The Myth of “8 glasses of water per day”

I have a friend who’s a bit passionate about nutrition, what with her father being a dietitian and all. When we talk about food, weird tidbits of trivia tend to enter our conversation, and I almost always learn something new from her. One recent conversation went like this:

Me: I need a glass of water.

Her: Why?

Me: I don’t know. Because doctors say so?

Her: Do they?

Me: Isn’t it eight glasses per day? Don’t they recommend 8 glasses per day?

Her: Really? And how many ounces are in that glass?

Me: I don’t know. Eight?

Her: Maybe. You had coffee this morning. There’s water in that. Should that count?

Me: No. The caffeine in it sort of, I don’t know, makes it less effective.

Her: Are you sure?

Me: No, not really.

Her: Don’t worry, the doctors aren’t sure either.

The eight glasses a day sentiment is prevalent because of two things.

1) Water is, at eight glasses, essentially harmless to us. It adds no calories, no protein, no carbohydrates. At most, it provides us with some minerals (which gives it its taste, by the way), without overdoing it. So adding it to our diet has no adverse effect upon us, aside from having to use the rest room more often.

2) It just sounds right, doesn’t it? We’re comprised of mostly water, so we need it for our bodies to work efficiently, right?

So when it comes to recommending nutritional advice, doctors will often respond to the “8 glasses of water” suggestion with some variation of “Yeah, sure, why not?” without fully understanding where the science had come from.

Jamie Hale, in his latest book Should I Eat the Yolk, points us to one Dr. Heinz Valtin, who wrote a review published by the American Journal of Physiology that stated:

Despite the seemingly ubiquitous admonition to “drink at least eight 8-oz glasses of water a day” (with an accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking.

So where and when did this advice start? Dr. Valtin continues:

According to J. Papai (65), P. Thomas has suggested a different origin for 8 × 8. Thomas reminds us that in 1945 the Food and Nutrition Board of the National Research Council wrote (31):

A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.

Thomas suggests that the last sentence was not heeded, and the recommendation was therefore erroneously interpreted as eight glasses of water to be drunk each day. The Food and Nutrition Board is currently reevaluating its recommendation.

In fact, Valtin concludes, not only are we getting our 8 glass of water per day through regular activities, there is evidence to support that some of us are overhydrating ourselves.