This is particular important advice for older people, even though Dr. Schoeller and his colleagues did not find that the older people in the study were dehydrated. In fact, the researchers were surprised to find that there was little difference in water intake or water output between the ages of 40 and 70, though water intake decreased after age 70.
"Dehydration in the elderly is still a concern," Dr. Schoeller cautions. "But it's not because they're bumping along with barely enough water intake. It's that when they're in a situation where the water need increases dramatically—like out in the sun at a picnic—they don't get as thirsty as young people."
Other people who may be at risk for dehydration include children, people with diabetes and people taking certain medications such as diuretics, which lower blood pressure, laxatives and some anti-psychotic and anti-epileptic drugs.
Still, a recent Institute of Medicine report also concluded that that most people "adequately meet their daily hydration needs by letting thirst be their guide." The report did offer general guidelines, recommending that women drink approximately 2.7 liters (91 oz) of total water a day and that men consume about 3.7 liters (125 oz). This water can be obtained from food or from fluids, including caffeinated and alcoholic beverages, according to the report's authors.
So while it's important to stay hydrated, unless you're exercising or out in hot or arid weather, the idea that healthy people have to drink water all day can probably be hung out to dry.
