There's no evidence for the oft-repeated assertion that people need at least eight glasses of water per day. The Institute of Medicine recommends that men consume the equivalent of about 125 ounces of water daily and that women consume about 91 ounces -- but that includes water intake from all foods and beverages. Most people get adequate hydration without "purposeful drinking" of fluids for hydration. According to the U.S. government's "Dietary Guidelines for Americans," "purposeful drinking" is only necessary for people "who are exposed to heat stress or who perform sustained vigorous activity."

Moisture in food accounts for about 20% of the average person's total daily water intake. An apple is 84% water. Bananas are 74% water. Broccoli is 91% water. Even foods that you might not think of as particularly moist -- a plain bagel (33% water), ground beef (56%), American cheese (39%) -- help your body meet its hydration needs.

It doesn't take long to get dehydrated -- especially in hot, humid weather. The National Athletic Trainers’ Association advises delaying or rescheduling vigorous outdoor exercise when it’s too hot and humid. Alternatively, you should take longer breaks, reduce the duration and/or intensity level, and make sure you're wearing minimal clothing or exercise gear.

It's hard to believe, but before about 1969, athletes were advised to avoid drinking fluids during exercise for fear of gastrointestinal problems or impaired performance. Now we know better. On average, you should take a rehydration break about every 20 minutes. Most people would stay adequately hydrated by drinking 5 to 10 ounces of fluid every 20 minutes -- but your exact need depends on factors such as level of exertion, whether you are indoors or outdoors, your age, gender, and weight.

It's also helpful to drink two cups of fluids (about 16 ounces) about two hours before a workout.

Alcoholic beverages have the most dehydrating effect, but most other beverages contribute to your overall level of hydration. Coffee and other caffeinated beverages do have some diuretic effect, causing your kidneys to form urine. But they are generally hydrating because of their water content. Juices, sodas, and other sugary beverages also are hydrating. Water is usually a better choice for hydration because it doesn’t contain additional calories.

The amount of fluid you need depends upon a number of factors, including:

• Age: Children need more fluids because they produce more urine. Older people may need more fluids because of specific health conditions or because they tend to lose their sense of thirst.
• Gender: Men need more fluids than women. (And pregnant women need more fluids than other women.)
• Weight: Heavy people need more water.
• Health: Diseases such as diabetes, cystic fibrosis, and kidney disease can increase your need for fluids.
• Environment: You need more fluids in extreme weather conditions (especially hot, humid, or cold) and at high altitudes.

You lose about 10 or more cups of water every day just living -- breathing, sweating, urinating, etc. That amount of water loss is usually replaced by normal levels of daily eating and drinking.

An easy way to monitor your hydration level is to check the color of your urine. The darker your urine, the less hydrated you are. Drink enough fluids to keep your urine a lighter color. If your urine is clear or pale, chances are you are well hydrated.

Other practical ways to monitor your hydration status include keeping an eye on your body weight (you lose weight as you lose water) and perspiration (the more you perspire, the more water you're losing).

Studies suggest that water could be a helpful in losing weight. A recent study suggested that by drinking water, overweight dieting women were able to promote weight loss by lowering total caloric intake and/or changing their metabolic rate. Another study showed that middle-aged and older women who drank water before a meal consumed fewer calories -- presumably because they already felt full -- and, consequently, they lost weight.

Both studies were short-term, however, and it's unknown if the results would have held up over a longer period of time.

Sodium is something your body needs when you're trying to rehydrate – either during or after exercise. That’s why sports drinks are often rich in sodium -- one of the “electrolytes” your body loses during exercise. Drinks and snacks with sodium also can stimulate thirst and help you retain fluids. Too much salt, however, can increase your blood pressure and worsen heart conditions in some people.

It is possible to drink too much water. While normally functioning kidneys in an adult can process anywhere from 20 -1000 ml of fluid per hour, it's not easy to overload them -- but it can be done. Excessive amounts of water, especially over a short period of time, can be life-threatening. Symptoms of too much water include weight gain, bloating, nausea, and vomiting. Acute cases of water intoxication can cause low blood sodium, which can result in headaches, confusion, seizures, and coma.

The International Marathon Medical Directors Association recommends that athletes drink no more than 31 ounces of water per hour during extended exercise.

There is water in every cell, tissue, and organ in your body. It helps transport nutrients, get rid of waste, keep your temperature at the right level, lubricate and cushion joints, keep your skin moisturized, and help with lots of other essential functions.

Thirst is one of the first warning signals you may be getting dehydrated, but you should not rely on thirst alone. Other early signs are: fatigue, flushed skin, faster breathing and pulse rate, and decreased exercise capacity. Later signs include weakness, dizziness, and labored breathing. Experts say that thirst occurs when at least 2% of your body weight is lost as sweat. Hydrating before and during strenuous activity is important.

If you think you're becoming dehydrated, you should move to a cool place and rehydrate. Drink fluids slowly, rather than quickly -- drinking too fast can stimulate urination, resulting in less hydration.

Water is usually sufficient to rehydrate properly. However, sports drinks may be preferable during especially vigorous or long-lasting exercise. The National Athletic Trainers' Association says "hydrating with a sports drink containing carbohydrates and electrolytes (i.e., sodium and potassium) before and during exercise is optimal to replace losses and provide energy." Athletes tend to replace only about half of the fluid lost when they drink water, so sports drinks may replace more lost fluids because athletes enjoy the taste.

The human body is mostly water: about 55% to 75%, on average (and depending on how well hydrated you are). That's about 10 to 12 gallons of water in your body!

Water makes up about 83% of blood, 73% of muscles, 25% of body fat, and 22% of bones.