The Wisdom of the Body

Your fluid balance depends on two key factors: sodium (salt) and water. They are essential to life. So your body has a sophisticated system to regulate salt and water. Special cells in the brain are in charge. If you need more water, they make you feel thirsty and tell your kidneys to retain water. If you have too much fluid, the brain cells tell your kidneys to put out (excrete) lots of water.

Normal kidneys can excrete as little as two pints of fluid per day, if the body is dry. Or as much as 20 quarts (it's true) if it's flooded. Even minor changes in water balance are enough to set the regulatory process in motion. For example, just a 1% deficiency of water or excess of sodium will produce a powerful sensation of thirst.

Under ordinary circumstances, you don't have to tally up the fluids you drink. Instead, let thirst be your guide: Drink when you're thirsty, but don't force fluids when you're not.

Fluid Requirements

On average, you need about 1 quart of water for each 1,000 calories you burn. For most men, that means about 2 quarts a day. About 60% of the body's needs are met by the water contained in food. In normal conditions, just 1 1/2 pints of fluid will make up the difference.

When You Need Extra Water

Thirst is a reliable guide to your fluid needs, but it has an important drawback: it's slow. By the time you feel thirsty, your body will be dry. You may suffer some of the consequences of dehydration, such as impaired concentration, irritability, headache and fatigue. These are temporary, but unpleasant. Fortunately, you can stay ahead of the game by anticipating special fluid needs. Think ahead and drink ahead.

When will you need extra water? Here are some factors to keep in mind:

• Exercise — When you exercise, your muscles generate heat. To get rid of that heat, you sweat. In warm, humid weather, you can lose up to 1 1/2 quarts per hour. To stay ahead, drink 6-8 ounces before you start working out, and take similar amounts at frequent intervals. After you finish exercising, spend some time at the water fountain before you shower. If you drink enough you won't feel thirsty — or, for that matter, grumpy or tired. You'll know you have enough when your urine is clear and plentiful. Or you can simply check your weight before and after exercise (minus your sweaty clothes, of course). For each pound of weight you lose, you'll need to drink a pint of fluid. And despite the popularity of sports drinks that contain sugar and potassium, plain old water is fine.

• Climate — It's obvious that you'll lose water in sweat when it's hot and/or humid. But you'll also need more water when it's cold and dry. That's because you lose water through your lungs every time you exhale. In an average climate, it amounts to a pint a day, but when the air is dry you'll lose substantially more water.

• Air travel — Dry air is responsible here, too. You'll enjoy your trip more if you drink even before you're thirsty.

How Illness Affects Thirst

Illness can increase a person's fluid requirements. For example:

• People who form kidney stones should drink at least 2 1/2 quarts a day to prevent recurrences.

• Urinary tract infections call for extra hydration. Diarrhea can cause excess fluid and mineral loss. Sports drinks are a good way to replace them.

• You'll also need more fluid every time you have a fever. That's because your body's metabolism speeds up by about 7% with each degree of fever.