— Sam Kass, White House Nutritional Policy Adviser in 2013
Water — what’s not to love? The oceans are full of it (300 million cubic miles in all!), it falls unbidden from the sky, we slosh around with 12 gallons of water inside us (if we weigh 150 lb.), we spend the first nine months of our lives immersed in it, we drink eight glasses a day of the stuff…
Oh wait, no we don’t. We’re practically dehydrated, according to many websites, which insist we’re not drinking enough of it, telling us that unless we down eight ounces of water — not coffee, tea, beer, juice, sodas, but water-water — we’re doomed to a life short and brutish. (Actually, since many of these “drink more water” sites and studies are financed by such unbiased paragons of nutrition as Coca Cola and Nestlé, maybe I should have left “sodas” off the list.)
So what’s with the so-called “8 x 8” rule: 8 glasses x 8 ounces = 64 oz.—half a gallon, about two liters—a day? Turns out, it was a mistake. Back in 1945, the Food and Nutrition Board (since subsumed into the National Academy of Science’s Institute of Medicine) suggested we should be drinking one milliliter of water for each calorie of food. For an intake of 2000 daily calories, that works out to two liters a day, hence the eight 8-ounce glasses legend. What got lost in translation from the 70-year old guideline was the comment, “Most of this quantity is contained in prepared foods.”
Which makes all the difference. When you allow for all the non-water water we consume—not just coffee and tea and all the other liquids, but fruits, vegetables, soups, meats, frozen foods, etc.—it turns out, on average, a typical woman in the US consumes about 91 ounces; a man, 125 ounces. That is, we’re doing fine. Most of us, most of the time.
(Unless you’re plagued by kidney stones, urinary tract infections, or clinical dehydration. But you already know that.)
But c’mon, what’s the harm in drinking too much water — no one ever died of it! Sadly, water intoxication does occur. It happened to a 28-year old woman who took part in radio station KDND’s “Hold Your Wee for a Wii” competition in 2007. And to a 21-year old CSU Chico male student who was forced to over-drink during a hazing in 2005.
Then there are marathon runners. Here’s the problem: we pee less when we’re exerting ourselves because our bodies produce extra vasopression, an anti-diuretic hormone. Doesn’t matter how much water we drink, we don’t excrete it—the vasopression tells our kidneys to conserve water, not pee it out. Makes all sorts of sense if you’re chasing gazelles across the savannah. Not so much if you’re running 26-odd miles, ingesting water every few miles at the marathon aid stations.
According to a 2005 study in the New England Journal of Medicine, about one in six marathon runners suffer from hyponatremia—their bloodstreams are essentially waterlogged—during and immediately after the race. Normally, each of our kidneys has the capacity to process and excrete about a quart of water every hour, but when the vasopression kicks in during heavy exercise, that volume is cut by up to 90%.
So what happens to all that extra water? Under extreme conditions, some of it ends up in our skulls. Trouble is, our skulls are full of brains, there’s no spare room up there, hence brain swelling, or edema (remind me to tell you about the time I thought I was going to die at 15,000 feet in Nepal) which can lead to seizure, coma, brain stem herniation, respiratory arrest, death even.
Which is not going to happen to most of us, or even most long-distance runners; that’s the extreme version of overindulging in water. But let’s use some commonsense here. Our bodies know what they’re doing! They’re finely tuned—you’ll feel thirsty long before you’re dehydrated. So let your thirst rule. And if you don’t feel thirsty, it’s probably fine to ignore that latest ad for Super-Energized Really Pure Mountain Spring Water. Not to mention a certain ill-informed White House Adviser.