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What is dehydration?

Dehydration is an excessive loss of water from the body tissues, often accompanied by an imbalance of sodium, potassium, chloride, and other electrolytes. It can occur whenever fluids are lost and not adequately replaced, especially when an individual does not drink enough fluids. Early dehydration has no symptoms; mild or moderate dehydration can cause symptoms that include: thirst, fatigue, muscle cramps, dizziness, and headache. Severe dehydration can cause more serious symptoms, such as confusion, low blood pressure, unconsciousness, shock, and may even lead to death.

The human body consists of about 60-70% water and must have a continuous supply in order to function. Water enters the body primarily through drinking liquids and secondarily as part of the food that we eat. It is absorbed by the intestines and carried throughout the body. Water comprises the fluids found inside of cells, in the spaces in between cells and tissues, in the lymphatic system, mucous membranes, and in the fluid portion of the blood within our veins and arteries. As needed, fluids can be shifted from one “compartment” or area into another.

Most water is filtered from the blood, reabsorbed, and recirculated several times by the kidneys. Excess water and dissolved wastes are made into urine and eliminated from the body during urination. Additional small amounts of water are continually lost through sweating, breathing, and in stool. The total amount of normal water loss ranges from 1,500 to 2,500 milliliters (mL) per day (about 50-85 ounces per day) based on the following sources:

Stool 50-100 mL/day
Evaporation/Exhaling 500-1000 mL/day
Urine 1000-2000 mL/day on average

Maintaining the balance and conservation of water within the body is a complex process. The kidneys are part of a feedback system that conserves or removes water by concentrating or diluting urine and by controlling the conservation of sodium. Sodium and other electrolytes – potassium, chloride, and bicarbonate – help regulate water balance at the cellular level by maintaining electrical neutrality and the body’s acid-base balance.

This feedback system and its components are vital in maintaining a healthy level of water in the body. Sensors in the body perceive and respond to increases and decreases in the amount of water and dissolved substances in the bloodstream. As the number of dissolved particles in blood (osmolality) increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, a specialized gland in the brain – the hypothalamus – secretes antidiuretic hormone (ADH). This hormone signals the kidneys to conserve water. Water moves from cells to the blood stream to maintain blood pressure and volume. If not corrected, body tissues dry out, causing cells to shrink and malfunction. As fluid levels decrease, a “thirst” response is triggered by the brain, signaling a person to drink more water. Working together, these feedback systems normally maintain a dynamic fluid balance.

Dehydration occurs when liquid/fluids are lost faster than they can be replaced. This can occur with excessive vomiting, diarrhea, sweating, use of diuretics (medications that increase urine production), and/or not taking in enough fluids through drinking or eating. This can worsen if the person also loses too much sodium (hyponatremia) or too little (hypernatremia) in relationship to the decrease in water. Prolonged dehydration can result in shock and damage to internal organs, particularly the brain, leading to confusion, coma, and possible death.

Anyone can become dehydrated, but the condition tends to be more serious in the young, elderly, and in those with underlying health conditions or weakened immune systems. Infants and children may have difficulty communicating thirst. Because they have faster metabolisms and higher body water content than adults, children also have different fluid requirements than adults. Sweating, vomiting, and diarrhea are common sources of rapid fluid loss in children, and worldwide, diarrheal illnesses are a serious health threat. According to the World Health Organization, diarrheal illness is the second leading cause of death in children under the age of five. The U.S. Centers for Disease Control and Prevention estimates that in developing countries, diarrhea kills 2,200 children every day.

In the elderly, dehydration is a common problem and has been linked to a number of adverse health outcomes. Experts estimate that more than 20% of older people living independently is the U.S. are dehydrated, and similar rates of dehydration have been recorded in older people living in residential treatment facilities in the UK.

About Dehydration


Large amounts of water can be lost in a short period of time with prolonged vomiting and/or diarrhea. One or both of these symptoms can be seen with a wide variety of conditions. Some examples include:

  • Gastroenteritis (“stomach flu”) – inflammation of the digestive tract; very common cause of vomiting and diarrhea in all age groups; may be linked to a foodborne or waterborne illness and caused by bacterial, viral, or parasitic infections of the digestive tract
  • An obstruction, for example, in the digestive tract
  • Drug overdose or toxicity
  • Malabsorption
  • Inflammatory bowel disease
  • Irritable bowel

Fluids may also be lost due to:

  • Fever
  • Burns
  • Intense or prolonged physical exertion and sweating such as may occur with athletes training in hot temperature and/or for long periods of time
  • Excessive urination – may occur with:
    • Some diseases that affect the body’s ability to concentrate urine and conserve water
    • Uncontrolled diabetes (diabetic ketoacidosis)
    • Use of medications such as diuretics
  • Excessive bleeding

A lack of fluids may also occur because there is insufficient intake. This can happen:

  • In infants who cannot communicate thirst
  • In people with inflamed mouths or sore throats who don’t drink enough because of the pain
  • In some elderly people who need assistance with access to water
  • Due to a lack of available water
  • With a decreased sense of thirst or appetite

Some rare causes include:

Signs and Symptoms

Signs and symptoms of dehydration will vary with duration of reduced fluid intake and from person to person.

