You’re supposed to drink 8 cups of water a day, right? Wrong. This idea, generally accepted and promoted throughout the country, has actually never held water.
The myth hits all the common notes of the spread of misinformation.
Pre-scientific or nonscientific origins? Check. Just prior to the establishment of modern medicine, in the mid- and late-1800s, many doctors and would-be doctors were advocating for “hydrotherapy”. Industrialization brought noise and dirt and pollution to everyday life, and, for many people, the allure of the natural — cold and hot springs, ocean water, rock minerals, etc. — was high. Members of the sanitation movement attributed many diseases (like cholera) to unclean living conditions. And a small subset of doctors believed that ingesting water in large quantities resolved all manner of ailments. There wasn’t much research to support these ideas. But, to be fair, at that time there wasn’t much research to support any ideas. Medicine based on sound science was just taking off.
The barest thread of scientific grounding? Check. In 1945, in a footnote in a report by the U.S.-sponsored Food and Nutrition Board’s Recommended Dietary Allowances, read, “a suitable allowance of water for adults is 2.5 liters daily in most instances.” This seems to imply that people should be drinking a lot of water. The footnote added, however, that, “Most of this quantity is contained in prepared foods.” In other words, you don’t have to drink 2.5 liters of water, y’all. It’s unclear, however, whether this footnote played any significant role in the popularization of the idea, as the idea of "8 glasses a day" was already circulating among followers of Charles Atlas and other health gurus in the early 1900s.
A vast oversimplification of a complicated process? Check. The amount of water you need to consume depends upon a lot of things: your level of physical activity, your overall caloric intake, the amount of water you consume from food (which, in many cases, is a lot), the environment you live in, and how sick you are (and in what ways). A single recommendation for the population at large is probably pointless. Even if 8 glasses a day did quantify the average person’s need for water intake, there’s so much variation to that need that the average is just not a very helpful number.
A central myth bundled with corresponding myths? Check. The advice to drink 8 glasses of water a day is usually linked to other wrong-headed ideas about hydration: for instance, that those glasses have to be water not tea or coffee (or soda). I can remember insisting to a friend that coffee and tea dehydrate you, counter-intuitively. But actually, counter-counter-intuitively, tea and coffee do, in fact, hydrate you.
A helpful push by big business? Of course. Companies that sell water like the idea that consumers should be drinking more water. Since most people report drinking far less than 8 cups of water a day, water-selling businesses (which are really soda-juice-and-all-drink-selling businesses) could latch onto the myth in order to convince more people to buy water.
A time-lag between scientific conclusion and action? Check. The earliest scientific article that I’m aware of batting this myth away was published in 2002. As far as I can tell, this created little to no media splash. Then, six years later, an opinion article in a scientific journal repeated the basic arguments from the 2002 article and this created at least some media response (a Slate explainer article from this time period is one of the best resources I found on the 8-glasses-a day myth).
But one of the most interesting aspects of the 8-cups-a-day myth is that it focuses attention on what is a non-problem for most of the population. If you just drink when you’re thirsty, you’ll do alright. Make sure to drink more when you sweat a lot. And… that’s it. Think of all of the time people have spent writing articles about drinking 8 cups of water a day; the anxiety many people have had over not drinking the right amount; the effort spent on public education campaigns. What was the point of it all?
This is probably the most succinct summary of the research I found: Rastogi, N. S. (2008, April 4). Who says you need to drink eight glasses of water a day? Slate Magazine. https://slate.com/news-and-politics/2008/04/who-says-you-need-to-drink-eight-glasses-of-water-a-day.html
For a more scholarly take, see: Valtin, H. (2002). “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8× 8”?. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology.
This is the opinion piece from 2008: Negoianu, D., & Goldfarb, S. (2008). Just add water. Journal of the American Society of Nephrology, 19(6), 1041-1043.
Here's a more recent news article on the same topic: O’Connor, A. (2012, April 16). Really? The Claim: Drink Eight Glasses of Water a Day to Protect the Kidneys. Well. https://archive.nytimes.com/well.blogs.nytimes.com/2011/11/07/really-the-claim-drink-eight-glasses-of-water-a-day-to-protect-the-kidneys/