Water plays a critical role in almost every major function of the human body, regulating body temperature, carrying nutrients and oxygen to cells, and aiding in digestion. But local and national authorities on health say most Americans don't drink nearly enough water.

"Water is perhaps the most overlooked ingredient in a healthful diet," said Felicia Busch, a member of the American Dietetic Association and author of a brochure published by the association that extols the value of water in the diet.

She recommends that people drink at least eight, 8-ounce glasses of water every day under normal circumstances. But she said, "An increase in activity, an illness, change of climate or change in diet can increase your body's need for water. In these situations, you have to compensate for an increase in lost fluids."

For example, Busch said a 125-pound woman who exercises for one hour should get an extra two cups of water in addition to the eight cups.

Vickie McDowell of the Cape Girardeau County Health Department agreed that water is important to healthy living and that many people just do not drink enough of it.

"It is pretty well standard that for a healthy diet you need eight, 8-ounce glasses of water a day," said McDowell.

"From my observations just out in the public people tend to want to drink soft drinks when water would be the best thing for them," said McDowell. "Very few people drink enough water because they are so used to flavored sodas and powdered drinks. Water is bland and there's not much taste to it."

McDowell said soft drinks don't quench thirst like water, and people often wind up drinking water after consuming other drinks because they are still thirsty.

Water is also important for people trying to lose weight. If someone is not drinking enough water, the body can't burn fat as efficiently as when someone is well hydrated.

Because water is perceived as bland, drinks that are perceived to be healthy seem to be increasing in popularity, especially bottled water. Sales of bottled water increase 25 to 40 percent during summer months as consumers look for something healthy but still flavored.

Analysts from the New York-based Beverage Marketing Corp., which tracks the U.S. beverage industry, say bottled water, ready-to-drink fruit juices and iced teas are quite popular now.

"Consumers are looking for all natural beverages that are perceived to be healthy," said Mike Bellas, a vice president of the company.

In the summer months, sales of all beverages tend to increase because consumers drink more liquid to stay hydrated. Industry analysts said more noncarbonated drinks are gaining popularity because of the wider variety of products available and more convenient packaging and lower costs.

McDowell said that water is especially important during summer months when people are working outside or exercising.

"It is especially important in the elderly and young children because they can dehydrate faster," said McDowell. "And people who work outside tend to lose fluids faster and need to replenish them. Their electrolytes get out of balance and that can cause health problems.

"It is important in the summer that folks consume more water because it is easier to dehydrate."

McDowell said, however, that an elderly person who has been placed on a fluid-restricted diet by a doctor should not necessarily consume more water in the summer months. "If a doctor has advised them to restrict fluids, they should follow the doctor's advice," said McDowell.

Dr. Coleen Kivlahan, director of the Missouri Department of Health, said six Missourians died from heat-related illness in 1993. She explained that the very young, the elderly and chronically ill are particularly at risk. The elderly and chronically ill tend to perspire less and are more likely to have health problems requiring medicines that work against the body's natural defenses to adjust to heat.

Two serious heat-related illnesses are heat stroke and heat exhaustion.

In cases of heat stroke, the outside temperature adds heat to the body faster than the body can deal with it, so the internal body temperature rises to the level of high fever.

In heat exhaustion, there is excessive loss of salt and body fluid when the body is overheated. Normal cooling by perspiration is prevented.

Seeking shelter in an air-conditioned building is the best way to combat the possibility of catching a heat-related illness, health officials say. And drinking plenty of fluids is also important.

While vacationing this summer the International Bottled Water Association suggests that travelers not forget water.

To stay well hydrated while on vacation, the association suggests carrying water in the car because, if spilled, it does not make a sticky mess and can be easily replenished along the way; when flying ask for bottled water instead of soft drinks or alcohol to help combat jet lag and dehydration from recirculated air, and drink one cup of water every hour in flight; when on a cruise, to help keep calories under control drink water instead of high-calorie drinks, and drinking water before a meal can curb appetite; when traveling in tropical countries take bottled water; and, when camping, make sure spigots used for water are drinking water. If unsure about a water source, boil it before drinking.

If exercising during the summer, the International Sports Institute says any type of physical exertion increases the body's need for water.

The institute says active people need two-thirds of an ounce of water per pound of body weight to meet their daily requirement for water. That means a person weighing 150 pounds needs a little more than 12 8-ounce glasses of water a day.

In order to feel well after a workout, proper fluid balance is essential.

The American Dietetic Association recommends starting with eight, 8-ounce servings of water daily and then adding two cups for every hour of exercise.

To help keep children in good shape during the summer, the association suggests offering water first instead of other beverages. Children who drink lots of juice or high-calorie beverages may not be hungry at meal time.

The organization also recommends:

-- Lower shelves of the refrigerator should be stocked with water, juice, fresh fruit and crunchy vegetables, which have a high water content and help replace lost fluids.

-- For children in sports activities, make sure coaches have plenty of water available during and after games and practices.

-- Older children should be encouraged to carry a bottle of water on their bicycles for when they get thirsty away from home.

-- Keep in mind that a 50 pound active child needs about a quart of water or other fluids each day to stay well hydrated.