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The Debunker: Eight Glasses of Water a Day? Seriously?

by Ken Jennings

If you’re in the Northern Hemisphere, August is, just like the human body, at least 60 percent made of water: vacations to the beach, weekend trips to the lake or pool, big pitchers full of refreshing ice water. August is also National Water Quality Month, as you probably know, and it’s when we all have our big get-togethers to celebrate World Water Week (August 26-31!) and Sea Serpent Day (August 7). It may be hot and dry where you are right now, but at least Ken Jennings can make it rain knowledge with his August Debunker column, deflating everything you think you know about H2O.

Water Myth #1: You Need Eight Glasses Full Every Day.

Of all the far-fetched nutritional information that parents and teachers feed to kids, this might be the most dubious. Eight glasses of water? That sounds like a vaudeville act or a Third World prison torture regimen, not a kindly bit of doctor’s advice. The home ec teacher who drummed this rule into me in the seventh grade called it the “8 x 8 rule”: eight 8-ounce glasses of water, every single day. That’s 64 ounces in total: a half-gallon a day. Who drinks that much water?

Not healthy people, says Heinz Valtin, a Dartmouth kidney specialist who studied the belief for a 2002 issue of the American Journal of Physiology. Valtin traces the myth back to a 1945 National Research Council pamphlet advising one milliliter of water for each calorie of food eaten, which would indeed be sixty to eighty ounces for most adults. But everyone seems to have missed the pamphlet’s next sentence: “Most of this quantity is contained in prepared foods” (my italics). In other words, your regular food intake is giving you most of the water you need without you ever taking a single sip of Dasani.

Valtin is quick to note that you might need more water if you’re in a hot climate, or very physically active, or prone to kidney stones. But everyone else can just take a drink if they’re thirsty, and not if they’re not. (“Thirst means you’re already dehydrated” is also a lie, says Valtin. So is “Urine should be clear!”) Drinking too much when you’re not thirsty can overtax your kidneys, leading to a possibly deadly state called “water intoxication.” So the next time your mother nags you to drink more water, tell Mom she’s all wet.

Quick Quiz: What 2002 sci-fi/horror film contains the line, “Daddy, there’s a monster outside my room. Can I have a glass of water?”

Ken Jennings is the author of Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.

Photo by Flickr member bfurnace. Used under a Creative Commons License.