humidity of the
environment. An “average” adult who expends 2,400 kilocalories daily loses
approximately 2.4 liters (about 10 cups) of water each day. Because we cannot
store water, the average adult needs to replace at least 2.4 liters of water a day.
Beverages, foods, and water produced by metabolic reactions supply the water
that the body needs. You should drink 8 to 10 glasses of water every day.
Metabolism typically provides about 400 milliliters to 500 milliliters, so the
remainder (1,500 ml to 2,500 ml) must come from beverages and foods.
Water Excretion
Our bodies continuously lose water through various routes:
In the lungs, water evaporates and exits in exhaled air.
Water also excreted through the skin by evaporation and perspiration.
In the GI tract feces carry water out of the body.
The kidneys excrete water in urine.
Typically daily water intake and output:
Intake:
a) Food 700-1000ml.
b) Drink 550-1500ml.
c) Metabolic 200-300ml.
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Output:
a) Kidneys (urine) 500-1400ml.
b) Skin 450-900 ml.
c) Lungs 350ml.
d) Feces 150 ml.
* People with diarrhea may lose several liters of water per day in feces.
Water plays a critical role in the elimination of wastes. Urea, a breakdown
product of protein metabolism, is a major component of urine. If we overconsume protein and salt, the kidneys have to work harder to eliminate excess
urea and sodium from the body. The more protein and sodium you consume, the
more fluid you need to consume and the more urine you are likely to produce.
Conclusion: Water intake comes from a combination of foods, fluids, and water
produced in normal metabolism. The main method of water excretion is in
urine. In addition, fluid is lost through the skin and lungs, and with feces.
Losses are higher when a person perspires heavily or is ill. Water is critical in
eliminating the body’s waste products.
Water Balance
Our bodies maintain water balance by mechanisms that control water intake
(e.g., thirst) and water excretion. The body works to closely regulate total body
water.
Regulation of Fluid Excretion
Our kidneys adjust the amount and concentration of urine in response to the
body’s hydration status. The kidneys can excrete a small volume of concentrated urine or a large volume of dilute urine while maintaining a relatively
constant excretion of solutes such as sodium and potassium. This ability to
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regulate water excretion without major changes in solute excretion is an
important survival mechanism, especially when water is in short supply.
When water intake is low, the kidneys conserve water. While continuing to
excrete solutes, they reabsorb water, thus decreasing urine volume and
concentrating the urine. When the body has an excess of water, the kidneys
form and excrete a large volume of dilute urine.
How do the kidneys know when to conserve water?
Osmoreceptors, special cells in the hypothalamus of the brain, are exquisitely
sensitive to very small increases in extracellular sodium concentration and thus
sense the body’s need for water. If the sodium concentration rises, these
receptors signal the pituitary gland to release antiduretic hormone (ADH). ADH
decreases water loss by causing the kidneys to reabsorb water rather than
excrete it in the urine. ADH signals the kidney to conserve water.
Regulation of Blood Volume and Pressure
The kidneys themselves have sensors that detect falling blood pressure. In
response, the kidneys release renin, an enzyme that splits off a small protein,
angiotensin I, from the blood protein angiotensinogen. Within seconds, enzymes
in the small blood vessels of the lungs convert nearly all angiotensin I to
angiotensin II. While angiotensin II is a powerful vasoconstrictor, it also acts
directly on the kidneys to decrease excretion of sodium and water. In addition,
this protein causes the release of aldosterone, a hormone from the adrenal
glands. Aldosterone also causes the kidneys to retain sodium, and since water
follows sodium, water retention increases as well. All of these processes work
to restore blood pressure and volume.
The ability of angiotensin II to act as a vasoconstrictor and increase blood
pressure is a life-saving measure. This acute short-term action helps compensate
for a severe loss of blood, such as occurs during a hemorrhage. Angiotensin II’s
effect on the kidney increases extracellular fluid volume and arterial blood
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pressure over a period of hours or days.
One the most important roles of the renin-angiotensin system is its response to
dietary sodium. It allows a person to consume either very small or very large
amounts of sodium without causing major changes in extracellular fluid volume
or blood pressure. Since water follows sodium, increased sodium intake
increases extracellular fluid volume and blood pressure. This reduces the
secretion of renin and production of angiotensin, leading to decreased retention
of sodium and water by the kidneys. The resulting excretion of water and
sodium returns extracellular volume and blood pressure to normal. A low
sodium intake triggers the opposite effects.
insensible water loss: The continual loss of body water by evaporation from
the respiratory tract and diffusion through the skin.
Osmoreceptors: Neurons in the hypothalamus that detect changes in the fluid
concentration in blood and regulate the release of antiduretic hormone.
Antiduretic hormone (ADH): A peptide hormone secreted by the pituitary
gland, it increases blood pressure and prevents fluid excretion by the kidneys.
Osmolarity: The concentration of dissolved particles (e.g., electrolytes) in a
solution expressed per unit of volume.
Renin: An enzyme produced by the kidney that affects blood pressure by
catalyzing the conversion of angiotensinogen to angiotensin I.
Angiotensinogen: A circulating protein produced by the liver from which
angiotensin I is cleaved by the action of renin.
Angiotensin I: A 10 amino-acid peptide that is a precursor of angiotensin II.
Angiotensin II: In the lungs, the 8-amino-acid peptide angiotensin II is formed
from angiotensin I. Angiotensin II is a powerful vasoconstrictor that rapidly
raises blood pressure.
Aldosterone: A steroid hormone secreted from the adrenal glands that acts on
the kidneys to regulate electrolyte and water balance. It raises blood pressure by
promoting retention of sodium (and thus water) and excretion of potassium.
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Thirst
Thirst is our most important stimulus for drinking. Why do we become thirsty?
