How Much Water is Enough? How Much is Too Much?

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The health tip about drinking eight 8-ounce glasses of water a day has been roundly dismissed by scientists, doctors, and upstanding bloggers for years, but this baseless myth refuses to die. Meanwhile, exercise-associated hyponatremia, a consequence of overhydration, is being reported across a broader range of athletes.

Aaron E. Carroll, professor of pediatrics at Indiana University School of Medicine, has confronted the 8-glasses-a-day myth more than most, having dismantled it in a 2007 paper published in the British Medical Journal and later in a book about medical misinformation. Now, a study recently published in the American Journal of Public Health has concluded that more than half of American children between 6 and 19 are “inadequately hydrated,” a finding Carroll says is total bunk. In light of this, he’s taken another stab at the whole 8 glasses/day myth.


The study in question used a measurement known as mean urine osmolality to gauge the urine concentration of more than 4000 children ages 6 to 19. A little more than half of the children surveyed had a urine osmolality of at least 800 mOsm/kg—a urine concentration above which the authors characterize as indicative of “inadequate hydration.”

As Carroll explains, “if you define ‘dehydration’ as a urine osmolality of 800 mOsm/kg or higher, the findings of this study are really concerning... The problem is that most clinicians don’t”:

...there’s very little reason to believe that children who have a spot urine measurement of 800 mOsm/kg should be worried. In fact, back in 2002, a study was published in the Journal of Pediatrics, one that was more exploratory in nature than a look for dehydration, and it found that boys in Germany had an average urine osmolality of 844 mOsm/kg. The third-to-last paragraph in the paper recounted a huge number of studies from all over the world finding average urine mOsm/kg in children ranging from 392 mOsm/kg in Kenya to 964 in Sweden.

That hasn’t stopped more recent studies from continuing to use the 800 mOsm/kg standard to declare huge numbers of children to be dehydrated. A 2012 study in the Annals of Nutrition and Metabolism used it to declare that almost two-thirds of French children weren’t getting enough water. Another in the journal Public Health Nutrition used it to declare that almost two-thirds of children in Los Angeles and New York City weren’t getting enough water. The first study was funded by Nestlé Waters; the second by Nestec, a Nestlé subsidiary.

It’s possible that there are children who need to be better hydrated. But at some point, we are at risk of calling an ordinary healthy condition a disease. When two-thirds of healthy children, year after year, are found to have a laboratory value that you are labeling “abnormal,” it may be the definition, and not their health, that is off.


Too Much Water

If you’re an athlete, overhydration may be a more pressing concern than underhydration. Hyponatremia, aka “water intoxication,” is the extreme result of a bodily imbalance between electrolytes (the minerals in your blood and body fluid that carry an electric charge) and water. The condition is more prevalent than people realize. From an article I wrote on the subject back in February:

Results from a study published in 2005 in the New England Journal of Medicine suggest nearly one in six participants in 2002’s Boston Marathon experienced some degree of hyponatremia. A 2006 study published in the British Journal of Sports Medicine states unequivocally that “exercise associated hyponatremia is due to overdrinking.” In the article, sports physiologists Timothy David Noakes and Benjamin Speedy lament the biomedical community’s slowness to acknowledge the risks over over-hydration in endurance athletes.


Last month, the British Journal of Sports Medicine published a new report that corroborates these observations. Gretchen Reynolds summarizes the report’s findings in a post published Wednesday at the NYT:

...according to the latest science, dehydration during sports is rarely if ever dangerous, but overhydration undeniably is.


Until recently, hyponatremia had been associated almost exclusively with marathon races and other prolonged endurance events, especially among slow racers, who tended to sweat little but drink copiously, often for hours on end. But as the new report, which presents updated hydration guidelines developed by a consortium of scientific experts, points out, exercise-associated hyponatremia “is now being reported in a more diverse set of sporting activities,” including half-marathons, sprint triathlons, Grand Canyon hikes, Bikram yoga classes, and, of course, team sport practices and games, especially football, at the professional, collegiate, and now high school level.


So how do you know if you’re getting enough water? The authors of the hyponatremia report conclude you should rely on your body’s “innate thirst mechanism.” In a nutshell: Listen to your body’s cues and exercise common sense. If you’re thirsty, drink. If you’re not thirsty, and especially if you’re an athlete, don’t feel compelled to drink.

[AJPH and BJSM via NYT]

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