Causes of dehydration 

Dehydration is caused by not drinking enough fluid or by losing more fluid than you take in. Fluid is lost through sweat, tears, vomiting, urine or diarrhoea.

The severity of dehydration can depend on a number of things, such as climate, level of physical activity and diet.

Types of dehydration

There are two types of dehydration. They are:

  • isotonic dehydration – where water and salt are lost in the same proportion as the water and salt in the fluid surrounding your cells; this type of dehydration is commonly caused by diarrhoea
  • hypernatraemic dehydration – which usually affects infants or children; 'hypernatraemic' means high levels of salt in the blood, so hypernatraemic dehydration is where a child loses relatively more water than salt – for example, when they have watery diarrhoea or excessive vomiting

There are several causes of dehydration, described below.


Dehydration is often the result of an illness, such as gastroenteritis, where fluid is lost through persistent bouts of diarrhoea and vomiting.


You can also become dehydrated if you sweat excessively due to a fever, exercise and sport, or carrying out heavy, manual work in hot conditions.

In these situations, it's important to drink regularly to replace lost fluids. It doesn't necessarily need to be hot for you to lose a significant amount of fluid from sweating.

Children and teenagers are particularly at risk because they may ignore the symptoms of dehydration, or not know how to recognise and treat them.


Dehydration can also occur as a result of drinking too much alcohol. Alcohol is a diuretic, which means that it makes you pee more.

The headache associated with a hangover indicates that your body is dehydrated. You should try to drink plenty of water when you have been drinking alcohol.


If you have diabetes, you're at risk of becoming dehydrated because you have high levels of glucose in your bloodstream. Your kidneys will try to get rid of the glucose by creating more urine, so your body becomes dehydrated from going to the toilet more frequently.

Read more about the different types of diabetes.

Who's at risk?

The groups of people most at risk of dehydration are:

  • babies and infants – their low body weight makes them sensitive to even small amounts of fluid loss
  • older people – they may be less aware that they're becoming dehydrated and need to drink fluids
  • people with a long-term health condition – such as diabetes or alcoholism
  • athletes – they lose a large amount of body fluid through sweat


It's possible to become over hydrated while exercising. This is known as hyponatremia and it's caused by low sodium (salt) levels in the blood. It can occur if too much water is drunk over a short time period.

Hyponatremia sometimes affects athletes whose blood sodium level is reduced through sweat and then diluted by drinking large amounts of water.

Symptoms of hyponatremia include nausea, vomiting and headache. In serious cases, the brain can swell, causing confusion, seizures, coma and, in rare cases, death.

Page last reviewed: 13/04/2015

Next review due: 13/04/2017