What am I? When there is a lack of me, people feel fatigued or lethargic. Mouth, lips and eyes feel flushed; and you may have headaches or dizziness. Constipation can become a problem, and the absorption of nutrients from food is inhibited.

Sounds like someone's missing a critical vitamin or mineral, doesn't it? Far from it. The substance that ameliorates all of the above is water, and people generally do not drink enough of it, said Lona Sandon, an instructor of clinical nutrition at UT Southwestern Allied Health Sciences School, who works closely with athletes.

On average, adults only drink about 3 glasses (24 ounces) of water a day. The amount needed is closer to 10 glasses (80 ounces), and active adults require even more. Many people likely don't understand that their bodies are continuously losing water, Ms. Sandon said. "We're losing it just sitting," she said.

The body loses water in urine, in perspiration and in respiration, Ms. Sandon said. And it also is lost during digestion. For ever calorie ingested the body needs about 1.5 milliliters of water to absorb it. "People ignore that their body is thirsty," Ms. Sandon said. "Even at the first sign of thirst, you're already dehydrated."

Adults sometimes believe they are getting water when they drink coffee or tea. But caffeine has a diuretic effect and pulls water from the body, Ms. Sandon said. Even children who drink soft drinks and other beverages aren't getting enough water, she said. Water isn't sweet--a taste children seem to crave--and generally not as easily available in machines that dispense soda. Ms. Sandon suggests bottled water because it often tastes better than tap water--although there is nothing harmful in most water than comes from the tap--and because it can be conveniently carried around.

"People make fun of us as we carry around our bottles," she said. "But we're getting properly hydrated. That's the biggest tip. Keep it handy." But water itself isn't the only way for people to get an ample supply of water into their systems. Another source is edible. Fruits and vegetables are a great source of water, Ms. Sandon.Summer temperatures in Texas make drinking water throughout the day imperative. But even in cold, dry weather, water evaporates off skin quickly. Humans must have many things to survive: protein, carbohydrates, fat, vitamins and minerals and water.

"It is the most essential nutrient for the body. We forget that," Ms. Sandon said. You can live for a month without food, but you can't live more than two to three days without water."