The origin of the statement is unknown.
Claim: You must drink eight glasses of water (about two liters) a day to maintain a healthy balance. (Other versions of the claim say six, or seven.)
Rating: PARTLY FALSE/NEEDS CONTEXT
Explanation:
This claim is very common—here's a reference on xkcd, and searching for it on DuckDuckGo yields plenty of articles on the subject.
An article by Heinz Valtin found "no evidence" for the claim, despite searching studies covering "thousands" of healthy adults.
Megan Schimpf, M.D. states here, "There is no medical evidence that drinking that much water is beneficial to your health. Yet I hear from many patients who believe it is necessary for good health. In fact, that tip was popularized by a widely known weight loss program, but there is even no medical evidence that it helps with weight loss. Drink only if you’re thirsty, and never feel as if you have to force yourself to drink more."
Tamara Hew-Butler, associate professor of exercise and sports science, says that "Not to burst anyone’s water bottle, but healthy people can actually die from drinking too much water. I am an exercise physiologist, and my research focuses on overhydration and how drinking too much water affects the body. Since water – and sodium – balance is essential to life, it is extremely rare for people to die from drinking too much – or too little – fluid. In most cases, your body’s finely tuned molecular processes are unconsciously taking care of you."
Arend-Jan Meinders/Arend E Meinders, in a paper found here, says: "Under normal circumstances of diet, exercise and climate the minimal urine output for healthy subjects is about 500 ml/day. Intake of more than 500 ml of fluids per day will result in the excretion of solute-free water. The recommended total daily fluid intake of 3,000 ml for men and of 2,200 ml for women is more than adequate. Higher fluid intake does not have any convincing health benefits, except perhaps in preventing (recurrent) kidney stones."
Instead, researchers recommend drinking water when you're thirsty. The amount of water each person need varies a lot, and so thirst is the best indicator (who knew?).
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