Dehydration is a deficiency of water in the body.
Dehydration occurs when the body loses more water than it takes in. Vomiting, diarrhea, the use of drugs that increase urine excretion (diuretics), profuse sweating (for example, during heat waves, particularly with prolonged exertion), and decreased water intake can lead to dehydration.
Dehydration is particularly common among older people because their thirst center may not function as well as that in younger people. Therefore, some older people may not recognize that they are becoming dehydrated. Certain disorders such as diabetes mellitus (see Diabetes Mellitus), diabetes insipidus (see Central Diabetes Insipidus), and Addison disease (see Addison Disease) can increase the excretion of urine and thereby lead to dehydration.
Dehydration is also common among infants and children because the amount of fluid lost during diarrhea or vomiting may represent a larger proportion of their body fluids than in older children and adults (see Dehydration in Children).
At first, dehydration stimulates the thirst center of the brain, causing thirst, a powerful motivator for people to drink more fluids. If water intake does not keep up with water loss, dehydration becomes more severe. Sweating decreases, and less urine is excreted. Water moves from inside the cells to the bloodstream to maintain the needed amount of blood (blood volume) and blood pressure (see About Body Water). If dehydration continues, tissues of the body begin to dry out, and cells begin to shrivel and malfunction.
Symptoms of mild to moderate dehydration include
In severe dehydration, the sensation of thirst may actually decrease and blood pressure can fall, causing light-headedness or fainting, particularly upon standing (a condition called orthostatic hypotension—see Dizziness or Light-Headedness When Standing Up). If dehydration continues, shock and severe damage to internal organs, such as the kidneys, liver, and brain, occur. Brain cells are particularly susceptible to more severe levels of dehydration. Consequently, confusion is one of the best indicators that dehydration has become severe. Very severe dehydration can lead to coma.
Dehydration can often be diagnosed from symptoms and the results of a doctor's examination. But sometimes doctors do blood tests for people who appear seriously ill or who take certain drugs or have certain disorders.
Dehydration normally causes the sodium level in the blood to increase (see Hypernatremia (High Level of Sodium in the Blood)). The reason is that although the common causes of dehydration (such as profuse sweating, vomiting, and diarrhea) result in a loss of electrolytes (especially sodium and potassium), even more water is lost, so the concentration of sodium in the blood rises.
Prevention is better than cure. Adults should drink at least 6 glasses of fluids daily (including fluid from eating foods high in water content, such as fruits and vegetables). Fluid intake should be increased on hot days and during or after prolonged exercise. Exercise, a high body temperature, and hot weather increase the body's need for water. Flavored sports drinks have been formulated to replace electrolytes lost during vigorous exercise. These drinks can be used to prevent dehydration. People should drink fluids with electrolytes before and during exercise as well as afterward. Before exercising, people with heart or kidney disorders should consult their doctors about how to safely replace fluids.
For treating mild dehydration, drinking plenty of water may be all that is needed. With moderate and severe dehydration, lost electrolytes (especially sodium and potassium) must also be replaced.
Treatment is also directed at the cause of dehydration. For example, when people have nausea or diarrhea, drugs to control or stop the vomiting or diarrhea may be used.
Oral rehydration solutions that contain appropriate amounts of electrolytes are available without a prescription. These solutions work well to treat mild dehydration, especially that caused by vomiting or diarrhea in children (see Treating Dehydration). Sports drinks do not necessarily contain enough electrolytes to be an adequate substitute for these solutions. However, when a person is vomiting, drinking fluids may not be sufficient to treat dehydration.
More severe dehydration requires treatment by doctors with intravenous solutions containing sodium chloride. The intravenous solution is given rapidly at first and then more slowly as the physical condition improves.
Last full review/revision January 2015 by James L. Lewis, III, MD