Dehydration happens when your body loses more fluid than you take in. When the normal water content of your body is reduced, it upsets the balance of minerals (salts and sugar) in your body, which affects the way that it functions.
Water makes up over two-thirds of the healthy human body. It lubricates the joints and eyes, aids digestion, flushes out waste and toxins and keeps skin healthy.
Some of the early warning signs that you're dehydrated include:
- feeling thirsty and lightheaded
- having dark coloured, strong-smelling urine
- passing urine less often than usual
A baby may be dehydrated if they have:
- a sunken soft spot (fontanelle) on their head
- few or no tears when they cry
- fewer wet nappies
- drowsiness
The body is affected even when you lose a small amount of fluid. Read more about the symptoms of dehydration.
Who is at risk from dehydration?
Anyone can become dehydrated, but certain groups are particularly at risk. These include:
- babies and infants, because they have a low body weight and are sensitive to even small amounts of fluid loss
- older people, because they may be less aware that they're becoming dehydrated and need to keep drinking fluids
- people with a long-term condition, such as diabetes or alcoholism
- athletes, because they lose a large amount of body fluid through sweat
What to do
If you're dehydrated, drink plenty of fluids. This can be water, semi-skimmed milk and diluted squash or fruit juice, but it's best to avoid fizzy drinks or caffeine if possible. If you're finding it hard to keep water down because you're vomiting or have diarrhoea, try drinking small amounts more frequently (perhaps using a teaspoon or syringe for an infant or child).
If you or your child are dehydrated due to watery diarrhoea or excessive vomiting (hypernatraemic dehydration), try not to drink only water as it's likely to further dilute the minerals in the body and make the problem worse. Try drinking diluted juice, squash or a rehydration solution (available from pharmacies).
If left untreated, severe dehydration can be serious and cause fits (seizures), brain damage and death.
Read more about treating dehydration.
When should I see my GP?
You should visit your GP if your symptoms continue, despite drinking plenty of fluids or if you suspect that your baby or toddler is dehydrated.
If your doctor suspects dehydration, you may be given a blood test or a urine test to check the balance of salts (sodium and potassium) in your body.
Contact your GP or out-of-hours service straight away if you have any of the following symptoms:
- feeling tired (lethargic) or confused
- dry mouth and eyes that don't produce tears
- not passing urine for eight hours
- dry skin that sags slowly into position when pinched up
- rapid heartbeat
- blood in your stools (faeces) or vomit
- low blood pressure (hypotension)
Also contact your GP if you or your child has diarrhoea that lasts longer than five days, or vomiting that lasts more than 24 hours in adults or 12 hours in a child.
How does dehydration happen?
Dehydration is usually caused by not drinking enough fluid or by losing fluid and not replacing it. The climate, the amount of physical exercise you're doing and your diet can also contribute to dehydration.
You can become dehydrated as a result of an illness, such as gastroenteritis, where you have persistent vomiting and diarrhoea, after sweating excessively from a fever, exercising in hot conditions or if you're urinating more frequently – for example, if you have diabetes.
Drinking too much alcohol can also cause dehydration, which is the main symptom of a hangover.
Read more about the causes of dehydration.
