One of the contributors to the Medical Informatics Student Blog has laid to rest one of the most frequent, stubborn medical claims I've come across:
The 8 glasses of water rule began when the Food and Nutrition Board recommened approximately "1 mm of water for each calorie of food" which would average about 64 to 80 ounces a day. However, the next part of the recommendation was "most of this quantity is conatined in prepared food." So, it seems that America only listened to the first half of this recommendation.
According to Family Practice News, 8 Glasses of Water a Day is no longer the Mantra. Most healthy individuals remain adequately hydrated by allowing their thirst to dictate their intake.
Additionally, I found that caffeinated drinks (coffee, tea, and soda) DO count towards your daily total.
The student provides the links are there to back it up, like this IOM report. But it can't be said any more emphatically than this pubmed abstract I'm excerpting:
The search included not only electronic modes but also a cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids. No scientific studies were found in support of 8 x 8. Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance.
This "8 glasses of 8 ounces" fallacy has undoubtedly contributed to the runaway bottled water industry, which is fundamentally ineffecient and surprisingly unsafe.