This myth is even propagated by doctors and other health professionals, who really should know better. But, and here's another indicator that this is a myth, when you ask the people "Why?" no one seems to have an answer. Oh, a doctor who believes this will give you some mumbo-jumbo about how the body requires that much water per day for its various functions, but if you persist in asking them for specifics, they will change the subject, or suddenly need to be somewhere else. Try it sometime.
It's the old one about drinking eight eight-ounce glasses a day. According to the myth, everyone should do so, apparently without regard to age, size, level of activity, and so forth. This alone makes it seem somewhat suspect to anyone who actually uses their brain. "What, everyone? So my toddler needs the same amount of water as Joe Sports Guy?"
As it turns out, though, this myth run rampant is as a result of some pretty typical pseudoscience. It goes like this: Around .03 ounces of water are required to digest one calorie. The "average" caloric intake is about 2000 calories. Using the formula .03 x 2000 =60, and then rounding it off for convenience, we arrive at the conclusion that the average person with the average caloric intake needs about sixty-four ounces of water to digest those calories. Which is true, as far as it goes.
However, "average" only accounts for people who truly eat the average amount of calories. In order to have an average, you must also have people whose caloric intake is more, and people whose caloric intake is less. Often, in the case of calories, this can be much more or much less. So, just going on the information we have so far, no, not everyone needs to drink 64 ounces of water a day.
But it gets worse: The real problem with this whole myth is that the formula completely fails to account for the fact that all foods contain water, and many contain enough water per calorie for us to digest them without any additional water intake. And if we eat foods that don't have enough water in them, or for some reason we don't drink with meals, (another silly thing some people believe) and we need more water to digest something, guess what happens? We get thirsty.
As I write this, there are millions of people out there blissfully over-hydrating themselves, toting around their little plastic bottles of water everywhere they go. Aside from the impact on the environment, with literally millions of these things going in landfills every day, there is also an impact on the people themselves, and it isn't good.
Yes, drinking too much water is harmful to your body. Not "might be," not "can be," but is. There are even medical terms for it, like "Water Intoxication." (I think they should have a fancier name, like "hydrotoxia," but apparently they don't) In any event, the short story is that too much water flushes things like water-soluble nutrients, vitamins and minerals from your body, along with electrolytes. Electrolytes simply put, allow your muscles and nerves to communicate and function properly. Sodium and potassium, among others, are the primary ions of electrolytes. Water soluble vitamins like B-complex and C get flushed out with water as well.
So, if you subscribe to the common "wisdom" that you should drink at least 64 ounces of water a day, that your urine should be clear, that you should eat a diet low in sodium, and take vitamin supplements, not only are you probably literally pissing away the money you spend on vitamins, but you are risking problems in the way the electric tissues of the body communicate with each other, and the way they behave.
Humans got along for millennia by simply drinking when we were thirsty, and now "science" has come along to tell us that our bodies can't actually tell when we need water. What is more, apparently Homo sapiens is the only species on Earth who must drink even when we are not thirsty. What complete rubbish.
I, for one, think this whole thing was started as a way to get people to buy something they would normally get for "free" out of the tap. Sure worked, didn't it?