Do You Need to Drink Eight 8-Ounce Glasses of Water a Day?

You’ve doubtless encountered the advice to drink eight 8-ounce glasses of water a day. I’ve heard it many times, on one occasion from a doctor. Feelers will likely act on this advice. It is, after all, what “they say.” Thinkers, however, will likely think twice before acting on it. For one thing, they will be reluctant to outsource their thinking to “them.”

Thinkers might not deem this claim worthy of their attention—they have more important things to think about—but if they do investigate it, their first step will be to figure out what, exactly, is being claimed. To do this, they will ask a number of questions:

• Do you in fact need to drink 64 (= 8 x 8) ounces of water a day to stay healthy?

• If this is the case, do you need to drink that water in eight separate acts, or can you instead drink smaller amounts of water more often or greater amounts less often?

• Do you need to drink the water from glasses, or would cups or thermoses be just as effective?

• Does the water you drink have to be “straight,” or can it be mixed with other things? Can it, for example, come in the form of Kool-Aid, lemonade, coffee, beer, wine, or milk (which is 87% water)?

• Can you eat rather than drink the water, say by eating watermelon (which is 92% water) or cucumber (which is 96% water)?

Having gotten clear on what, exactly, is being claimed, Thinkers might track down the source of the claim. Turn to Google Books Ngram Viewer, and they will find that the phrase “eight glasses of water a day” was first used in 1907, that its use peaked in 1998, and that it has subsequently declined. This sounds plausible. At the turn of the millennium, it wasn’t unusual for people to carry around a bottle of water.

Continue their research, and Thinkers might encounter the water consumption recommendation that the U.S. National Research Council’s Food and Nutrition Board released in 1945. It recommended daily consumption of 64 ounces of water. It didn’t, however, speak in terms of drinking eight 8-ounce glasses of water; indeed, the 64 ounces included the water obtained by eating foods. Thinkers will also discover that the current scientific consensus is that how much water you should consume in a day depends on a number of factors. In particular, you need more water if

• You are sweating, as the result of exercising, having a fever, or being out in the heat.

• You are in a dry environment. This will be the case if you are in a desert or at altitude. (Jet airplanes, which routinely fly at more than 30,000 feet, typically pressurize their cabins to 8,000 feet, meaning that it makes sense to drink additional water on a plane flight, even though you aren’t hot or physically active.)

• You are a child or an older adult.

More recent research indicates that adult men and women typically require a total daily water intake from all foods and liquids of 125 and 91 ounces, respectively. It also indicates that adult men and women typically have a total daily water intake of 117 and 93 ounces, respectively. Consequently, despite what “they say,” you probably don’t need to supplement your diet with additional water—and certainly not with 64 ounces of additional water.

Nevertheless, there are circumstances under which you should make a point of drinking more water, such as if you are exercising in the heat. You should also monitor your body’s hydration signals. If you are thirsty, drink water; likewise if your urine is amber or brownish.

So much for water. What about apples? Consider the claim that “an apple a day keeps the doctor away.” This advice has been circulating since at least 1913, and it seems to be a repackaging of a less memorable rhyming adage dating back to 1866: “Eat an apple on going to bed, and you’ll keep the doctor from earning his bread.”

Is this sound medical advice? Despite being 86% water, apples have nutritional value. They don’t contain as many minerals and vitamins as many other fruits, but they can be a significant source of fiber, especially if you eat the peel. In 2015, however, scientists found that consumption of apples was not statistically associated with a reduction in the number of visits to a doctor.

Like the advice to “drink eight 8-ounce glasses of water a day,” the advice to “eat an apple a day” is both memorable and repeatable. A medically sounder bit of advice, however, would be to “eat lots of fruit.” Although it is admirable for its brevity, it doesn’t rhyme, and as a result it has as little chance of going viral as, say, the exhortation to “think more and better.” Sigh!

Need more food for thought? Click here for my past essays, listed by title. Want to listen to rather than read my essays? Click here for narrations of my essays on becoming a Thinker, and click here for narrations of my essays on “applied thinking.” Narrations of my essays on becoming a Thinker are also available on your favorite podcast app. To play them, open that app and search for “Thinking More and Better,” and add episodes to your playlist.