People tend to follow what they hear.
For instance: The rule of thumb is that everybody needs to drink at least 8 glasses of water per day. It makes sense — water is one of the healthiest things you can put into your body, roughly 60% of which consists of H2O, already. And we know water can do everything from helping you maintain a normal body temperature to lubricating joints to ridding your body of waste. It can always be a great way to curb your appetite, making you feel full before bolting for the fridge or pantry to sneak an extra snack between meals. And of course, it keeps you hydrated.
But do you really need to drink 8 glasses of the stuff every day?
The short answer is "no." The more complicated answer, according to Registered Dietitian Caroline Fox, is that the actual recommended amount differs for everyone.
"More recent guidelines on fluid intake can be found from the Institute of Medicine and Dietary Reference Intakes.," says Fox, who works at the Tufts Medical Center Weight and Wellness Center. "They recommend 2.7 liters (11 cups) a day for women; 3.7 liters for men."
The trick is that water is not the only external source which can hydrate us, so the exact amount of water necessary for hydration status is more "fluid" than it seems!
"Hydration doesn't come from drinking water alone," says Fox. "It can come from different types of fluids, such as soups, tea, coffee or even gelatin. And perhaps even more importantly, a majority of your water intake should come from food products. Especially high water content foods like vegetables and fruits." Fox does agree that drinking adequate water, calorie-free beverages, or nutrient dense beverages such as reduced-fat milk and soymilk is a good way to support healthy weight as opposed to drinking beverages with excess & added sugars and non-nutritive calories.
So where did the myth of the 8 glasses come from? Fox says it likely stems from a decree from the U.S. Food and Nutrition Board issued back in the 1940s. It suggested that everyone drink 2.5 liters (84.5 ounces) per day — so not far off from today's standard. But even back then, that same recommendation clarified that a majority of that water comes, not from a glass, but from "prepared foods."
Like we said: People tend to follow what they hear. And even back then, they tended to hear what they wanted to hear. While the recommendation of 6-8 glasses water per day has some merits as a standard guideline, it is more or less an arbitrary number.