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Hydration

/haɪˈdreɪ.ʃən/ · hy-DRAY-shun noun
The process of taking in enough fluid to replace the water the body loses every day, and the resulting state of having adequate fluid in the body. Water contributes to the maintenance of normal physical and cognitive functions and to normal temperature regulation.

What it is and how it works

Roughly half to sixty per cent of an adult's body weight is water. It is lost all the time: in every breath, through the skin, in sweat and in urine. Hydration simply means replacing those losses well enough that the body's systems can keep working comfortably.

The body is good at this. When fluid runs low, the brain releases a hormone that tells the kidneys to hold on to water, and thirst appears. When there is plenty, the kidneys let more go. This regulation is why most healthy people do not need to measure their drinks precisely.

Water's officially recognised roles are broad: it contributes to the maintenance of normal physical and cognitive functions and to the maintenance of normal thermoregulation. Sweat cools the body only because there is water to sweat.

Black foam roller and a green reusable water bottle on a wooden floor
A reusable bottle by the door. Having water within reach makes drinking regularly easier.

How much is "enough"?

The European Food Safety Authority gives adequate intakes of about 2.0 litres of total water a day for women and 2.5 litres for men, in moderate climates and with moderate activity. "Total water" includes the water in food, which usually provides around a fifth of the total. Soup, fruit, vegetables, yoghurt and cooked grains all contribute.

Needs go up with heat, physical work, exercise, fever, pregnancy and breastfeeding. They differ from person to person, so these figures are reference points rather than targets to hit to the millilitre.

Signs worth noticing

  • Thirst is a reliable signal for most healthy adults, though it becomes weaker with age.
  • Urine colour: pale straw-coloured urine usually indicates adequate hydration; dark yellow suggests drinking a little more. Some vitamins and foods change the colour too.
  • Headache, tiredness and difficulty concentrating can appear with mild dehydration, but they have many other causes.

Older people, young children and anyone who is unwell with vomiting, diarrhoea or fever are more vulnerable to dehydration and may need closer attention or medical advice.

In everyday life

  • Keep a glass or bottle in sight at your desk or in the kitchen.
  • Drink with meals. It is an easy habit to anchor.
  • Tea and coffee count. Despite their reputation, moderate amounts contribute to fluid intake; see caffeine.
  • Long or hot exercise may call for some sodium as well as water; see electrolytes.
  • Eating more fiber? Drink a little more too; see fiber.

Common confusions

"Everyone must drink eight glasses of water a day." There is no single figure that suits everyone. Food water and other drinks count, and needs vary.

"If you feel thirsty, you are already dehydrated." For healthy adults, thirst is an early, normal prompt rather than an emergency.

"Clear urine is the goal." Completely clear urine all day can simply mean you are drinking more than you need.

"More water is always healthier." Drinking very large volumes quickly, particularly during long endurance events, can lower blood sodium. People with heart or kidney conditions may be given specific fluid limits by their doctor; those come first.

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Added Sep 12, 2026 · Filed under Everyday habits