Dehydration

What happens in dehydration

About 50 to 60% of an adult's body weight is water, and more in babies. Water sits inside cells and outside them, in the blood and tissues. When you lose fluid:

1. Blood volume falls, so the heart beats faster to keep blood pressure up.
2. The kidneys conserve water and make less, more concentrated urine.
3. The brain triggers thirst and releases antidiuretic hormone.
4. If losses continue, blood pressure drops, organs get less blood, and the kidneys can be injured.

Doctors describe dehydration by what is lost alongside the water:

Correcting high or low sodium too quickly can cause brain swelling or nerve damage, which is why severe cases are managed carefully in the hospital.

Causes

• Gastrointestinal losses: gastroenteritis, food poisoning, cholera and other diarrheal illnesses are the leading cause worldwide, especially in children.
• Fever and infection, which raise water loss through breathing and the skin.
• Heat and exercise: heavy sweating can lose over 1 liter (about 34 oz) an hour. See heat exhaustion and heat stroke.
• Reduced intake: sore mouth or throat, nausea, dementia, disability, or no access to water.
• Kidney losses: high blood sugar in uncontrolled diabetes, diabetes insipidus, diuretics, adrenal insufficiency.
• Skin losses: burns and severe skin disease.
• Alcohol, which suppresses antidiuretic hormone.

Risk factors

• infants and young children
• adults over 65
• chronic illness: diabetes, kidney disease, heart failure
• athletes and outdoor workers in hot weather
• living at high altitude
• people who cannot easily get a drink themselves

Symptoms

Early dehydration causes thirst, dry mouth, dark urine, reduced urination, tiredness and headache. As it worsens, dizziness on standing, rapid heartbeat, sunken eyes, confusion and very low urine output develop. Our signs of dehydration page sets out symptoms by age and what to do at home.

How dehydration is diagnosed

• History: how much fluid has been lost and taken in, number of vomits and stools, urine output.
• Examination: pulse, blood pressure lying and standing, mouth moisture, skin turgor, capillary refill time (how quickly color returns after pressing a fingertip), and mental state. In children, recent weight change is the most accurate measure.
• Blood tests: sodium, potassium, urea and creatinine (kidney function), glucose and bicarbonate in moderate or severe cases.
• Urine tests: concentration (specific gravity), and ketones or glucose.

Treatment

Oral rehydration therapy

Oral rehydration solution (ORS) is the treatment of choice for mild and moderate dehydration in both children and adults. The glucose in it helps the gut absorb sodium and water together. It works even during ongoing diarrhea.

• Give small, frequent amounts. In children, around 50 to 100 ml/kg over 3 to 4 hours is a typical target for moderate dehydration under medical guidance.
• Replace ongoing losses after each loose stool or vomit.
• Continue breastfeeding, and restart normal food once rehydrated.
• If oral fluids fail, a nasogastric tube can deliver ORS.

Medicines

A single dose of an anti-nausea medicine such as ondansetron, given by a clinician, helps many children and adults keep oral fluids down. Antidiarrheal medicines are not used in young children.

Intravenous fluids

Severe dehydration, shock, inability to drink, or dangerous sodium or potassium levels need IV fluids, usually saline or a balanced salt solution, given quickly at first and then adjusted to blood results.

Treating the cause

Examples include insulin and fluids for diabetic ketoacidosis, cooling for heat stroke, stopping or adjusting diuretics, and treating infections.

Complications

• Acute kidney injury: reduced blood flow to the kidneys, usually reversible with prompt treatment.
• Electrolyte disorders: low or high sodium (hyponatremia), and low potassium, which can cause weakness and heart rhythm problems.
• Hypovolemic shock: life-threatening low blood volume.
• Seizures: from sodium disturbances.
• Heat injury: dehydration raises the risk of heat stroke.
• Urinary tract infections and kidney stones with chronic low fluid intake.
• Falls and confusion in older adults.

Prevention

• Drink regularly through the day. Most healthy adults need roughly 2 to 3 liters (8 to 12 cups) of total fluid a day from drinks and food, more in heat and with exercise.
• Use pale urine as your guide.
• Before and during long exercise in heat, drink fluids, and use drinks with electrolytes for sessions longer than about an hour.
• Start ORS early in a stomach bug, especially for children.
• Offer drinks regularly to older relatives and people with dementia.
• Ask your doctor about pausing diuretics or certain diabetes medicines during vomiting illnesses ("sick day rules").

Frequently asked questions

What causes dehydration?

The most common causes are vomiting and diarrhea, fever, sweating in heat or exercise, and not drinking enough. High blood sugar, diuretics, alcohol and burns can also cause it. Babies and older adults are most vulnerable.

How is severe dehydration treated?

Severe dehydration is treated in the hospital with intravenous fluids to restore blood volume quickly, with blood tests to guide the correction of sodium and potassium. The underlying cause, such as infection or diabetic ketoacidosis, is treated at the same time.

Can dehydration damage your kidneys?

Yes. Significant dehydration reduces blood flow to the kidneys and can cause acute kidney injury, which usually recovers with rehydration. Repeated or prolonged dehydration is linked to kidney stones and may contribute to long-term kidney problems.

Are sports drinks good for dehydration?

Sports drinks help replace fluid and some salt during long, intense exercise. For dehydration from vomiting or diarrhea, they contain too much sugar and too little sodium, so oral rehydration solution is better, especially for children.

How much water should you drink to avoid dehydration?

Needs vary with size, activity and climate. A practical guide is to drink enough that your urine is pale yellow throughout the day, which for most adults means about 2 to 3 liters of total fluid daily, including water in food.