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Note: All links within content go to MayoClinic.com

A professional athlete — strong, fit and apparently in excellent health — collapses during a workout on a hot day and never recovers. The cause of death is dehydration (hypohydration), which occurs when the body doesn't have enough water to carry out its normal functions. Although stories of high-profile athletes succumbing on the playing field grab headlines, such cases are rare. On the other hand, millions of ordinary people — many of them infants and older adults — die worldwide of dehydration every year.

What's more, even mild dehydration — as little as a 1 percent to 2 percent loss of body weight — can cause symptoms such as weakness, dizziness and fatigue and may have a negative effect on long-term health. Severe dehydration, usually defined as a loss of 9 percent to 15 percent of body weight, is a life-threatening medical emergency.

In the simplest terms, dehydration occurs when you lose more water than you take in. Even a slight imbalance causes serious problems because water is essential to human life: It forms the basis for all body fluids, including blood and digestive juices; it aids in the transportation and absorption of nutrients; and it helps eliminate waste.

You can usually reverse mild to moderate dehydration by increasing your intake of fluids, but severe cases need immediate medical treatment. The safest approach is not to become dehydrated in the first place. You can do that by monitoring your fluid loss during hot weather, illness or exercise, and drinking enough liquids to replace what you lose.

Unfortunately, thirst isn't a reliable gauge of the body's need for water, especially in children and older adults. A better barometer is the color of your urine: clear or light-colored urine means you're well-hydrated, whereas a dark yellow or amber color usually signals dehydration. Other signs and symptoms vary, depending on the degree of dehydration, which may be mild, moderate or severe.

Mild to moderate dehydration is likely to cause:

• Excessive thirst
• Sleepiness or tiredness — children are likely to be less active than usual
• Dry mouth
• Decreased urine output — fewer than six wet diapers a day for infants and eight hours or more without urination for older children and teens
• Few or no tears when crying
• Muscle weakness
• Headache
• Dizziness or lightheadedness

Severe dehydration, a medical emergency, can cause:

• Extreme thirst
• Extreme fussiness or sleepiness in infants and children; irritability and confusion in adults
• Very dry mouth, skin and mucous membranes
• Lack of sweating
• Little or no urination — any urine that is produced will be dark yellow or amber
• Sunken eyes
• Shriveled and dry skin that lacks elasticity and doesn't "bounce back" when pinched into a fold
• In infants, sunken fontanels — the soft spot on the top of a baby's head
• Low blood pressure
• Rapid heart beat
• Fever
• In the most serious cases, delirium or unconsciousness

If you're an average adult, every day you lose close to 2.5 liters (more than 10 cups) of water simply by sweating, breathing and eliminating waste. You also lose electrolytes — minerals such as sodium, potassium and calcium that maintain the balance of fluids in your body. And all this is before you even think about mowing the lawn on a sticky afternoon, working out at the gym or rearranging the living room furniture.

Even when you're active, you normally can replenish what you've lost through the food and liquids you consume, but sometimes you eliminate more water and salts than you replace. The result is dehydration — your system literally dries out. Because water makes up about 60 percent of your body weight, this can cause physiological changes that may affect your health, even if you don't have noticeable signs or symptoms.

Just a small reduction in body fluids and electrolytes, for instance, leads to a lower circulating blood volume. As a result, your heart has to pump harder to maintain adequate blood flow to your vital organs, and your body is less able to control blood pressure, distribute nutrients and eliminate waste.

What's more, because blood flow to your skin is reduced and you have less water in your system, you don't perspire or dissipate heat the way you normally would. In extreme cases, this can cause your body core temperature to soar, leading to heat exhaustion and possibly heat stroke — a potentially life-threatening condition in which your body temperature climbs to 104 F or more, sometimes reaching as high as 107 F.

Not all cases of dehydration have such serious consequences, but even a 2 percent loss of body weight can affect athletic performance, and a 3 percent to 5 percent loss adversely affects reaction time, concentration and judgment. What's more, dehydration is insidious; these effects often occur before you're aware of them.

How dehydration happens
Sometimes dehydration occurs for simple reasons: You don't drink enough because you're sick, busy or just not thirsty, or because you lack access to potable water when you're traveling, hiking or camping.

