the 8-glasses-a-day myth

By Alison Campbell 07/02/2010

I was at the gym yesterday when I read something in a women’s mag that quite put me off my stride on the cross-trainer. (In my defence, I’d forgotten to take a book & the only other reading material on offer was car magazines.) The offending article contained the following factoids: you need to drink at least 8 glasses of water a day, if you don’t drink it your cells will be sloshing in dirty recycled water, tea & the like won’t do – but ‘lemon water’ will & is especially good because it will alkalise your blood & help you metabolise fats (which your cells store to protect them against acids…).

Hmmm. Let’s look at all this through the lens of science – or to put it another way

funny pictures of cats with captions

First up – the idea (very widely promoted) that you have to drink at least 8 glasses of water a day in order to stay healthy is actually a myth, not science. Yes, our bodies contain a considerable amount of water – around 60% of body weight, with a fair bit of variation between the individual organs (& between men & women), & we do need to replace that which is lost through sweating, urination, breathing, & in our faeces. These losses can amount to 2-3 litres per day. But that definitely doesn’t mean you have to drink at least 8 glasses of water a day on top of everything else – any fluids are OK and there’s a fair amount of water in food as well, particularly in fruit & vegetables. Not only do we get water directly from various foods, but there’s also a certain amount of water generated through the metabolism of the carbohydrates they contain. (It is possible to drink too much water. This can lead to ‘water intoxication’, when the body becomes hyponatraemic: cellular fluids become so dilute that normal biochemical processes don’t work properly, a state that can be fatal.) In normal circumstances, your body’s normal homeostatic mechanisms operate to maintain water balance: when you enter a slightly dehydrated state, not only are you stimulated to drink but your kidneys reduce the volume of urine they produce. Be slightly over-hydrated, & urine volume increases. (I can still remember getting a lecture about this in Christchurch airport, prior to flying down to Antarctica. The air is so dry down there that it’s easy to become a bit dehydrated, & we were told all about the warning signs in terms of urine volume & also colour – very pale straw, Good; brown, Bad…)

What about the concept that if you don’t drink those 8 glasses a day, your cells will be bathed in ‘dirty’ recycled water? Again, no. This one shows a basic misunderstanding of how kidneys do their job. And they’re certainly not flushing out yesterday’s ‘used’ water & replacing it with the nice fresh stuff you drink today, not in the way that article implied. Water is water, as far as your cells are concerned, & it doesn’t come stamped with a use-by date.

The lemon juice one I found rather puzzling. It’s hard to see how lemon juice (which is acidic) is going to ‘alkalise’ anything… In addition, your blood pH is already regulated in a rather tight range: between 7.35 & 7.45. Treatment for some illnesses (eg kidney stones) may involve the use of urinary alkalinisers – but these have minimal effects on blood pH. Urine pH is increased all right – as your body excretes excess bicarbonate ions in order to keep the blood pH at its normal closely regulated level. And a fat burner, lemon juice ain’t – my Significant Other will have to find another excuse for adding lemon to his G&Ts!

0 Responses to “the 8-glasses-a-day myth”

    • What galls me is that the people giving this advice genuinely seem to believe it. And because they’re called ‘nutritionist’ or some such, the readers probably think, oh that’s OK then, s/he must know what s/he’s talking about. Sigh.