The only sure sign of life on another planet would be evidence of water. Water is an indispensable nutrient for all known forms of life. A fascinating fact is that all living things need the same concentration of water in their bodies to survive. The healthy adult human is 60 to 65 percent water by weight; so is the elephant, the mackerel, the bullfrog, the 'possum, and the dandelion. Only when you understand how this precious substance is used by the body can you fully appreciate the concepts of health and disease.
To put it simply, your heart is at one end trying to pump all the water out of your body, while your kidneys are at the other end trying to hold it all in. If either succeeds, you die. Normal concentrations of minerals and other substances are maintained by drinking fluids and by excreting water and various chemical substances. A water-retaining hormone called vasopressin controls water excretion in your kidneys and sweating in your skin. Astoundingly, your kidney filters about 170 liters of water a day, but most of it is reabsorbed (about 69 liters, thank heavens). So we excrete only a liter or two of water as urine each day.
In healthy young people, mineral substances and water are automatically balanced, thanks to an acute hormonal sensitivity to even slight changes in blood concentrations. At low blood concentrations, vasopressin is suppressed, allowing the excretion of a large amount of very dilute urine. When the mineral concentration is high, the amount of urine excreted is minimized by the "water" hormone.
As you age, this mechanism doesn't work as well; it becomes less responsive to changes in the blood. Several things are responsible: thirst itself decreases; the kidneys become less responsive to vasopressin; and vasopressin secretion increases. Of these three factors, which can you do something about? Thirst! Even normally, thirst is experienced late in water depletion. By the time you feel thirsty, you are already dehydrated. This little-known fact is even more true as you age. Though you are fully capable of requesting and obtaining water, you will experience thirst less and less as you age.
One study explored the effect of 24 hours of water deprivation in active, healthy older men (67-75 years) compared to younger men (20-31 years). The older men lost the same amount of weight but showed large increases in concentrations of salts in the blood. Their urine analysis reflected inadequate excretion of salts. Nevertheless, the older men were not thirsty. In fact, all failed to correct their water deficit after two hours. Conclusion: people over forty, if left to their own thirst mechanisms, are likely to become dehydrated and to lose blood volume, even though they may appear healthy. People whose mental state has been compromised are particularly at risk. Many immobilized people with communication problems inhabit a virtual desert simply because they cannot ask for water.
Even for people without communication problems, dehydration is the most common age-related cause of health disturbances. It can lead to fatigue, a drop in blood pressure, perceptions of overall body weakness, and even fainting. Elderly people dehydrate easily in hot weather but also during fever, infection, diarrhea, or vomiting. Many older people arrive in emergency rooms near death, not for lack of proper medication or care but for lack of water.
Not only is water a necessary nutrient, it is also a catalyst for drug efficacy. All medications, liquids included, should be taken with a full glass of water. ("Water," by the way, does not mean coffee, tea, or juice. These beverages may contain caffeine or other chemicals, be high in sugar or calories, and interfere with drug action.) Drinking a full glass of water allows the medications to dissolve more quickly and be more readily absorbed, while reducing the possibility of stomach irritation. Thus, water-plain old water-can be one of the most useful agents for assuring proper drug action. In fact, most laxatives work because they are taken with plenty of water. Constipation follows insufficient water intake, and, therefore, laxatives simply do not work without enough water.
Many drugs, especially diuretics (which are used to treat heart failure), deplete body water volume. Unfortunately, many people report that they deliberately cut down on fluid intake to "help" their medications. They think that if their diuretic pills are supposed to get rid of excess water, then decreasing water intake will be that much better. But in doing so, they put themselves at risk of falling blood pressure, confusion, pseudodementia, and kidney problems.
Often older people ignore their thirst because drinking does not relieve persistent dry mouth or because they're afraid of becoming incontinent. Some nursing homes, concerned with inadequate water intake by their residents, ask doctors to prescribe four to six glasses of water a day for their patients. Once the order is written, nurses must give water, then witness and document the fluid intake.
Water, then, has many virtues:

What You Can Do
You must make an effort to drink plenty of water each day. To maintain optimum hydration, you should drink at least a quart of water (preferably two quarts) every day. Six to eight 8-ounce glasses of water a day are the minimum you should be drinking for good health.

Designer Water: Is It Any Better than Tap Water?
Once upon a time, when you went to a restaurant and requested water, you got a glass of ordinary tap water. Not any more. Ask for water, and the waiter recites a list of choices longer than the daily specials-water that comes from France, from Italy, from almost every spring and stream on the planet; water with minerals or without salt; water that sparkles or just sits quietly in your glass. When you ask for the wine list in New York's best restaurants, the waiters provide one for waters as well. Still, water is water, and designer water is no different from undesigned water. Actually, it may be of poorer quality than what flows from your tap.

The Muddied Image of Bottled Water
Not long ago, a batch of Perrier water was contaminated with the toxic chemical benzene, prompting a recall of 160 million bottles. This incident led to concern about bottled water in the United States. A year-long investigation of bottle-water manufacturing practices ensued, with some surprising results. The General Accounting Office found that the Food and Drug Administration (FDA) had never adopted standards for contaminants in bottled water comparable to those for tap water, which are regulated by the Environmental Protection Agency. Yes, that's right. Bottled water can legally contain more toxic substances than tap water.
The General Accounting Office also found that the FDA did not monitor bottling facilities closely enough to make certain that bottles complied with the few regulations that had been set. The FDA admits its regulation of bottled water is "imperfect," but they cite "no reason to question the safety of bottled water. The FDA considers bottled water to have a low potential for contamination or for causing sickness."
To its credit, the water-bottling industry is beginning to regulate itself. Bottlers belong to a trade group called The International Bottled Water Association. Members voluntarily undergo yearly unannounced plant inspections by a nonprofit testing and certification organization known as the National Sanitation Foundation.
There is certainly room for improvement in the regulations regarding bottled-water safety, but so far bottled water has caused no apparent health problems. Still, the General Accounting Office's report underscores the fact that, despite consumer protections, bottled water is no purer than tap water.

From Live Long, Die Fast: Playing the Aging Game to Win, by John H. Bland, M.D. © 1997 by John H. Bland, M.D. Excerpted by arrangement with Fairview Press. $14.95. Available in local bookstores, or call 800-544-8207.ß