Water

Around sixty per cent of an adult body, and the medium every biochemical reaction takes place in. Needs vary far more than the familiar "eight glasses" suggests — with size, activity, climate, illness and what else you eat and drink.

Around 60% of body weight. The medium every reaction happens in, the transport for nutrients and waste, and the mechanism of temperature control.

• Drinking water
• Tea and coffee — yes, they count
• Fruit and vegetables
• Soups and milk

3.7 L total (men) · 2.7 L (women), including water from food for most adults. The upper limit, past which harm becomes likely, is None established.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Dehydration. That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: Older adults, hot weather, illness with fever or vomiting, and endurance exercise.

Not drinking enough for the circumstances: heat, exercise, fever, vomiting and diarrhoea. Older adults feel thirst less reliably, and some medications increase losses.

Thirst, dark urine, headache, fatigue and poor concentration. Older adults lose the thirst signal, which is why dehydration is common and easily missed in them.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Assessed clinically rather than by a single test — skin turgor, blood pressure, and urine concentration, supported by sodium and kidney function.

Food supplies roughly a fifth of intake, more with plenty of fruit and vegetables. Urine that is pale straw rather than dark is a reasonable everyday guide.

Electrolyte solutions help when losses are heavy — prolonged sweating, vomiting, diarrhoea — and are unnecessary otherwise. Drinking very large volumes of plain water quickly can dangerously dilute blood sodium.

Drinking far more than the kidneys can excrete dilutes blood sodium and causes hyponatraemia — confusion, seizures, and occasionally death. It has killed marathon runners and people attempting water-drinking challenges.