Should you drink eight glasses of water a day?

We’ve all seen those haunted souls sipping water obsessively as though they may at any moment crumble to the floor in a pile of dust. You may be one of them. Even if you aren’t eternally clutching a water bottle there’s a good chance you try to guzzle eight glasses a day to ward off dehydration.

Yet when US physiologist Heinz Valtin set out in 2002 to find evidence to support the commonly recommended daily eight glasses — roughly two litres — he couldn’t find any.

There are a few studies hinting that high water intake may help prevent some kinds of cancer, heart disease and other disorders, but nothing yet compelling. On the other hand water can be harmful if you drink enough of it, though this is most serious when things are already out of kilter thanks to disease or drugs.

Though the rule probably sprang from genuine nutritional advice, it has drifted far from its scientific roots, gathering an entourage of highly dubious claims.

One such addition is the literal interpretation of water to mean pure water or nothing. Not only does this ignore food (which, like the human body, is largely water) but also coffee, tea, soft drinks and alcohol. Caffeine is particularly frowned upon since, as a diuretic, it is supposed to be like taking two steps back as you take one forward. Recent research suggests this isn’t true, at least for regular caffeine consumers. Even beer, in moderation, may be okay.

Nor is there anything scientific about the common belief that by the time you are thirsty you are already dehydrated. Far from justifying the eight glasses rule, the body’s dependence on water provides it with a strong evolutionary incentive to keep water under control on its own — without relying on the fickle brain and its woolly ideas. And thirst turns out to be an extremely reliable signal which kicks in well before dehydration occurs.

Yes, water is important, but most healthy people, most of the time, have no good reason to drink any more than what their bodies tell them to.

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