It's our body's vital fuel, a health drink from Mother Nature. It's calorie-free, inexpensive and easily obtained. Yet few people follow the old fashioned advice to drink eight glasses of water a day.

Most people drink when they are thirsty, but the beverage of choice tends to be some drink other than water. Americans drink two or three glasses of plain water a day, according to a U.S. Department of Agriculture survey conducted in the late 1970s. Based on an analysis of all fluid intake by adults, it totals to about two quarts of water a day, and this includes water from foods and from other beverages.

Because there is so much water in your food, it's usually unnecessary to swallow two quarts of plain water every day. However, people with special problems such as kidney conditions might be exceptions.

Americans drink eight gallons of bottled water a year, roughly two ounces or a quarter-cup a day, according to the International Bottled Water Association. Californians drink three times the national average of bottled water, downing 24 gallons a year, or nearly a cup a day. Climate and seasons of the year play a role in one's thirst also, and just as we tend to perspire more in the summer months, we also tend to drink more water.

Boosting intake of plain water makes good sense, many experts concur, because water eases digestion and regulates body temperature. Water also bathes the cells and accounts for about 60 percent of body weight. And it can help us exercise longer and more efficiently. Drinking water can ward off constipation and maybe even crankiness.

And since it's a natural appetite suppressant, water can help us lose weight and keep it off. It can help keep skin healthy, although it won't necessarily banish acne.

Who should drink water? We all should, but pregnant women, nursing mothers and athletes should be especially careful to drink a sufficient amount. When it is hot or humid, upping water intake is also wise. Certain workers seem to have a more difficult time developing the water-drinking habit. Among those who don't normally drink enough water are teachers, airline attendants and nurses.

Drinking fluids, particularly, water, during exercise reduces cardiovascular stress and improves performance. After a strenuous workout, you must replace the fluids you have lost. Otherwise, you will suffer chronic dehydration. Drink water before, during and after exercising, and remember that water reduces body temperature thus making the whole exercise process safer.

Water can be especially helpful for people with a history of kidney stones because it dissolves calcium in the urine, reducing the risk of stone formation. Among physicians, urologists are  most likely to extol the virtues of water. Drinking water mostly before 6 P.M. can reduce the likelihood of nocturnal bathroom visits.

It is interesting to note also that water helps prevent urinary tract infections, both for men and for women. Too busy to count how many glasses a day you drink? There are other ways to calculate if your intake is sufficient. For example, dark-colored urine suggests you aren't drinking enough water. Get into the habit by starting with a glass of water with every meal, then work in a cup between meals.