WASHINGTON\ -- ''Drink at least eight glasses of water a day'' is an adage some obsessively follow, judging by the people sucking on water bottles at every street corner -- but the need for so much water may be a myth.
Fear that once you're thirsty you're already dehydrated? For many of us, another myth. Caffeinated drinks don't count because they dehydrate? Probably wrong, too.
So says a scientist who undertook an exhaustive hunt for evidence backing all this water advice and came up mostly, well, dry. Now the group that sets the nation's nutrition standards is studying the issue, too, to see if it's time to declare a daily fluid level needed for good health -- and how much leaves you waterlogged.
Until then, ''obey your thirst'' is good advice, says Dr. Heinz Valtin, professor emeritus at Dartmouth Medical School, whose review of the eight-glass theory appears in this month's American Journal of Physiology.
It's about time for all the attention, says Pennsylvania State University nutritionist Barbara Rolls, a well-known expert on thirst. ''There's so much confusion out there.''
Much of it centers on where you should get your daily water.
''There's this conception it can only come out of a bottle,'' and that's wrong, notes Paula Trumbo of the Institute of Medicine's Food and Nutrition Board, which hopes to decide by March whether to issue the first official water-intake recommendation.
In fact, people absorb much water from the food they eat. Fruits and vegetables are 80 to 95 percent water; meats contain a fair amount; even dry bread and cheese are about 35 percent water, says Rolls. That's in addition to juices, milk and other beverages.
And many of us drink when we don't really need to, spurred by marketing, salty foods and dry environments, Rolls says.
''For most of us, that's not going to matter -- you're just going to need to go to the bathroom more,'' she says.
But for people with certain medical conditions, chugging too much can be harmful, sometimes fatal, Valtin warns. Even healthy people -- such as teenagers taking the party drug Ecstasy, which induces abnormal thirst -- can occasionally drink too much. So-called water intoxication dilutes sodium in the blood until the body can't function properly.
Conversely, no one disputes that getting enough water is crucial. Indeed, the elderly often have a diminished sensation of thirst and can become dangerously dehydrated without realizing it. People with kidney stones, for example, require lots of water, as does anyone doing strenuous exercise.
But the question remains: How much water does the typical, mostly sedentary American truly need? And what's the origin of the theory, heavily promoted by water sellers and various nutrition groups, that the magic number is at least 64 ounces?
Valtin, who has spent 40 years researching how the body maintains a healthy fluid balance, determined the advice probably stems from muddled interpretation of a 1945 Food and Nutrition Board report.
That report said the body needs about 1 milliliter of water for each calorie consumed -- almost 8 cups for a typical 2,000-calorie diet -- but that ''most of this quantity is contained in prepared foods.''
That language somehow has morphed into ''at least'' 64 ounces daily, Valtin says. (One Web site's ''hydration calculator'' even recommends a startling 125 ounces for a 250-pound couch potato.) And aside from the American Dietetic Association's advice, few of the ''drink more water'' campaigns targeted to consumers mention how much comes from food.
Valtin couldn't find any research proving the average person needs to drink a full 64 ounces of water daily.
Also, contrary to popular opinion, he cites a University of Nebraska study that found coffee, tea and sodas are hydrating for people used to caffeine and thus should count toward their daily fluid total.
--That thirst means you're already dehydrated. That can be true of the elderly, and studies of marathon runners and military recruits in training have found that some focus so intently on strenuous exercise that they block thirst sensations until they're in trouble. But Rolls did hourly hydration tests to prove that drinking when thirsty is good advice for the rest of us.
--That water blocks dieters' hunger. Studies show water with food can help you feel full faster, but that just drinking water between meals has little effect, Rolls says.
So how much do we need? Until the Institute of Medicine sets a level, ''if people obey their thirst and they are producing urine of a normal yellow color, that's a safe sign,'' Valtin concludes.
EDITOR'S NOTE -- Lauran Neergaard covers health and medical issues for The Associated Press in Washington.
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