Other causes of dehydration include:

• Diarrhea. Severe, acute diarrhea — that is, diarrhea that comes on suddenly and violently — can cause a tremendous loss of water and electrolytes in a short amount of time. If you have vomiting along with diarrhea, you lose even more fluids and minerals. Children and infants are especially at risk. Worldwide, more than 1.5 million infants and children die of dehydration resulting from diarrhea every year — 300 to 500 of them in the United States. Most of these deaths occur in the first year of life.
• Exercise. Every day, athletes of all ages and skill levels — pint-sized soccer players, treadmill joggers, professional cyclists and tennis players — experience some degree of dehydration. The reason is simple: you lose water when you sweat. And although it makes sense that you sweat more in hot weather, you can also become dehydrated in winter, especially if you wear layers of insulated clothing or work out in an overheated gym. Humidity compounds the problem because it increases sweating and inhibits cooling. Although no one who is active is immune, preteens and teens who participate in sports are especially susceptible, both because of their smaller body weight and because they may not be experienced enough to know the warning signs of dehydration. Young athletes are also at risk if they don't become acclimated to heat and humidity before a workout or if they exercise too strenuously in an effort to make the team.
• Fever. In general, the higher your fever, the more dehydrated you become. If you have a fever in addition to diarrhea and vomiting, you lose even more fluids.
• Increased urination. This is most often the result of undiagnosed or uncontrolled diabetes, a disease that affects the way your body uses blood sugar and that often causes increased thirst and more frequent urination. Another condition, diabetes insipidus, is also characterized by excessive thirst and urination, but in this case the cause is a hormonal disorder that makes your kidneys unable to conserve water. Certain medications — diuretics, antihistamines, blood pressure medications and some psychiatric drugs — as well as alcohol can also lead to dehydration, generally because they cause you to urinate or perspire more than normal.
• Long airplane flights. The air in the cabins of most commercial airplanes is drier than the Sahara Desert, with humidity levels hovering around 10 percent or less. Compounding the problem for some people is alcohol — a readily available in-flight beverage. The longer the flight and the more alcohol you drink, the more dehydrated you become — it takes 8 ounces of water to make up for every 1.5 ounces of alcohol you consume. Older adults and people who have diabetes or who take drugs that increase urination are particularly at risk. The best advice when flying is to bypass the alcohol cart and drink bottled water instead. Sugary drinks, coffee and tea aren't ideal substitutes. Although new research suggests that caffeine-containing beverages aren't as dehydrating as once thought, water is still your best bet. And don't worry about having to climb over your seatmates to get to the bathroom — preventing dehydration is worth a little social embarrassment. In addition to all the other problems it can cause, a lack of fluids combined with arid airplane air dries out your nasal passages, increasing your risk of upper respiratory infections.
• Hot, sunny climates. It makes sense that you perspire more — and therefore lose more fluids — in hot climates. High humidity makes matters worse because it prevents sweat from evaporating quickly.
• Burns. Doctors classify burns according to the depth of the injury and the extent of tissue damage. Third-degree burns are the most severe, penetrating all three layers of skin, and often destroying sweat glands, hair follicles and nerve endings. People with third-degree burns or extensive first- or second-degree burns experience profound fluid loss, and the resulting dehydration can be life-threatening.

Given the right circumstances — a transatlantic plane flight, a stretch of hot weather or a bout of punishing exercise — anyone can become dehydrated. But certain people are at greater risk, including:

• Infants and children. Worldwide, dehydration secondary to diarrhea is the leading cause of death in children. Infants and children are especially vulnerable because of their relatively small body weights, low sweating capacity, and high turnover of water and electrolytes. They're also the group most likely to experience diarrhea. In the United States, diarrhea remains one of the most common pediatric illnesses.
• Older adults. As you age, you become more susceptible to dehydration for several reasons: Your body's ability to conserve water is reduced, your thirst sense becomes less acute and you're less able to respond to changes in temperature. What's more, older adults, especially those in nursing homes or living alone, tend to eat less than younger people do and sometimes may forget to eat or drink altogether. Disability or outright neglect may also prevent them from being well nourished. These problems are compounded by chronic illnesses such as diabetes, by hormonal changes associated with menopause and by the use of certain medications. The gravity of dehydration in seniors was brought home in August 2003, when thousands of older adults in Europe died during an unprecedented heat wave.
• Endurance athletes. Anyone who exercises can become dehydrated, especially in hot, humid conditions or at high altitudes. But athletes who train for and participate in ultramarathons, triathalons, mountain climbing expeditions and cycling tournaments such as the Tour de France are at particularly high risk. That's because the longer you exercise, the more difficult it is to stay hydrated. During exercise, your body can absorb about 24 to 32 ounces of water an hour, but you may lose twice that amount in hot weather. With every hour, your fluid debt increases. Dehydration is also cumulative over a period of days, which means you can become dehydrated with even a moderate exercise routine if you don't drink enough to replace what you lose on a daily basis.
• People with chronic illnesses. Having uncontrolled or untreated diabetes puts you at high risk of dehydration. But other chronic illnesses also make you more likely to become dehydrated. These include kidney disease, cystic fibrosis, alcoholism and adrenal gland disorders. Even having a cold or sore throat makes you more susceptible to dehydration because you're less likely to feel like eating or drinking when you're sick. A fever increases dehydration even more.
• People living at high altitudes. Living, working and exercising at high altitudes — generally defined as 8,000 feet to 12,000 feet (about 2,400 meters to 3,600 meters) — or very high altitudes — 12,000 feet to 18,000 feet (about 3,600 meters to 5,400 meters) can cause a number of health problems. One is dehydration, which commonly occurs when your body tries to adjust to high elevations through increased urination and more rapid breathing — the faster you breathe to maintain adequate oxygen levels in your blood, the more water vapor you exhale.

