I bet you've heard it said that you need to drink at least eight glasses of water each day in order to stay properly hydrated. Perhaps you've also read that by the time you feel thirsty you're already in an advanced state of dehydration, or that most of us are chronically dehydrated. Chances are also good that you've been told that drinking caffeinated beverages like tea and coffee cause you to lose more fluid than you take in.
The dehydration myth has become so firmly entrenched in our collective consciousness that it will probably come as a surprise to learn that there is very little scientific support for any of these notions.
Drinking Water Is a Good Habit
Look, as excesses go, drinking a lot of water isn't a bad one. It won't make you fat. It won't rot your teeth or give you a hangover. Drinking lots of water can temporarily assuage hunger pangs, which dieters find useful. It can also help prevent kidney stones in those susceptible to them. Drinking more water can ease constipation an-- ironically -- also alleviate water retention.
The body has a fairly efficient mechanism for getting rid of excess water, so under normal circumstances it's hard to get yourself into trouble by drinking water -- except the kind of trouble that happens when you find yourself in bumper-to-bumper traffic on the interstate and the next rest stop is 50 long miles away. Yikes!
Still, as a nutritionist, I feel compelled to point out that most people can stay perfectly hydrated on significantly less than eight glasses of water a day.
A Drop of Truth in a Sea of Misunderstanding
So what is the basis for the idea that nothing less than two liters of water a day will keep us from multiple-organ failure? Like most urban legends, this one does contain a drop of truth. The average person needs about two liters, or approximately eight glasses, of water a day to replace what is lost through normal biological functions like breathing, sweating and urinating.
But that doesn't mean that you need to drink two liters of water. In fact, hypothetically, you don't have to drink any water at all. For one thing, you can easily get a liter or liter and a half of water just from the food that you eat, especially if you eat lots of fruits and vegetables, which are up to 97 percent water.
Surprise! Drinking Caffeinated Beverages Won't Dehydrate You
Secondly, contrary to another widely held nutritional myth, coffee, tea, sodas and other caffeinated drinks are not dehydrating. Caffeine can act as a diuretic, increasing urine output, but you still end up taking in more fluids than you lose. If you don't drink caffeinated beverages regularly, drinking a cup of coffee ends up being the equivalent of drinking about 2/3 of a cup of water. In other words, drinking coffee will hydrate you -- just not quite as efficiently as water will.
If you regularly drink caffeinated beverages, however, the diuretic effects of the caffeine are almost negligible. In other words, if you drink coffee every day, your body retains the same amount of fluid from a cup of coffee as it does from a cup of water.
There are a few situations when dehydration is a legitimate concern. People involved in sustained, strenuous exercise or spending extended periods of time in very hot or dry conditions need a lot more fluids to stay adequately hydrated. (Although, when you're sweating a lot, it's really important to replace sodium as well as fluids to prevent a potentially serious condition called hyponatremia.) The thirst reflex also declines with age and the elderly are at elevated risk of dehydration.
But barring ill health, extreme conditions, or intense physical activity, most people will stay well hydrated by eating a reasonably healthy diet and drinking water or other non-alcoholic beverages when they are thirsty. As a rule of thumb, if you are peeing several times a day and your urine is pale in color, you are doing fine.