Medical myths: Drinks with caffeine in will dehydrate you

Dr Max Pemberton

Where did the myth come from?

Lots of beverages such as tea, coffee and fizzy drinks contain caffeine. It's what makes them popular as a "pick-me-up", as caffeine is a known stimulant. But caffeine is also thought to be a diuretic, which causes kidneys to make more urine.

It's therefore assumed that caffeine causes the body to produce more urine than it would do normally, thus dehydrating you. It sounds sensible, doesn't it? It's not true, though. 

What's the truth?

The idea that drinks such as tea or coffee have a particularly strong diuretic effect appears to come from a study done in the 1920s.

Conducted on just three patients, it seemed to suggest that caffeinated drinks acted as a diuretic. But since then further studies have failed to show a significant effect on fluid balance within the body.

In fact, studies have shown that people who get the majority of their liquid from tea or coffee are no more dehydrated in blood tests than others.

So, there's nothing to worry about?

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