Sunday, July 6, 2008

The 8 glasses of water myth

You've all heard and read - over and over again - that we need to drink 8 glasses of water every day to keep ourselves healthy. Does anyone remember seeing any references for this assertion? Any scientific evidence whatsoever?

No, you don't. Because there aren't any. It's a load of crap.

There was a report from the National Research Council in 1945 that stated that we need about 1 millilitre of water per calorie of food intake, which runs to about 2 - 2.5 litres of water (8-10 8 oz. glasses per day). However, the next sentence states that this amount is already found in prepared food. Humans aren't the only organisms on the planet that are around 95% water.

For the data: here's a short excerpt from the British Medical Journal examining this (along with 9 other science myths, a great read - here's a truncated version from the Globe and Mail if you don't have BMJ access). Here's an article on Dr. Heinz Valtin M.D., kidney specialist, Professor Emeritus, and author of a (fairly) recent invited review on the subject. Finally, there is his exhaustive review of the literature, for which you can see the abstract here on PubMed, and the actual review is to be found here at the American Journal of Physiology.

Among the things he finds in the literature over the course of this review: 1) Caffeinated beverages are not dehydrating to those who take them regularly. Maybe a little if you haven't taken any caffeine in weeks. 2) Feeling thirsty doesn't mean you're dehydrated. It means you're on the road there - thirst occurs when the concentration of dissolved substances of blood has risen 2% due to water loss. Dehydration starts at 5%. 3) Dark urine doesn't mean dehydration. It means orange pee.

Among the things he doesn't find: ANY scientific evidence that you need 8 glasses of water (and only water, not coffee, juice, beer, whatever).

So rejoice, coffee, beer, and cola drinkers. Stop feeling guilty about whether you get enough water. If you're thirsty, drink something. If you're not, don't worry about it. Evolution is smart enough to program organisms to know when they need water.


jordin said...

This is only kind of related... but drinking too much water has a terrible effect of intoxication, that can lead to death. This "addiction" (i have this in quotation more because i dont think you can be addicted to water, but more the intoxication) is treated by living in a controlled environment and being weighted every few hours to ensure one isnt over drinking.

Ian Vitro said...

This is totally true, the psychiatric condition is known as psychogenic polydipsia. Water intoxication is REALLY bad though, and seems to occur as a result of cell death - whereas in good ol' beer intoxication, cell death just accompanies the pleasant feelings.

jordin said...

I'm so glad that you found the name for it. I am so out of touch of anything that involves something past grade 2 right now its sad.

Sam said...

Aren't we more like 73% water, rather than the oft-quoted 95%? I certainly don't FEEL like I'm 95% water, except perhaps just after taking my morning coffee(s).