The Medical Minute: Water, water everywhere -- how much should you drink?

May 19, 2004

By John Messmer
Penn State Milton S. Hershey Medical Center

You've heard it before -- drink eight glasses of water (8 ounces each) per day. If you've tried to do that, you know it takes some effort. Perhaps you've wondered: does that include the coffee I have each morning? Is it OK to have a diet cola or must it be water? Is bottled water better than tap water? What about sports drinks?

There has been much speculation in medical circles as to the origin of the "eight-a-day" recommendation. It probably comes from formulae used to determine how much fluid a hospitalized patient needs. One estimate is 1 milliliter of water per 1.2 calories of food consumed (assuming intake equals amount of calories burned). On this basis, the average lean adult who consumes about 2,500 to 3,000 calories per day has a water requirement of two to two-and-a-half liters per day. Two liters is 68 ounces or eight and one-half 8 ounce glasses. Another way of estimating fluid is more cumbersome but results in the same volume. But doctors use this to determine intravenous replacement -- there has never been an official medical recommendation as to a daily water requirement!

The Institute of Medicine recently reviewed the recommendations for water intake. While there is scientific give and take on the precision of the recommendations, the report of the Food and Nutrition Board of the Institute of Medicine gives a wide range of usual water consumption. Its recommendations depend on an individual's circumstances. In a cool climate, a sedentary person needs less than an athlete exercising on a hot, humid day. A soldier on patrol in Iraq needs more than someone sitting in an air conditioned office. On any given day, you may need only a liter of water while on another, you may need 4 liters.

You can get that water by drinking it from the tap or from spring water, but other beverages will work just fine. Conventional wisdom says caffeine, because it is a diuretic, makes coffee, tea and cola inadequate for fluid needs. However, it has been shown that the diuretic effect is mild and short lived so all liquids count about the same, even alcoholic beverages within reason. You get water from food also. Meat and fish are about half water, bread is more than one-third water and broccoli is 90 percent water. Sports drinks include a little salt and a little sugar. In intense exercise, sports drinks are absorbed slightly faster than water, but for the average American, they are unnecessary.

Water makes up about 60 percent of our body's weight -- about 70 percent of muscle and 10-40 percent of our fat is water. It helps maintain body temperature, carries nutrients and salts used for function of muscles and nerves, and is an integral part of our body's structure. Still, how can you know you are getting enough water? The simple answer is if you are not thirsty, you are doing fine -- as long as you are at room temperature and not exercising or doing heavy physical labor. As the air temperature or your exertion increases, increase your intake before feeling thirsty because you might get lightheaded or develop heat exhaustion before thirst develops.

Some situations are special. People who have had kidney stones should drink to keep their urine very dilute to reduce the risk of stones forming. Singers and public speakers have better vocal production when they are very well hydrated. Women who have had frequent bladder infections may reduce the chance of recurrence by increasing fluid intake although that has not been proven.

Some people advocate keeping our urine pale in color as a measure of adequate hydration. That may not be completely reliable since many foods and vitamins affect the color of urine, but if you are making enough urine to fill your bladder every three to four hours or so during daytime, you are getting enough to drink.

So, if you like chilled bottled water, go for it. It's OK to drink milk, tea, coffee, soda or sports drinks if you like -- liquid is pretty much all the same when it comes to hydration. Drink enough to prevent being thirsty, drink more if it's hot or you are exercising or working hard. It's not necessary to count glasses anymore.

(Media Contacts)

Last Updated March 19, 2009