If you're a healthy adult, you can usually treat mild to moderate dehydration by drinking more fluids. Get immediate medical care if you develop severe signs and symptoms such as extreme thirst, no urination for eight hours, shriveled skin, dizziness and confusion.

Treat children and older adults with even greater caution. Call your family doctor right away if your child:

• Develops severe diarrhea, with or without vomiting or fever
• Has had episodes of vomiting for more than 12 hours
• Has had moderate diarrhea for five days or more
• Can't keep down fluids Is irritable and much sleepier or less active than usual Has any of the signs or symptoms of mild or moderate dehydration

Go to the nearest hospital emergency room or call 911 if you think a child or older adult is severely dehydrated. You can help prevent dehydration from becoming severe by carefully monitoring someone who is sick and giving fluids at the first sign of diarrhea, vomiting or fever and by encouraging children to drink plenty of water before, during and after exercise.

Your doctor can often diagnose dehydration on the basis of physical symptoms such as little or no urination, sunken eyes and skin that lacks its normal elasticity and resilience when pinched. If you're dehydrated, you're also likely to have low blood pressure, especially when moving from a prone to a standing position, a faster than normal heart rate and reduced blood flow to your extremities.

To help confirm the diagnosis and pinpoint the degree of dehydration, you may have other tests, such as:

• Blood tests. These may be used to check your electrolytes, especially sodium and potassium; to look for signs of concentrated blood; and to evaluate how well your kidneys are working.
• Urinanalysis. The color and clarity of your urine, the presence of carbon compounds (ketones) and your urine's specific gravity — that is, the mass of the urine as compared with equal amounts of distilled water — all help show whether you're dehydrated and to what degree. A high specific gravity, for example, indicates significant dehydration.

If it's not obvious why you're dehydrated, your doctor may order additional tests to check for diabetes and for liver or kidney problems.

Hypovolemic shock is one of the most serious complications of dehydration. It occurs when low blood volume causes a drop in blood pressure and a corresponding reduction in the amount of oxygen reaching your tissues. If untreated, severe hypovolemic shock can cause death in a matter of minutes.

Other life-threatening complications include:

• Swelling of the brain (cerebral edema). Most often, the fluid you lose when you're dehydrated contains the same amount of sodium your blood does (isotonic dehydration). In some instances, though, you may lose more sodium than fluid (hypotonic dehydration). To compensate for this loss, your body produces particles that pull water back into the cells. As a result, your cells may absorb too much water during the rehydration process, causing them to swell and rupture. The consequences are especially grave when brain cells are affected.
• Seizures. These occur when the normal electrical discharges in your brain become disorganized, leading to involuntary muscle contractions and sometimes to a loss of consciousness.
• Kidney failure. This potentially life-threatening problem occurs when your kidneys are no longer able to remove excess fluids and waste from your blood.
• Coma and death. When not treated promptly and appropriately, severe dehydration can be fatal.

The only effective treatment for dehydration is to replace lost fluids. The best way to do that depends on your age, the severity of your dehydration and its cause.

Treating dehydration in sick children
Your doctor can offer specific suggestions for treating dehydration in your child, but some general guidelines include the following:

• Use an oral rehydration solution. Unless your doctor advises otherwise, use an oral rehydration solution such as Pedialtyte or Ricelyte for infants and children who have diarrhea, vomiting or fever. These solutions contain water and salts in specific proportions to replenish both fluids and electrolytes. They also contain glucose or another carbohydrate such as rice powder to enhance absorption in the intestinal tract. Oral rehydration products are readily available in most drugstores, and many pharmacies carry their own brands. The secret is to begin giving fluids early in the course of an illness instead of waiting until the situation becomes urgent.

  In an emergency, or when traveling abroad, you can make your own oral rehydration solution by mixing 1/2 teaspoon of salt, 1/2 teaspoon of baking soda and 3 tablespoons of sugar in a quart of pure water. Be sure to measure accurately because incorrect amounts can make the solution less effective or even harmful.

  In most developing countries you also can buy packets of a powdered oral rehydration solution, WHO-ORS, originally developed by the World Health Organization to treat diarrhea and dehydration in infants with cholera. Reconstitute the powder in bottled or boiled water according to the directions on the package.

