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updated 3/4/2004 7:40:54 PM ET 2004-03-05T00:40:54

How much water does a person need? The surprising answer: Nobody knows.

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A new study makes an important first step in finding the answer. For the first time, a multicenter research team has found a way to measure water intake and urine output in healthy people going about their normal lives.

The findings offer several surprises, report University of Wisconsin-Madison researcher Aarthi Raman and colleagues in the February issue of the American Journal of Physiology-Renal. Perhaps the biggest surprise is that one piece of often-repeated advice -- elderly people need eight 8-ounce glasses of water a day -- may be wrong and even harmful.

"We did not find any evidence of dehydration in the 70- to 79-year-old group, despite the majority of individuals having intakes of less than the commonly used suggestion of eight 8-ounce. glasses of water each day," Raman and colleagues conclude. "Furthermore, [this recommendation] ... may not be prudent because the elderly have an elevated risk of overhydration."

Elderly people, they suggest, should increase their water intake only when they are feeling hot or feverish.

How much is enough?
There is no more important nutrient than water. Surprisingly, nobody knows exactly how much a person needs. That's because it's devilishly hard to measure how much water a person consumes in the form of beverages and food, how much stored water they use, and how much water they excrete in urine and other body fluids.

Raman's team used a clever approach. They gave 458 40- to 79-year-old men and women a special form of hydrogen-labeled water. The labeled water spreads evenly through the body, allowing researchers to calculate how much is retained and how much is lost through urine.

The findings:
Individuals vary widely in how much water they need. Without getting dehydrated, the people in the study took in as little as 1.2 liters of water a day and as much as 7.7 liters a day. On average older people took in less water than younger adults yet they did not have evidence of dehydration. The differences in water intake between age groups were small. Thirty eight percent of the people in the study took in less water than usually is recommended, yet they did not become dehydrated. Urine accounts for 66 percent of total body water output, not the 50 percent of water output previously assumed.

"It is likely that individual behavior and not the physiological differences we investigated account for most of these inter-individual variations," Raman and colleagues conclude.

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