As a developing embryo, you nestled in a watery bed and your body weight averaged around 80 percent water, an average that dropped to approximately 74 percent by birth. As an adult, you gurgle along at somewhere around 62 percent water overall, and healthy blood, the red tide of life, surges at between 85 and 90 percent water. Water puddles inside every one of your cells, and flows through the microscopic spaces between cells. In water, oxygen and nutrients float to all parts of your body, and waste products are carried away. When your kidneys remove wastes from your body, those wastes have to be dissolved in water. If your internal water level drops too low, the strain on your kidneys can be damaging.

Digestion and metabolism are water-based processes, and water is the primary lubricating element in your joints. Sweat, almost entirely water, evaporates from your skin, helping to regulate body core temperature in the heat. The water in your blood carries heat from warmer body parts to cooler areas of your anatomy when it's cold. You even need water to breathe, your lungs requiring moisture to expedite the transfer of oxygen into blood and carbon dioxide out of blood.

If you're not drinking enough water to maintain a healthy fluid balance--water ingested equal to water used (the WIWU Principle)--you can alter, and harm, every physiological function of your body. Too bad, as a spokesman for the International Sportsmedicine Institute once said, "many Americans live from day to day in a dehydrated" state--that is, they don't drink enough water."

The water in your body, the fluid that keeps you alive and well, leaves you at an alarming rate. An average person on a normal day loses between three and six liters of water. One to two liters rushes out as urine, and another one-tenth liter in defecation. (Note: Severe diarrhea can increase fluid lost in defecation to an astounding 25 liters in a 24-hour period.) Moisture lost from the act of breathing would fill from one to two liter water bottles a day, and that rate increases in dry air.

And then there's sweat. Your perspiration totals one to two liters on an average day, but that amount can climb to one to two liters per hour during periods of vigorous exercise. Compared to watching TV all day, one hour of exercise may demand approximately a 50 percent increase in the amount of water your body uses.

How can you tell you're running low on water? At first the signs are subtle. Your urine turns light yellow. You'll get a mild headache, and then start feeling tired. Down merely a liter and a half, your endurance may be reduced 22 percent and your maximum oxygen uptake (a measure of heart and lung efficiency) can be lowered 10 percent. Remember, that can happen after one hour of strenuous work. And your thirst mechanism, that feeling of "Gosh, I need a drink of water!," doesn't kick in until you're about a liter and half low. Down three to four liters can leave your endurance decreased to 50 percent and your oxygen uptake reduced close to 25 percent. By now, if you're observant, you'll have noticed your urine has turned a dark yellow. With your mind occupied by other things--say lugging a heavy pack uphill--you may suddenly find yourself seriously dehydrated: disoriented, irritable, rapid pulse, completely pooped.

Medically, dehydration can be classified into three levels.

Mild: dry mucous membranes (lips and mouth), normal pulse, darkened urine, mild thirst.

Moderate: very dry mucous membranes, rapid and weak pulse, darker urine, thirst.

Severe: very, very dry mucous membranes, an altered level of consciousness (drowsy, lethargic, disoriented, irritable), no urine, no tears, and shock (indicated by rapid and weak pulse, rapid breathing, and pale skin).

So, what's a body to do? Drink, of course, is the answer--but What, How Much, and When? Well, you can certainly get by on plain water. It's worked to keep human bodies functioning for uncounted years. Since internal water is used faster than the need for replenishment is felt, water should be consumed at a disciplined rate. There's considerable benefit from starting each day with an ingestion of a large volume of water, about half a liter. Following that, the International Sportsmedicine Institute recommends 1/2 to 2/3 ounce of water per pound of body weight per day, ingested periodically throughout the day. Figured in liters, that's about three to four liters per day for the average-sized person. Drink water with meals and snacks, to encourage digestion, and suck down a few swallows before bedtime to replace what you'll lose in sleep.

When exercising, you should be pounding down water on an even more disciplined schedule. Since the human body can only absorb so much water at one time, the rate of ingestion should be matched, as closely as possible, to the rate of absorption. Most of us have been classified for years into a rate-of-absorption range of one-quarter liter per one-quarter hour. Recent research indicated some of us can do better, absorbing as much as one-quarter liter in 10 minutes. That means, for maximum efficiency and well-being, drink about one-quarter liter of water every 10 to 15 minutes during periods of intense exercise. And cool water gets absorbed faster than warm water. In some conditions you will lose water faster than you can replace it. In those conditions, rest breaks, during which fluid is consumed, become important.

Is water the ideal fluid replacement drink? Maybe not always. Ellen Coleman, RD, MA, MPH, of the Gatorade Sports Science Institute, says an ideal fluid replacement drink should meet four prerequisites: 1) it should not cause gastrointestinal discomfort when consumed in large quantities, 2) it should be rapidly absorbed, 3) it should provide energy to muscles, and 4) it should taste good.

Ten years ago it was widely accepted that a drink containing more than two or three percent sugar considerably slowed the rate of absorption. Current research has shown that, even though stomach emptying is slowed somewhat by sugar, the absorption rate from the small intestine is increased by slightly sweetened drinks. Tests indicate that a drink of six to eight percent sugar--specifically glucose, maltodextrin, or sucrose--gets absorbed as fast as plain water during periods of hard exercise with the added benefit of supplying energy for improved physical performance. Fructose ingestion does not seem to improve absorption or performance.

What about electrolytes (e.g., sodium and potassium) that are lost in sweat and found in many sports drinks? A balanced diet meets even the most active person's requirements for electrolytes, but there are small advantages to having a tiny bit of salt in a sports drink. For one, salt helps you retain water during exercise and, for another, salt stimulates the need to drink. Too much salt, of course, would be counter-productive, causing you to need more water than normal. Actually, most sports drinks are low in salt. Gatorade Thirst Quencher, for example, has about 110 mg of sodium per serving, the same as a cup of two-percent milk.

There are some beverages that, even though they contain water, just don't work well, and may be counter-productive, for hydration. Caffeinated drinks, such as coffee, stimulate fluid loss through urination. Three cups of coffee supply only about two cups of water to your body. Alcohol is a toxin that draws water out of your cells to "water down" increasing blood toxicity." You must drink eight ounces of plain water just to balance one ounce of alcohol. Fruit juices contain too much sugar to meet your body's demands for fluid during exercise.

So drink up, plain water or a sports drink, and remember the WIWU Principle which is still best monitored by using the old maxim: Drink enough to keep your urine clear and copious.

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