Our individual need for water depends on numerous factors. An individual's metabolism, activity level, body size, environment (humidity level and altitude, most significantly), quality of health, age, and pregnancy/breastfeeding are the major factors. In general, we want to replace the fluids we lose in a day, and intensive activity (with its accompanying sweat) will increase the amount of fluid we need.
The human body can last weeks without food, but only days without water. The body is made up of 55-75% water. Mature adults are about 70% water; this drops to about 60% in the elderly and continues to drop into very old age. Water forms the basis of blood, digestive juices, urine and perspiration.
Loss from lungs and skin accounts for 50% of water loss; losses from urine and feces account for the rest of the total. The drier our climate, the more water we tend to lose, but this does not make as large a difference as one might think.
For prolonged, intensive exercise and/or significant water intake, it's essential to balance salt/electrolytes with water.
Those who are ill can require more water, depending on their condition and any treatments they are receiving. For example, people with kidney disease, kidney stones, a history of bladder cancer, or a tendency for urinary tract infections are usually advised to drink more water.
Most of us can safely rely on our body's instincts to tell us when we need water. It has become a common belief that we need "2 liters or 8 glasses" of water daily and that beverages containing caffeine or alcohol do not count because they increase the excretion of water or have a diuretic effect. As far as we know, this belief has never been scientifically tested, and there is little evidence to support it.
Drinking an extreme excess of water can lead to water intoxication, with symptoms varying from headache, blurred vision, cramps, convulsions... to death.
Chronic over-consumption of water can cause the relative concentration of important electrolytes in the blood – such as sodium – to drop. This condition, called hyponatremia, in turn forces water out of the bloodstream and into cells, causing them to swell. This does not matter much for a muscle cell, but can be catastrophic when it's a brain cell and there's no extra space to expand into. Replacing plain water with 'balanced' drinks such as Gatorade® will help in that respect.
Drinking too much water can also cause excessive loss of the body's essential minerals via urine.
When you drink lots of fluid at a meal, you are substantially diluting the stomach acid and diminishing its ability to effectively digest your food. This would suggest that many cases of GERD, gas, stomach upset and other common complaints might be addressed simply by NOT drinking so much water throughout the day and refraining entirely from drinking while eating.
It should also be borne in mind that the stomach is one of the first lines of defense in the immune system. Bacteria and yeast that are regularly consumed along with food can be quickly and easily dispensed with in a very acidic stomach, preventing what might otherwise become a short term bout of food poisoning or a possible longer term GI tract infection. So, again, allowing undiluted stomach acid to do its work may be helpful.
Mild hyponatremia is treated by reducing water intake and monitoring the use of diuretics.
If the kidneys are not allowing the body to get rid of excess fluid, fluid intake may need to be restricted so that the kidneys don't have to work as hard. Fluid restrictions are usually only used in severe cases or if the renal failure has developed into end-stage renal failure.
Occasionally, the quantity of fluid a patient is allowed to drink is restricted.
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