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Is 8 Glasses of Water Per Day Overkill?
An anonymous reader writes: "David Harris reports in his science news that the American Journal of Physiology today published a study dispelling the 8x8 myth. That is, the recommendation to drink 8 eight-ounce glasses of water per day has no scientific evidence behind it. The paper also mentions the risks of drinking too much water and explodes some other urban myths of water drinking such as the 'thirsty means dehydrated' and 'dark urine means dehydrated' myths."
The part about water in caffeine drinks counting towards you're recommended intake struck me as most contrary to my own anecdotal expereience--it seems like whenever I drink anything caffeinated, I'm thirsty for water shortly afterwards. I suppose this discrepancy is either in my head, a bizarre artifact of my own physiology, or a misinterpretation of the abstract linked to here.
1) It is true that 8 glasses of water need not be drunk each day. It *is* true that the equivalent, two liters of fluid, must be consumed -- but food contains 30-50% of the water you need each day.
2) It is true that "being thirsty means you're already dehydrated" is an exaggeration, but it is an intentional one. It means "you're well on the way to being dehyrated". The exaggeration is, I suppose, intended to catch ID10Ts who ignore thirst even during 10-km runs in 110 degree weather. Thirst does kick in well after you have lost plenty of fluid, and once ADH (anti-diuretic hormone, the hormone that controls urine production and volume status) has already risen. These facts, I suppose, are the origin of the common anecdote.
3) "Dark urine doesn't mean you are dehydrated" -- not true. It *is* true that it is difficult to judge from color alone. Despite our 16 million color resolution, it is hard for people to subjectively grade color without experience (i.e. working in a urinalysis lab); thus, judging hydration by urine color alone will have a high error rate and often be inconclusive. Other factors, such as urine contaminants, might make the test less accurate. Along with other clues, however (i.e., being sweaty, thirsty, and tired), it may be useful.
4) "Water intoxication", a peculiar form of hyponatremia (low blood sodium concentration), is *not* mythical. However, it *is* much rarer than dehydration. It takes a special kind of person (read: mentally ill patient) to drink the 25+ glasses of water per day that it requires to achieve this.
5) Studies have shown (sorry, I don't have links, its 0130 around here) that caffinated beverages such as Coke hydrate at about 2/3 the rate of pure water. That is, at Coke concentrations, the caffeine dose is sufficient to cause you to lose 1/3 of the water you just drank as part of the Coke.
6) These indicators -- thirst and dark urine -- are not "myths" and not useless (although they are far from perfect). I happen to know first-hand that some of the best doctors in the United States use these observations in their daily assessments of patients.
7) "Absence of evidence is not equal to evidence of absence." --paraphrased from Carl Sagan
This fellow has done very little experimentation on his own, but simply collected the lack of research combined with evidence from his own student experiments over the years. While this is a provocative article, it is intended to stimulate research, and NOT to be the definitive work on the subject. What's needed (as he states at the end, in A CALL FOR DIALOGUE) is more and better (and better funded, pretty please)nutritional research!
Disclaimer: IANAD (but I am in medical school and work with 'em in hospitals every day; see address.) And yes, I am a geek. Been programmin' since age four (BASIC on a c64), researchin' at MIT. Good enough for youz?
Your friendly neighborhood nitpicker