Dr. Heinz Valtin of Dartmouth Medical School in New Hampshire answered that question in a review published online by the American Journal of Physiology on August 8, 2002. Valtin, a kidney specialist and author of two widely used textbooks on the kidney and water balance, could not find a single paper that supports this recommendation. Furthermore, there is little indication where it began.

Back in the 1940s, the Food and Nutrition Board of the National Research Council wrote that one should attempt to drink one milliliter of water per calorie consumed. So, based on a 2,000 calorie diet, that would be two liters, or about 8 8-oz glasses. But in the very next sentence they added “and much of this can be gained from the solid food we eat.” Nearly everything we eat contains water; even bread is 30% water. And since ALL beverages contribute to water intake (in spite of what you’ve heard), it is unlikely the average individual, under normal circumstances, needs to be concerned about a water shortage. It is likely though, that the bottled water industry has adopted this concept and enhanced sales by creating a culture around “drink 8 glasses of water a day” (or else?)

There are conditions, activity levels and lifestyles where plenty of water is called for, and conversely, drinking lots of water carries certain risks. Rather than take the space to spell these out, please read more about this at CBS News: http://www.cbsnews.com/stories/2002/08/10/health/main518276.shtml, and Dartmouth’s website: http://www.dartmouth.edu/~news/releases/2002/aug/080802.html.