The human body is made up of 70% water. Water is essential to the normal working of the body. It lubricates joints and eyes, aids digestion, flushes out waste and toxins and keeps skin healthy. Dehydration is when the normal water content of the body is reduced. This leads to a change in the vital balance of chemical substances in the body, especially sodium (salt) and potassium. The function of many cells depends critically on these substances being maintained at the correct levels; serious effects follow any change and dehydration can result in shock, then death.

Long term dehydration is bad for skin, kidneys, liver, joints and muscles and can cause cholesterol problems, headaches, reduced blood pressure (hypotension), fatigue and constipation.

Dehydration is caused by insufficient intake of water or by losing a lot of fluid and not taking in enough to replace it. It commonly results from an illness with persistent diarrhea and vomiting, as well as excessive sweating from a fever, heavy work in hot surroundings or heatstroke.

Dehydration can also occur as a result of drinking too much alcohol. The headache associated with a hangover indicates that your body is dehydrated. This is why it is important to drink plenty of water when you have been drinking alcohol.

Babies, infants and elderly people need urgent treatment if they are dehydrated.

You can avoid dehydration by drinking plenty of water. The Food Standards Agency recommends that if you live in the UK (or somewhere with a similar climate) you should drink approximately 1.2 litres (6-8 glasses) of water every day. Your intake of water should increase if you are ill with sickness and/or diarrhea.

When exercising, you should drink up to one litre of water per hour of exercise, on top of your normal daily amount. This should be increased if you are exercising in warm conditions, as you will dehydrate more quickly.         

In hot weather, you will sweat more and lose fluid from your body. Make sure you are drinking enough water to replace lost fluids.