Water makes up over two-thirds of the healthy human body. It is essential for lubricating the joints and eyes, aiding digestion, flushing out waste and toxins and keeping skin healthy.

Dehydration occurs when the normal water content of your body is reduced, upsetting the delicate balance of minerals (salts and sugar) in your body fluid. Many of your body’s cells depend on these minerals being maintained at the correct levels to function properly. Dehydration is generally caused by not drinking enough fluid, or by losing fluid and not replacing it. You can lose fluid through vomiting or diarrhoea (for example, if you have gastroenteritis), through sweating (for example, if you have a fever or are exercising in hot conditions), or through urinating too frequently, if you have diabetes. Drinking too much alcohol can also cause dehydration, which is the main symptom of a hangover.

There are two types of dehydration:
• Isotonic dehydration. This is when you lose the same proportions of water and salt as those in the fluid surrounding your cells. It is the type of dehydration most often caused by diarrhoea.
• Hypernatraemic dehydration. This usually happens in infants or children. ‘Hypernatraemic’ means high levels of salt in the blood, so hypernatraemic dehydration is when a child loses relatively more water than salt – for example, when they have watery diarrhoea and/or excessive vomiting.

At-risk groups:
People who are particularly at risk from dehydration are:
• 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 of becoming dehydrated and the need to keep drinking fluids,
• people with a long-term condition, such as diabetes or alcoholism, and
• athletes, because of the amount they sweat.

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NHS Choices
NHS Choices