Early dehydration has no symptoms. Mild to moderate dehydration may cause few to no noticeable symptoms, but individuals may experience symptoms that include:

  • Increased thirst
  • Dry and/or sticky mouth
  • Less frequent urination, and urine may be darker yellow, indicating concentration
  • Constipation
  • A decrease in tears
  • Mildly sunken eyes in an infant
  • Fatigue
  • Dry skin
  • Headache
  • Feeling lightheaded or dizzy

Severe dehydration can cause increasingly serious signs and symptoms, such as:

  • Lack of tears and sweating
  • Lack of urine – little to no urine produced and dark yellow in color
  • Intense thirst
  • Extremely dry mouth and mucous membranes
  • Dry skin that lacks elasticity
  • Sunken eyes in an infant
  • Sunken fontanelles in an infant (the soft spots on top of the head)
  • Rapid breathing
  • Increased heart rate
  • Low blood pressure
  • Confusion and, in critical cases, unconsciousness


A diagnosis of dehydration is frequently based upon clinical signs and symptoms, and appropriate treatment is given. Laboratory testing is typically not required for mild to moderate dehydration, but a variety of non-laboratory evaluations may be used to assess an individual with more serious signs and symptoms.

Non-Laboratory Evaluations

These may include an evaluation of:

  • Urine output and production of tears
  • Examination of dryness of skin and mucous membranes
  • Breathing rate – is it rapid?
  • Heart rate – is it rapid?
  • Blood pressure – is it low?
  • Skin turgor – when a fold of skin is pinched and then released, does it bounce back in shape or only slowly relax?
  • Capillary refill rate – is it slower than normal? For this evaluation, pressure is applied to a patient’s nail bed until it turns white, indicating blood has been forced out. Then the pressure is released. The time that it takes for the nail bed to turn pink again, indicating return of blood, is observed.
  • Do eyes appear sunken and, if so, to what degree?
  • State of consciousness

Laboratory Tests

In cases of severe dehydration, laboratory testing is frequently ordered to identify electrolyte and acid-base imbalances, to evaluate kidney function, and general health status. If imbalances and/or organ dysfunction are found, then serial testing may be performed to monitor the person over time and their response to treatment. Testing may include:

If the cause of dehydration is apparent, then usually no other testing is necessary. However, a variety of tests may be performed when the cause is unknown, to diagnose and address underlying conditions, such as those associated with prolonged diarrhea and/or vomiting.

A wide variety of other tests may be done depending on what is suspected to be the underlying cause of the signs and symptoms, such as:

Treatment and Prevention

Where possible, it is best to prevent dehydration by drinking adequate fluids, especially when losing water through perspiration, and making up for losses as they occur. Those involved in extended physical exertion, for instance, should make sure that they drink water before, during, and after their activities. The equivalent of 8 glasses of fluids per day for women and 12 glasses of fluids per day for men (including that derived from foods with high water content – vegetables, fruits) is adequate to maintain proper hydration.

Mild dehydration can be treated in most healthy adults by drinking more water. Moderate to severe dehydration can be more complicated to treat, especially if there is also an electrolyte imbalance with excess or a deficient sodium (hypernatremia or hyponatremia) level. With moderate to severe dehydration, plain water is usually insufficient because it will not restore a person’s electrolyte balance.

Oral rehydration solutions (ORS, commercially available under a variety of brand names) that contain small but very specific concentrations of salt and sugar are often used to slowly return a person’s fluid and electrolyte balance to normal. ORS are NOT the same thing as, for example, sports drinks, energy drinks, or flavored waters; the types and proportions of ingredients are different.

There can be serious complications, such as fluid accumulation in the brain (cerebral edema), if someone is significantly dehydrated and rehydration is done too rapidly. In addition, someone who has been vomiting may have difficulty retaining large amounts of fluid. Typically, very small amounts of fluids are given frequently, such as every few minutes for several hours, and the person’s health status is monitored.

In some cases, a person may need to be hospitalized and have intravenous (IV) fluids administered, with careful monitoring until both fluid and electrolyte balances are restored.

For some individuals, an underlying condition that causes vomiting, diarrhea, and/or other problem exacerbating dehydration will also need to be addressed.

View Sources

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(April 30, 2015) Hooper L, et al. Clinical symptoms, signs and tests for identification of impending and current water-loss dehydration in older people. Cochrane Database of Systematic Reviews. Cochrane Library. Available online at: Accessed 6/8/2016.

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