Hydration and kidney health

How your kidneys use water

The kidneys act as the body's filters, cleaning waste products and excess salts from the blood and turning them into urine. To do this well, they need enough fluid passing through them. When you are well hydrated, your kidneys can flush out waste and keep the body's water and salt levels balanced. When you become dehydrated, the body holds on to water, urine becomes more concentrated and darker, and the kidneys have to work harder. Staying reasonably hydrated helps prevent this strain, supports normal kidney function, and lowers the risk of some problems such as kidney stones and urine infections. Good hydration is one of the simplest everyday things you can do to look after your kidneys.

How much should you drink?

For most healthy adults in the UK, a common guide is around six to eight glasses or cups of fluid a day, though needs vary with body size, activity, the weather and health. This does not have to be plain water — lower-sugar drinks, milk, tea and coffee all count towards your fluid, and water-rich foods contribute too. A practical way to judge is by your urine: pale straw-coloured urine usually means you are well hydrated, while dark yellow urine suggests you need to drink more. You will naturally need more when it is hot, when exercising, or when unwell with fever, vomiting or diarrhoea. Rather than fixating on an exact number, aim to drink regularly and respond to thirst and urine colour.

Spotting dehydration

Dehydration happens when you lose more fluid than you take in. Early signs include feeling thirsty, a dry mouth and lips, tiredness, a headache, and dark, strong-smelling urine passed less often than usual. As it worsens, people can feel dizzy or light-headed, especially on standing, and may pass very little urine. Babies, young children and older adults are more vulnerable and can become dehydrated more quickly, so they need extra care in hot weather or during illness. Illnesses causing vomiting or diarrhoea are common culprits. Most mild dehydration is easily corrected by drinking more, using oral rehydration salts if fluid losses are large. Severe dehydration, with confusion, fainting or passing no urine, needs urgent medical help.

When more is not better

While staying hydrated is good, drinking excessive amounts of water is not automatically healthier and, in rare cases, can be harmful by diluting the salts in the blood. This is uncommon but can happen with very large intakes over a short time, for example during some endurance sports. More importantly, people with certain conditions — such as advanced kidney disease, heart failure or particular hormone problems — may be advised to limit their fluids, because their bodies cannot handle large amounts. If you take water tablets (diuretics) or have been told to watch your fluid intake, follow that individual advice rather than general rules. For most healthy people, though, drinking to satisfy thirst and keep urine pale is a sensible, safe approach.

Everyday habits that help

Simple habits make good hydration easy. Keep a drink within reach and sip through the day rather than waiting until you are very thirsty, as thirst can lag behind need, especially in older people. Have a drink with each meal and carry water when out or exercising. Drink more in hot weather and when you are active or unwell. If you find plain water dull, add a slice of fruit, or choose lower-sugar drinks, milk or tea. Cutting down on sugary and heavily sweetened drinks is better for your teeth and overall health. Alongside hydration, not smoking, keeping blood pressure and blood sugar controlled, and using medicines like anti-inflammatory painkillers only as advised all help protect your kidneys over the long term.