The Hydration Myth

Q: People say you need eight glasses of water per day, but I’m having trouble keeping up. Do I really need to drink that much water?

A: Water is important—it makes up 50 to 70 percent of your body. Without it, the blood vessels that carry oxygen and nutrients from place to place would shrivel up like dry riverbeds. Your kidneys could not eliminate wastes, and you couldn’t regulate your body temperature. If you were to add up all the water your body loses each day— as perspiration, urine, exhaled moisture, etc.—, it would be somewhere between 1 and 3 liters— that’s the equivalent of losing between four and 13 8-ounce glasses of water every day.

To stay well hydrated, the U.S. government estimates that women need 2.7 liters (11 glasses) of water and men need 3.7 liters (16 glasses) each day. Sound like a lot? Here’s why: The water your body needs does not have to come in glass after glass of drinking water.

In fact, if you’re trying to put away eight glasses of water a day, you’re probably getting more than necessary. This is because you get much of the water you need through the foods you eat. A morning bowl of oatmeal brings you the roughly 8 ounces of water used to cook it, even before you splash anything on top. A cup of soup at lunch delivers another 8 ounces of water. A serving of broccoli, spinach, or similar vegetable contains about 5 more ounces, as does a cup of rice.

Other foods are not much help. An egg brings you only about 1 ounce of water, and a slice of pizza delivers less than 2 ounces. If you snack on an apple or pear, you gain about 5 ounces of water, but snacking on a bag of potato chips gets you virtually none. Some health writers have encouraged us to drink eight glasses of water daily over and above the water in our foods.

In 2002, Dartmouth College researcher Heinz Valtin, MD, tried to track down a source for this eight-glasses-a-day recommendation. Not only was he unable to find its source, he could not find any evidence supporting it. University of Pennsylvania researchers Dan Negoianu, MD, and Stanley Goldfarb, MD, went on a similar quest in 2008. They concluded that drinking extra water does not help the body eliminate toxins, lose weight, improve the skin, or relieve constipation, and that the eight-glasses-a-day notion is a well-intentioned myth.

Q: How can I tell if I’m dehydrated?

A: Thirst is a good indicator, except during exercise. Some people have suggested that thirst is not a good guide, and that by the time you feel thirsty, you’re already dehydrated. Not true. Thirst sets in when your hydration status is still within normal boundaries, well before you reach dehydration. So if you are thirsty, drink. Other indicators are darkly colored urine, dry mouth, and morning headaches. Now, having extra water can help prevent kidney stones and urinary tract infections. Even so, don’t overdo it. An extra glass or two a day beyond your normal intake is more than enough to keep you well hydrated. If you were to massively overdo it with water, you could develop hyponatremia—a very low level of sodium in the bloodstream. Because sodium is needed for maintaining brain and heart function, people who drink far too much water can run into serious problems.

Q: What about caffeine and alcohol in beverages? Do they contribute to dehydration?

A: Very little. Both have a slight, temporary diuretic effect, but overall, the water in these beverages compensates for any increased urine output they cause. What can cause dehydration is intestinal illness. It’s no surprise that diarrhea leads to significant water loss. An even more common—and rapid—cause is exercise. When you are active, you need more water to make up for losses in perspiration.

The American College of Sports Medicine recommends being sure you’re adequately hydrated at least four hours before you start exercising. Then, have about 1/2 to 1 cup of water every 20 minutes during exercise. If you’ve lost enough water to show up as weight loss during your exercise, drink 2 to 3 cups of fluid for every pound lost.