Every day people around the globe drink 1.6 billion cups of coffee and around twice as many cups of tea.

They enjoy the taste and the fact that the caffeine wakes them up. But when we’re exhorted to drink six or eight glasses of water a day (a disputed figure that I’ve discussed previously), it’s usually emphasised that drinks like coffee and tea don’t count towards your daily liquid total because they’re dehydrating. Or so we’re told. What’s the evidence?

Although tea and coffee contain many different substances the one on which most research focuses is caffeine. Even then there is so little research on the topic, that one of the most frequently mentioned studies was conducted way back in 1928 with a sample of just three people. The three men were studied over the course of two winters. Sometimes they were required to drink four cups of coffee a day; sometimes they drank mainly tea and at other times they abstained or drank water laced with pure caffeine. Meanwhile the volume of their urine was measured regularly. The authors concluded that if the men consumed caffeine-laced water after a two month period of abstinence from both coffee and tea, the volume of their urine increased by 50%, but when they drank coffee regularly again they became inured to its diuretic effects.

Very large doses of caffeine are known to increase the blood flow to the kidneys and to inhibit the absorption of sodium which explains why it could act as a diuretic, dealing with the sodium which hasn’t been absorbed. But the exact mechanism is still a matter of debate.