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Ask Tufts Experts September 2012 Issue

Q: Can coffee or tea help take the place of the recommended eight daily glasses of water?

Answer :  Yes, says Health & Nutrition Letter editor Irwin R. Rosenberg, MD, coffee and tea “count” toward meeting your body’s fluid requirements. The notion that the diuretic effects of caffeinated beverages such as coffee and tea cancel out their water content was disproven as far back as 1928, according to the Institute of Medicine (IOM). More recently, in 2000, researchers at the University of Nebraska’s Center for Human Nutrition compared the hydration effects of various beverages on 18 healthy adult males and found no significant differences whether the drinks were carbonated, diet or contained caffeine. They concluded: “Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated by the results of this study.”

Dr. Rosenberg adds, “There is no daily requirement for eight glasses of water. That, too, has been pretty much debunked.”

That widely held belief about water consumption probably originated with a 1945 finding that people need 64 ounces of fluids (eight eight-ounce glasses) daily. But an important part of that recommendation has gotten lost: Those 64 ounces include the fluids in food as well as coffee, tea and soda.

Actual fluid needs vary widely among individuals, Dr. Rosenberg explains, and depend upon calorie intake and energy expenditure through exercise, among other factors. For most people, according to the IOM, “fluid intake, driven by thirst... allows maintenance of hydration status and total body water at normal levels.”

Older people, however, often have a reduced sensation of thirst, so it’s easier to miss the warning signs that you’re dehydrating. Older persons also tend to have lower reserves of fluid in the body and to drink insufficient water following fluid deprivation to replenish their body water deficit. Because of their low water reserves, Dr. Rosenberg says, it may be prudent for the elderly to learn to drink regularly when not thirsty and to moderately increase their salt intake and eat foods high in potassium when they sweat. And yes, coffee and tea can help make sure you don’t get dehydrated.

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