  Whatever alternative you chose, be sure to give enough solution. Your doctor may suggest specific amounts, depending on your child's age and degree of dehydration, but a general rule of thumb is to keep giving liquids slowly until your child's urine becomes clear-colored. When your child is vomiting, try giving small amounts of solution at frequent intervals — 1 teaspoon every minute, for instance. If your child can't keep this down, wait 30 to 60 minutes and try again. Room temperature fluids are best.

• Continue to breast-feed. Don't stop breast-feeding when your baby is sick, but add an oral rehydration solution as well. If you give your baby formula, try switching to one that's lactose-free until diarrhea improves — lactose can make diarrhea worse. Your doctor may also suggest substituting an oral rehydration solution for the formula for 12 to 24 hours.
• Avoid certain foods and drinks. The best liquid for a sick child is an oral rehydration solution — plain water doesn't provide essential electrolytes, and although sports drinks replenish electrolytes, they replace those lost through sweating, not through diarrhea or vomiting. Avoid giving your child salty broths, milk — especially boiled milk — sodas, fruit juices or gelatins, which don't relieve dehydration and which may make symptoms worse.

Treating dehydration in sick adults
Most adults with mild to moderate dehydration from diarrhea, vomiting or fever can improve their condition by drinking more water. The operative word is water. Coffee, tea and other beverages that contain caffeine may temporarily increase dehydration, and fruit juices and sodas can make diarrhea worse.

Treating dehydration in athletes of all ages
For exercise-related dehydration, cool water is your best bet. There's no need for salt tablets — too much salt can lead to hypernatremic dehydration, a condition in which your body not only is short of water but also carries an excess of sodium. Some people prefer sports drinks containing electrolytes and a carbohydrate solution, but they're not essential unless you're exercising strenuously for an hour or more.

Treating severe dehydration
Children and adults who are severely dehydrated should be treated in a hospital emergency room where they can receive salts and fluids through a vein (intravenously) rather than by mouth. Intravenous hydration provides the body with water and essential nutrients much more quickly than oral solutions do — something that's essential in life-threatening situations.

Preventing dehydration sounds easy enough: consume lots of fluids and foods high in water such as fruits and vegetables. But questions as to how much fluid, what kind and when to drink muddy the waters. Unfortunately, determining appropriate water intake isn't an exact science, especially because so much depends on your age, physical condition, activity level, locale and your own unique physiology.

Some general guidelines exist, however. The most familiar is probably the 8 by 8 rule — that is, drink at least eight 8-ounce glasses of water a day. Though this recommendation isn't backed up by much scientific evidence, it may work for you.

A better idea may be the replacement plan. Most adults lose about 2.5 liters of water a day through breathing, sweating and elimination. Food accounts for about 20 percent of your fluid intake, so consuming 2 liters of water a day (a little more than 8 cups) in addition to your normal diet should replace what you lose. On the other hand, the Institute of Medicine recommends more — 13 cups of liquids a day for men and about 9 cups for women.

Perhaps the best overall recommendation is to make a conscious effort to keep yourself hydrated and to make water your beverage of choice. That might mean drinking water with every meal and between each meal; taking water breaks instead of coffee or tea breaks; and substituting sparkling water for alcohol.

These guidelines are rough estimates and don't take into account special circumstances such as strenuous exercise, heat and humidity, pregnancy, breast-feeding, and illness. In those cases, you may need to drink much more.

• Exercise. In general, it's best to start hydrating the day before strenuous exercise. Producing lots of clear, dilute urine is a good indication that you're well hydrated. Two hours before an endurance event such as a marathon or half-marathon, drink 2 cups of water. One to 2 cups of water is usually adequate before shorter bouts of exercise. During the activity, replenish fluids at regular intervals, and continue drinking water or other fluids after you're finished.

  Keep in mind that drinking too much can not only cause bloating and discomfort, it may lead to a potentially fatal condition in which your blood sodium becomes too low (hyponatremia). This occurs when you drink more fluids than you lose through sweating.

• Environment. You need to drink additional water in hot or humid weather to help lower your body temperature and to replace what you lose through sweating. You may also need extra water in cold weather if you sweat while wearing insulated clothing. Heated, indoor air can cause your skin to lose moisture, increasing your daily fluid requirements. And altitudes greater than 2,500 meters (8,200 feet) can also affect how much water your body needs.
• Pregnancy or breast-feeding. Women who are pregnant or breast-feeding need additional water to stay hydrated and to replenish lost fluids. The Institute of Medicine recommends that pregnant women drink 2.3 liters (nearly 10 cups) of liquids a day and that women who breast-feed consume 3.1 liters (about 13 cups).
• Illness. Start giving extra water or an oral rehydration solution at the first signs of illness — don't wait until dehydration occurs. And although they might sound appealing, traditional "clear fluids" such as ginger ale or other sodas contain too much sugar and too little sodium to replenish lost electrolytes.