Indeed, water is as important as oxygen for sustaining life. You could survive up to six weeks without food, but not more than a few days, maybe a week tops, without water. Even when you are inactive, your body loses more than a quart of water every day — through urine, perspiration, and sweat. And most days you lose 2.5 quarts. You also lose water when you exhale – up to one or two glasses a day in the form of vapor. With all that water going out, you need to replace what's lost. For most people, that's done without a second thought: You get thirsty; you drink. Simple.

"Normally, your body does an amazing job of maintaining proper fluid balance," says Robert H. Shmerling, M.D., associate professor of medicine at Harvard Medical School.

With help from your kidneys, your body holds onto water when your system is a bit low and gets rid of it when there's excess. But what happens if the balance shifts to the point where your body becomes depleted of water? In other words, there's more going out than coming in. That's when you're at risk of dehydration, a potentially life-threatening condition if not treated promptly.

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"The best way to determine if you're getting enough water every day is to listen to your body," Dr. Shmerling says. "Are you excessively thirsty? Are you urinating less frequently than normal? Are you feeling unwell? Are you dizzy when you stand up? Is your mouth dry? While there are reasons other than dehydration that can cause these symptoms, if you can answer 'no' to all these questions, then you're probably getting enough water."

Your body usually can cope with extreme changes in water intake. But that's not true for everyone all the time, Dr. Shmerling says. Certain factors increase the risk of dehydration:

Activity

People who exercise strenuously, particularly in hot or humid weather, need to drink more water. It's also wise to avoid exercising outdoors during the hottest period of a summer day.

Weather

People living in hot, humid climates, or those experiencing a summer heat wave, need more water than usual. Vacationers take note: Don't forget to factor in the weather of your destination. A Chicago native may run three miles every day at home without taking water along, but the same trek on the beaches of a balmy Caribbean island could lead to trouble.

Age

According to the American College of Sports Medicine, as we age, the body's ability to conserve water is reduced. The body excretes more water (through urination and perspiration) than is taken in. In addition, those 65 or older may not feel thirsty even when their bodies need water. This need for water is so crucial that the Food Guide Pyramid made especially for seniors features water as the base of the structure. So be sure to drink your eight glasses of water a day before you feel thirsty.

Health

Anyone who is sick with fever, diarrhea, nausea and/or vomiting should increase water intake. Kids and the elderly are at additional risk.

Medications

People who take diuretics ("water pills") such as furosemide and hydrochlorothiazide must monitor themselves closely for signs of dehydration. "These medications are used to help reduce high blood pressure," Dr. Shmerling says. "But if the dose is too high or you start to drink less for any reason, dehydration can follow."

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Most of the time, dehydration doesn't reach the danger level. "While it's still mild, your body detects dehydration and leads you to behaviors that correct the problem," Dr. Shmerling says. "Either you get thirsty and replenish lost fluids (by drinking water) or you reduce fluid loss (by cooling off and resting)." However, mild dehydration can become more severe if fluid isn't restored. As you lose more water from your body, you may experience symptoms such as dry mouth, flushed skin, fatigue, headache and impaired physical performance. "Eventually, dehydration can lead to low blood pressure, which reduces the ability of your body to circulate oxygen and other nutrients throughout your body," Dr. Shmerling says. "The end result may be dizziness, loss of consciousness, rapid heart rate, kidney failure and finally death."

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To prevent dehydration, you need to take in plenty of fluids throughout the day — at least eight 8-ounce glasses — even if you don't feel thirsty. Follow these eight tips (one for each glass you should drink) from Dr. Shmerling and the American Dietetic Association to achieve the fluid intake your body needs.

1. Take a water break frequently throughout the day.

2. Always take a sip when you pass a water fountain.

3. Carry a bottle of water with you in your car.

4. Keep a sport bottle with you at work.

5. Start a meal with a cup of soup.

6. Drink a beverage with each meal.

7. Drink more when exercising, especially if it's hot or humid. Drink one or two glasses of water or diluted fruit juice (one part juice to one or two parts water) about 30 minutes to an hour before you begin. Drink another glass or more when you're finished. For more intense and longer workouts, increase your fluid intake to 8 ounces every 20 to 30 minutes. (Skip the high-calorie sports drinks unless you're working out strenuously for at least 90 minutes.)

8. Consider foods as sources of water, too. Lettuce, watermelon, broccoli, grapefruit, carrots and apples are all more than 80-percent water by weight. Low-fat cottage cheese, yogurt, potatoes and canned drained tuna all contain more than 70-percent water by weight.

Don't include beverages containing caffeine (coffee, colas) or alcohol when tallying your daily fluid intake. These act as diuretics, causing water loss.

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Drink a minimum of eight 8-ounce glasses of water per day, experts say. That might seem like a lot of water, but when you break it down throughout a typical day, getting your eight is actually fairly easy. Try this sample schedule, then make changes to it to best fit your specific day's events. Drink one 8-ounce glass of water:

1. With breakfast
2. Mid-morning
3. With lunch
4. Mid-afternoon
5. Right before eating dinner
6. With dinner
7. Evening
8. Bedtime

You can add additional glasses of water at each meal. And don't forget to "soak up" some water before, during and after any exercise or activity you engage in.

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