"while we may find it hard to believe,
a few cups of coffee a day is as hydrating as water"
The Birmingham University researchers called for health advice to be updated to reflect their findings.
Sports scientist Sophie Killer said that the idea that coffee dehydrates stems from research done on samples of caffeine over 80 years ago.
However, caffeine and the many other compounds in coffee interact with each other and so the 1928 study isn’t necessarily relevant to daily life.
Miss Killer said: ‘It is estimated that 1.6billion cups of coffee are consumed worldwide every day, thus it is of interest to know whether coffee contributes to the daily fluid requirement or whether it causes low-level chronic dehydration.’
To find out, the researcher asked 50 healthy men to drink either four mugs of water or coffee a day for three days and then switch.
The men ate the same food during the two parts of the study and were banned from vigorous exercise and alcohol.
Tests of blood and urine samples showed the men were just as well hydrated when they drank coffee and when they had water.
They also passed the same amount of urine, the journal PLOS ONE reports.
The European Food Safety Authority recommends that women should drink about 2.8 pints of fluid and men should drink about 3.5 pints of fluid per day.
However, some experts say that coffee doesn’t count towards this. Others advise that every cup of coffee or tea drunk is matched with a glass of water to protect against dehydration.
But Miss Killer said that her industry-funded study shows that coffee doesn’t dehydrate – at least when consumed in moderate amounts.
She said: ‘It is a common belief that coffee consumption can lead to dehydration and should be avoided, or reduced, in order to maintain a healthy fluid balance.
‘The advice provided in the public health domain regarding coffee intake and hydration status should be updated to reflect these findings.’
Dr Frankie Phillips, of the British Dietetic Association, said a small, strong coffee such as an espresso might dehydrate.
However, any diuretic effect of caffeine in a normal cup of coffee is more than balanced by the amount of water in the drink – leading to hydration.
She added that a couple of cups of coffee in the morning could ‘add something useful’ to someone’s day.
Tea drinkers can also take heart, with a similar, earlier, study concluding that it also does not bother the bladder more than plain water.