The idea that the average person requires eight glasses of water each day to maintain good health has become widely accepted, but one Dartmouth Medical School physician says the advice seems to lack any grounding in science.
Heinz Valtin, the former Chair of Physiology at he Dartmouth Medical School, reported his findings on the popular recommendation -- often known as "8 x 8," for the suggested eight, eight-ounce glasses per day -- in an invited review recently published by the American Journal of Physiology.
"This rule is found everywhere all over the world," Valtin said, "but after 10 months of searching I have found no published evidence to support it."
While Dartmouth students can often be seen toting water bottles across campus, sipping from them at every available opportunity, Valtin said that much of the body's required daily intake of water is in fact supplied by food and that such attentiveness to hydration is usually unnecessary.
"You get lots of water from solid food, especially fruits and vegetables," he said. The advice to drink eight glasses per day may have originated in a report from a prestigious scientific organization, the Food and Nutrition Board, which in 1945 recommended "one milliliter of water for each calorie of food."
In the following sentence, the guidelines stated that much of this daily supply can be gained from prepared food, but Valtin said this crucial fact may have been missed and the report misconstrued to indicate the amount of water the average person should drink each day.
Valtin, who has authored three textbooks on the kidney and water balance in the body, said he found it "virtually impossible that evolution over hundreds of thousands of years has left us with a chronic water deficit."
He stressed, however, that his findings apply only to "healthy adults in temperate climates leading sedentary lives."
Some diseases such as kidney stones may require an increased water intake, Valtin said, while drinking large amounts of water is advisable in hot weather or when participating in vigorous athletic activity. Surprisingly, he said, studies have shown that even caffeinated drinks such as soft drinks and coffee can count towards the daily water total.
Another piece of popular wisdom that Valtin investigated is the often-stated belief that once thirst sets in, the body is already dehydrated. He explained that the body has a "very sensitive, very quick and very accurate" system for maintaining the body's water balance that will stimulate thirst well before a stage of "meaningful dehydration" is reached.
Peer-reviewed studies using randomized experiments have supported this view, Valtin explained, confirming that blood concentrations are in fact within the normal range when thirst initially occurs.
Additionally, Valtin said, drinking mass quantities of water is not as risk-free as most people assume. The condition of water intoxication -- which causes mental confusion, seizures and sometimes death -- can result when more water is consumed than the body can excrete. The drug Ecstasy, which can induce intense thirst, has the potential to cause such a condition, he said.
"There have been reports of 16-year-old girls who drank themselves to death after taking Ecstasy," Valtin said. "It's not a rare or unheard of occurrence."
While Valtin said he has yet to find any solid evidence to support the "8 x 8" mantra, he stated his hope that the report would help spur debate on the topic.
"The tone is conciliatory," Valtin said of his review. "It's an invitation for dialogue."
