The myth that we need eight glasses of water a day

So we no longer need to add water. At least not to ourselves in the amount of eight lots of 8oz a day as advised by the website NHS Choices and numerous health writers. We should all raise our glasses to Dr Margaret McCartney, a GP from Glasgow, writing in the BMJ, who has revealed what most doctors already know: there is no scientific evidence that we need so much water. None. At all.

We humans are pretty good at regulating our own water intake. When we need to replace fluid there's this highly accurate mechanism called thirst that prompts us to have a drink. And the best drink to have is free, clean tap water. Ingenious, eh?

The thirst mechanism's so efficient there's no good evidence for how much we need to drink on average a day. It depends on how active we are, how much water is in the food we've eaten, how hot it is and if we have any medical conditions.

A review by Heinz Valtin, a physiologist from Dartmouth Medical School in the US, found no references for what's called the 8x8 advice but did find a recommendation from the Food and Nutrition Board of the National Research Council in 1945 saying a lot of fluid is already available in the food we eat. Valtin points out that this message is often forgotten. He measured how much he drank a day and it was about 1 to 1.5 litres.

Drinking extra water is said to reduce urinary tract infections, improve skin tone, help with weight loss (fill up with water first), reduce headaches and fatigue, eliminate constipation and improve concentration. There's no robust evidence for any of this. The kidneys are wonderful things (that don't need flushing with lots of water) and will make concentrated urine to save water.

Manufacturers of bottled water may push the message that more water is better. But there's no evidence that bottled water is better than tap water. You can even drink other fluids and they work too (but may have more sugar).

So will you all stop carrying round those little bottles of water and sipping from them during meetings? Because now we know it's all cobblers, it's going to be even more annoying.

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