Six to eight cups – and it does not have to be water
Most people should aim to drink enough during the day for their urine to be a clear pale yellow colour, the NHS says.
The rule that everyone should drink eight glasses of water a day had no scientific study behind it, a review of the evidence by physiologist Heinz Valtin found.
The review was published in 2002. Dartmouth’s Geisel School of Medicine, which reported it that August, described the eight-glass rule as advice with “no supporting evidence”.
The NHS advises a range rather than a single figure: people should aim to drink six to eight cups or glasses of fluid a day.
It does not all have to be water. Lower-fat milk and sugar-free drinks count too, and those include tea and coffee.
Valtin’s review found that dilute alcoholic drinks such as beer, if taken in moderation, probably count towards fluid needs as well.
The European Food Safety Authority (EFSA) works its figures out differently. In a podcast episode titled Thirsty work, published in August 2023, the authority explained that they cover water “from food and beverages, not only beverages”.
For adults, that means around 2.5 litres a day for men and 2 litres for women.
Water needs “may change dramatically depending on certain conditions, for example, climate and physical activity”, the authority added.
The NHS says more fluid than usual is needed by people who are:
- pregnant or breastfeeding
- in a hot environment
- physically active for long periods
- ill or recovering from illness
Valtin’s conclusion had limits. It applied to healthy adults in a mild climate who spend much of the day sitting down.
The review also found large amounts of fluid advisable to treat or prevent some diseases, such as kidney stones. The same goes for special circumstances such as hard physical activity, long plane flights or hot weather.
Berkshire Healthcare NHS Foundation Trust tells people with heart failure to manage their fluid intake as advised by their heart function nurse.
The trust also asks patients to record their weight every day and to contact the nurse if it rises or falls by two kilograms (4-5lb) within three days.
North Bristol NHS Trust’s advice for kidney patients is that a doctor, nurse or dietitian can advise how much fluid to have.
Drinking less can help those patients breathe more easily, prevent ankle swelling, keep blood pressure stable and help prevent faintness or cramp on dialysis, the trust says.