the 8-glasses-a-day myth

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!

6 thoughts on “the 8-glasses-a-day myth”

  • I always enjoy it when you rip these bogus claims up. I knew you didn’t need 8 glasses of water a day… because I’ve gotten on fine without it for all my life. But now I know exactly why you don’t. Thanks! 🙂

  • Jim Thomerson says:

    I’ve had some leg problems. I was doing something and my legs were shaking so bad, I wondered if I could finish the task. I managed to do so. I sat down and drank about a half liter of water. In about 15 minutes I got up and repeated the task and my legs were fine. I have read that people over 40 are prone to dehydration without realizing it.
    Back in the 80’s it was legal to drink and drive in Texas; not to drive drunk, you understand. I was in Ft. Worth during a heat wave. The public radio repeatedly made the announcement that for each can of beer, you should drink a glass of water.

  • Alison Campbell says:

    I have read that people over 40 are prone to dehydration without realizing it. Being of an advanced age myself, I’ll have to look into that one. Cartainly one of the science-based sites I visited in writing that post indicated that older folks needed a higher water intake, although part of that would have been related to body mass given that their table started off with toddlers 🙂 Mind you, The SO would say that with my (weak) tea-drinking habit, it shouldn’t be a problem!

  • Hooray, a link I can give to the lecturing folk who sit there virtuously chugging away at their gigantic bottle of water!
    Like Mark I had read that it was a myth (some nutritionalist pulled a figure out of the air when challenged some time or something similar) but didn’t know exactly why.
    Although, not being a smoker (i.e. not having an excuse to run outside every 10 minutes for a ciggy), maybe the result of being an excessive drinker of water is a good reason to leave ones desk numerous times during the day 🙂

  • herr doktor bimler says:

    Nice review article from Heinz Valtin (2002): “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”? Am. J. Physiol. Integr. Comp. Physiol. 283, R993-R1004.
    The author shreds the story, and carries out some detective work to trace how the figure was pulled out of the air. Available on the WWW.
    A couple of years ago, the 8-Glasses story was being heavily promoted around primary schools by the Kidney Foundation (who on close inquiry turned out to be relying on sports-drinks companies for much of their funding, though that may have changed). I had some arguments with my daughter’s school on their acceptance of junk science.
    The Foundation is still promoting National Drink Water Week through schools, though they seem to have dropped the recommended minimal intake.
    As with much of this kind of pseudoscience dietary advice, there is the meta-narrative that you cannot rely on the sensations of thirst or hunger from your own body to tell you what it needs, and you should instead follow the instructions of the Appropriate Authorities.

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