Modern Mythology: 8 to 10 Glasses of Water a Day
Health and diet websites and books, not to mention endless email forwards, tout many unproved (and some downright erroneous) health benefits of water. Some of these are easy to be skeptical of, such as the claim that drinking two glasses of water in the morning “activates” the internal organs. As though our organs are inactive when we sleep. Another myth that seems particularly persistent, however, is that we all need to drink 8 to 10 glasses of water a day.
Where did this idea come from? About ten years ago, Dartmouth kidney specialist Heinz Valtin reviewed the medical literature and came up empty-handed. The only potential source he could find was a 1945 recommendation from the National Research Council’s Food and Nutrition Board. If this is indeed where the myth started, an important sentence was missed: “most of this quantity can be found in prepared foods.”
Just the idea of taking health advice from the 1940s conjures up images of this guy:
But even if the recommendation is unsupported, many people still think that you basically can’t drink too much water. When someone dies from water intoxication
Drinking more water than your body needs, even if you don’t take it to extreme levels, means more fluid for your kidneys to filter, and the filtering mechanisms are these itty bitty balls of capillaries called glomeruli. So when your kidneys have to work harder to eliminate excess water, it’s these delicate capillaries that are doing all the work.
Too much water also throws off the balance of sodium and water in your body, so your kidneys work to regain that balance. But if they can’t eliminate water fast enough, you risk a condition called hyponatremia. The excess water is drawn to the higher sodium concentrations in the blood and cells, causing swelling. In extreme cases (water intoxication), this swelling in brain cells is particularly dangerous, and potentially fatal, because the skull doesn’t exactly have a lot of give the way our other tissues do.
Athletes have to be particularly careful. Physical stress causes our bodies to produce more of a hormone called vasopressin, which signals the kidneys to conserve water. So drinking excess fluids while the kidneys are in conservation mode increases the danger of developing hyponatremia.
So how much water should we drink? It depends. A one-size-fits-all health recommendation is usually worthy of your skepticism. Individuals vary in diet, body size, medical conditions, how much they sweat and how much they exercise, not to mention the effects of weather, and so forth. There really isn’t a magic number.
But the good news is, you don’t need one. The human body has evolved a rather amazing mechanism for ensuring we get enough water: thirst. And don’t worry: you’re not already dehydrated by the time you’re thirsty. That, too, is just more modern mythology.
Image credits: Davide Guglielmo, Kidney Dialysis Information Centre, Mitarart