HOW MANY TIMES have you seen a young woman toting around a large plastic container filled with pure spring water--a commodity more precious than fuel at the pump--from the hills of Colorado, Pennsylvania, the Alps or some such high elevation? Is she really constantly thirsty? Is her need for water really three or four or more quarts per day? Does it make her skin more radiant? Is it just plain healthy? Will she become dehydrated unless she forces the water down at a steady pace? Should she follow the mayor of Philadelphia's trademark advice and "Don't forget to drink your water"? Correct answers: No, no, no, no, no, and sometimes.

The supposed health benefit of consuming large volumes of water has become one of those urban myths that even some physicians have come to endorse without real insight into the science underlying water intake and its effects on the body.

Several Nobel Prizes, the latest in 2004, have been awarded for discoveries that explain how humans perfectly regulate the total amount of water in the body. The marvelous system that regulates the body's water content nearly perfectly prevents any excess, unless individuals consume enormous amounts (greater than a quart per hour for several hours) or they have a condition that impairs the normal robust capacity of their kidneys to eliminate over 20 quarts per day. As soon as a few ounces of extra water are consumed, a master hormone called anti-diuretic hormone (the name derives from the hormone's capacity to stimulate the kidney to retain water) falls to undetectable levels and allows the kidney to excrete, in a matter of minutes, all of the surplus water ingested. This system--the hormone originates in the brain and acts upon a specific part of the kidney--exists in virtually all animals; It can lead to the retention of virtually all water ingested, if the body has a true water deficit, or the excretion of as much as 20 to 25 quarts of water per day, if intake of water is that excessive. It maintains perfect balance over the years so that total body water remains within a percent or so of the baseline for as long as one is healthy.

Our young woman toting around her bottle of water can only retain a few extra ounces in her body no matter how quickly she drinks it. Moreover, the amount she can retain is truly only a drop in the bucket. The svelte 5'9" woman who weighs, say 125 lbs., has about 75 lbs. of water (about 35 quarts) in her body. The extra water retained in a few sips hardly increases the body's content of water and even then, the excess is rapidly eliminated in the urine. Therefore there is no possibility that consumed water can make a sustained difference in anything but how often she needs to find a ladies room.

SO WHY DOES THE BODY need water at all? Since we lose water continuously through the skin by the process of evaporation, repletion is a requisite. This process of evaporative loss through the skin is not sensed (the technical name is insensible loss) and serves the purpose of eliminating excess heat from the body. This process may become observable as perspiration if the ambient temperature is uncomfortably hot or if excess body heat is generated through intense muscular activity, as skin losses of fluid under these conditions may rise to several quarts per day. But normal daily skin losses of water occur at rate of about 1 quart per day in a cool, air-conditioned environment to which most of us are accustomed.

There is also an obligate loss of body water as urine. But if we do not consume an unusual diet, we are only obliged to produce about 1/2 quart of urine per day to allow our kidneys to excrete the products of waste that are generated from a typical diet.

Given these two sources of water loss, most people in temperate climates would need to consume about one and one half quarts of fluid per day to replace the obligate losses of water from the body. Since most glasses contain about 12 oz. of fluid, about 5 glasses of fluid each day is sufficient to maintain a stable water content of the body. Of course this includes all the fluid in coffee, lattes, soda, or anything else that is liquid and consumed each day.

IF YOU DO NOT KEEP UP WITH LOSSES, nature has a sensing system in place called thirst. Under almost all circumstances it works very effectively to alert an individual when water losses have produced even a minor degree of water deficiency. Thirst is first perceived in the mouth but it is a highly complex function that involves a number of hormones and brain functions, all of which serve to signal fluid intake. And one need not worry if the fluid is not pure water--the fluids humans consume are almost entirely composed of water. As soon as a few extra ounces of fluid are consumed, the system rapidly and efficiently eliminates any overage.

So is there any benefit to water-bottle toting and forced intake? To those who suffer from kidney stones and those with recurrent bladder infections, some incremental fluid intake is beneficial. But the idea that everyone who exercises needs to force fluids in order to be adequately hydrated and avoid calamity is simply not true. In fact, as was recently reported in the New York Times--just before the running of the New York Marathon--the excess consumption of water during situations such as a 26 mile run is dangerous. Recommendations to consume water at a rate faster than about a pint every half hour are excessive, as recently reported in the European Journal of Applied Physiology. The reason for this is that the body actually generates water through the metabolism of fats and starches and sugars during intense exercise so that large volumes of excess water are not needed and, if consumed too quickly, can overcome the kidneys' robust excretion capacity and lead to a dangerous dilution of body fluids, brain swelling, loss of consciousness, and even serious brain injury.

And by the way, there is no evidence that the few ounces of extra water in the system improve the luster of your skin, either. Given the fact that those few ounces distribute themselves uniformly in the 35 quarts throughout the body--including the liver, muscles, brain, and skin--you cannot notice them. The idea that drinking water can "flush out impurities from your system" is an idea that also belongs in the toilet. The kidneys handle water in a manner that separates the amount of water excreted from the elimination of the waste products of metabolism as well as salt, potassium, calcium and the many other components of the urine. Drinking all that water dilutes the urine but does little else.

So water-bottle-toters of the world unite and liberate yourselves from the belief that drinking all that water keeps you healthy or attractive. Unless you are engaged in really intense work in a hot environment, drink when you're thirsty--it's nature's way. If you are running a marathon, drink in moderation as the guidelines suggest. And if you do get a little dry and thirsty during your 20 minutes of spinning at the gym, a trip to the oh, so inexpensive water fountain will get you and your body fluids back to where they need to be.

Stanley Goldfarb MD is associate dean of clinical education at the University of Pennsylvania School of Medicine and a nephrologist.

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