The four major stimuli for thirst are:
1. Increased osmolarity of the fluid surrounding the osmoreceptors in the
hypothalamus.
2. Reduced blood volume and blood pressure.
3. Increased angiotensin II.
4. Dryness of the mouth and mucous membranes lining the esophagus.
* Older people and infants are particularly vulnerable to dehydration. The
sensitivity of the thirst response declines with age, putting the elderly at high
risk. Infants need to take in a large amount of water relative to their size because
a large proportion of their body weight is water. Breast milk or infant formula
provides an appropriate amount of fluid for infants. People who care for
children and elders must remember to give them fluid often.
Conclusion: The body has mechanisms that balance water among
compartments and regulate total body water. Antidiuretic hormone (ADH)
stimulates water reabsorption in the kidneys, while aldosterone stimulates the
kidneys to reabsorb sodium (and thus water). Thirst is not a reliable indicator to
avoid dehydration when fluid losses are high, such as during hot weather or
heavy exercise.
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Substances That Affect Fluid Balance
1) Alcohol
Alcohol is a diuretic—a drug that increases fluid loss through increased
urination.
Alcohol increases fluid loss by suppressing ADH production, and excessive
alcohol consumption can cause dehydration. Symptoms of mild to moderate
dehydration include thirst, weakness, dryness of mucous membranes and
dizziness.
2) Caffeine
Caffeine is a strong diuretic. Like alcohol, it inhibits ADH activity.
3) Diuretic Medications
Diuretics are some of the most common medications. They can help lower
blood pressure, decrease high intracranial pressure, or reduce excess pressure in
the eye. On the other hand, diuretics can significantly disrupt sodium and
potassium balance.
Dehydration
Dehydration is a major killer worldwide—infants and the frail elderly are
especially vulnerable.
Gastrointestinal infections are primarily responsible. These infections cause
diarrhea and prolonged vomiting, leading to excessive water loss.
Unless treated rapidly, a person who loses an amount of water equal to 20
percent of body weight is likely to become comatose and die.
Burns also can cause deadly dehydration. Extensively damaged skin cannot
protect the body and prevent excessive fluid loss.
Dehydration diminishes physical and mental performance. Early signs of
dehydration include fatigue, dry mouth, headache, and dark urine with a strong
odor Change in urine color reflects the body’s attempt to conserve water by
increasing water reabsorption in the kidney.
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Water consumption is the primary treatment for dehydration. Oral-rehydration
solutions also can be used; typically these consist of simple ingredients
including clean water, sugar, and table salt. Oral rehydration may be sufficient
for mild dehydration, but intravenous fluids and hospitalization may be
necessary for moderate to severe dehydration. Diarrhea and prolonged
vomiting, which cause heavy fluid and electrolyte losses, can be fatal unless the
person is rapidly rehydrated with electrolyte solutions.
Conclusion: Alcohol, caffeine, and diuretic medications increase urinary fluid
losses. Chronic ingestion of these substances may contribute to dehydration.
Dehydration occurs when fluid loss exceeds fluid intake; it is a potential consequence of gastrointestinal disease, burns, and heavy sweating. Treatment
involves replacing fluids, along with electrolytes if the condition is severe.
Minerals
Minerals are not changed during digestion or when the body uses them. Unlike
many vitamins, minerals are not destroyed by heat, light, or alkalinity.
Minerals play many essential roles in the body.
Some minerals, such as
magnesium, participate in the catalytic activity of enzymes. Others serve a
structural function; for example, calcium and phosphorus are among the
minerals that make our bones hard. Minerals are categorized as major or trace
minerals, based on the amount needed in the diet and the amount of the mineral
in the body. This classification of minerals is unrelated to the mineral’s
biological importance. For example, iron is a trace mineral but it plays a critical
role in many major metabolic reactions.
Drinking water can sometimes be a significant source of several minerals such
as sodium, magnesium, and fluoride.
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Bioavailability of Minerals
A variety of factors affect the bioavailability (the amount available to the body)
of minerals from foods. The body absorbs many minerals in proportion to its
needs. For example, a calcium-deficient person more readily absorbs calcium
than does a person with normal calcium levels. Overloading one mineral by
taking supplements can affect the absorption of other minerals. Minerals like
calcium, iron, and magnesium (which all have similar chemical properties,
including a 2+ charge) compete for absorption. Fiber content of food also can
affect mineral bioavailability. High-fiber diets can reduce the absorption of iron,
calcium, zinc, and magnesium. Phytate in grain fibers binds minerals,
sequestering them and carrying them out of the intestine unabsorbed. Oxalate,
found for example in spinach, binds calcium; consequently decrease the
absorption of calcium.
Major minerals Major minerals are required in the diet and present in the
body in large amounts compared to trace minerals.
Trace minerals Minerals required in the diet and present in the body in very
small quantities, less than 100 mg each.
Phytate; A phosphorus-containing compound in the outer husks of cereal
grains that binds with minerals and inhibits their absorption.
Oxalate: An organic acid in some leafy green vegetables, such as spinach,
that binds to calcium to form calcium oxalate, an insoluble compound the
body cannot absorb.
Conclusion: Minerals are essential inorganic elements. Those that we need and
store in larger amounts are called major minerals, and those we need in very
small quantities are the trace minerals. A wide variety of foods contain
minerals. Physiological needs, competition with minerals, and fiber content of
food all affect mineral bioavailability.
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Functions of Sodium
Sodium is the major cation in extracellular fluid and a critical electrolyte in the
regulation of body fluids. It acts in concert with potassium, the major cation in
intracellular fluid, and chloride, the major anion in extracellular fluid, to
maintain proper body water distribution and blood pressure. Nerve transmission
and muscle function require sodium. Sodium also helps control the body’s
acidity, and aids the absorption of some nutrients such as glucose.
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