"Should I drink this much of the river?" Sorry kid, science has no idea. (Cristina Mittermeier / iLCP)

A new campaign by Michelle Obama encourages Americans to drink more water. But how much more water you need to be drinking, or whether your water drinking is already up to snuff right now, turns out to be a nearly impossible question to answer.

The Centers for Disease Control does not issue guidelines for how much water Americans should aim to consume as it does with the food pyramid, which tells us how much to eat of different foods, and exercise guidelines, which advise us on how much physical activity we should work into our days.

You've likely heard at some point that its prudent to drink eight cups of water a day. But that's not part of any federal guideline right now, and tracking down where the number came from turns out to be surprisingly tricky.

Some think the recommendation grew out of a footnote to the 1945 U.S. Dietary Guidelines. The footnote "recommended that, as the average male diet would consume 2,500 kilocalories (10,467 kilojoules), this diet would require 1 mL of water for each kilocalorie; consequently 2,500 mL of fluid should be ingested on a daily basis," according to Spero Tsindos, a Canadian scientist who has published research on the history of water consumption guidelines.

"This recommendation was repeated in the 1948 revision," Tsindos said, "with no reference or authority cited in the calculation."

But there is a a bit of disagreement among those who have studied the history of water consumption recommendations -- many people, it turns out -- about whether that footnote is actually the source of the eight-cups-a-day mantra. Another researcher cites a 1974 book, Nutrition for Good Health, with popularizing this idea.

In neither case was the researcher able to find any academic backing for health benefits accrued from consuming eight glasses of water each day.

But that advice on how much to guzzle is still popular, and some of that appears to have to do with the water industry, which, of course, encourages people to drink as much water as possible. Margaret McCartney looked into the origins of a British campaign called Hydration for Health for the British Medical Journal.

The campaign recommended that "1.5 to 2 litres [about eight cups] of water daily is the simplest and healthiest hydration advice you can give.”

"Hydration for Health has a vested interest: it is sponsored and was created by French food giant Danone," McCartney writes. "This company produces Volvic, Evian, and Badoit bottled waters."

Recommended levels of water consumption have, meanwhile, disappeared completely from our national guidelines. That might have something to do with the sheer complexity of measuring water consumption. As one CDC Web site points out, you can get a significant amount of hydration from eating foods such as fruits or vegetables or soup.

This makes it hard for any of us to estimate the amount of water that we're chugging each day and to sort out whether we need to heed the first lady's call to "Drink up," or keep on sipping (and snacking) as usual.