A fall in the water content of the body. Sixty per cent of a man's body weight is water, and 50 per cent of a woman's; those proportions need to be maintained within quite narrow limits to ensure proper functioning of body tissues. Body fluids contain a variety of mineral salts (see ELECTROLYTES) and these, too, must remain within narrow concentration bands. Dehydration is often accompanied by loss of salt, one of the most important minerals in the body.

The start of ‘dehydration’ is signalled by a person becoming thirsty. In normal circumstances, drinking water will relieve thirst and serious dehydration does not develop. In a temperate climate an adult will lose 1.5 litres or more a day from sweating, urine excretion and loss of fluid through the lungs (known as ‘insensible loss’). In a hot climate the loss is much higher – up to 10 litres if a person is doing hard physical work. Even in a temperate climate, severe dehydration will occur if a person does not drink for two or three days. Large losses of fluid occur with certain illnesses – for example, profuse diarrhoea; POLYURIA in diabetes or kidney failure (see KIDNEYS, DISEASES OF); and serious blood loss from, say, injury or a badly bleeding ULCER in the gastrointestinal tract. Severe thirst, dry lips and tongue, TACHYCARDIA, fast breathing, light-headedness and confusion are signs of serious dehydration. Output of urine is reduced.

Prevention is important, especially in hot climates, where it is essential to drink water even if one is not thirsty. Replacement of salts is also vital, and a diet containing half a teaspoon of table salt to every litre of water drunk is advisable. If someone, particularly a child, suffers from persistent vomiting and diarrhoea, rehydration therapy is required and a salt-and-glucose rehydration mixture (obtainable from pharmacies) should be taken. For those with severe dehydration, oral fluids may be insufficient and the affected person might need intravenous fluids and, admission to hospital, where fluid intake and output can be monitored as can the levels of ELECTROLYTES and rehydration measures safely controlled.