Science Myth #1 : Do we need to drink 8 glasses of water a day?

My research involves reading a lot of bad science misinformation. People sometimes ask me for examples of this misinformation so I compiled a list of the most annoying ones I come across. They're annoying because people repeat the myth with confidence but without providing much evidence.

Here is the first myth I would like to tackle:

Health magazines often mention this myth, but it is also mentioned in news too and even on Mayo Clinic's website. Often these sources are explaining the importance of staying hydrated and they are correct that in some special cases people need to monitor their water intake. However, the recommendation of drinking 8 glasses of water is not a great guideline. Sources that advise it rarely provide scientific sources for why the average healthy adult needs to drink eight glasses. For example, a couple of weeks ago an article was titled "Here's why you need eight glasses of water every day". No where in the article does it mention why we need eight. Even if it's suggested use as a "rule of thumb," why is it anywhere near eight? Which scientific evidence suggests eight is a meaningful guideline?

The odd advice does not rest with just journalists. Mayo Clinic's staff write: "You've probably heard the advice, 'Drink eight 8-ounce glasses of water a day.' That's easy to remember, and it's a reasonable goal." Mayo Clinic is basing its advice on a finding by The National Academies of Sciences, Engineering, and Medicine, but they misunderstood the report. Here is what the report's own panel chair actually says: " We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water. " As the chair explains, the report even "concluded that on a daily basis, people get adequate amounts of water from normal drinking behavior -- consumption of beverages at meals and in other social situations -- and by letting their thirst guide them."

It is important to note that the report is referring to total water and that includes "water from juice, milk, coffee, tea, soda, fruits, vegetables, and other foods and beverages as well.". In other words, it doesn't have to be just water. The National Academies of Sciences, Engineering, and Medicine report is consistent with other research publications that explicitly deny requirements to drink eight glasses. Again, the studies do acknowledge that there are situations where large intakes of water "are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates." The requirement to drink more water in these scenarios seems to be common sense. Sure, maybe people need to be adived to drink more water if they have a disease that dehydrates them, or they’ve been exercising heavily, or the day is especially hot — however, that advice doesn’t translate to advising the general population to drink eight glasses every day regardless of circumstance.

In conclusion, the advice for drinking eight glasses of water does not pertain to general situations for the average healthy adult. Many sources continue to make this claim and there is even a patent on a jug that helps people make sure they drink eight glasses. However, the average person should just drink when they’re thirsty. There’s a huge complex network inside the body of the average healthy adult that is designed specifically to inform them when they need water, when they’re dehydrated, and when they need to release water. Personally, I’d trust that inner-advice over a health magazine.