Solvent and transport medium, temperature regulation, blood volume and metabolic reactions.
Switzerland age 18–65: about 2.0 L/day for women and 2.5 L/day for men as total water; needs rise with heat, exercise and illness.
Water and unsweetened drinks, plus water-rich foods such as fruit, vegetables, soups and yoghurt.
Thirst, dark urine, headache, dizziness and performance decline can occur; severe dehydration is an emergency.
Older adults and children dehydrate more easily; fluid loss from diarrhoea, vomiting or heat can become dangerous quickly.
Urine colour can offer a rough clue but is also affected by medicines, foods and vitamins.
Heat, endurance exercise, fever, diarrhoea, diuretics and impaired thirst.
Assess thirst, body-weight changes, urine and clinical context together.
Electrolytes are situational for prolonged exercise or major losses; water plus a normal diet is usually enough day to day.
Extremely high water intake over a short period can dangerously dilute sodium (hyponatraemia).