Forget about your 8 glasses a day and take a pass on the salt, please

Conventional wisdom holds so strongly that drinking eight glasses of water a day is essential to good health that bottles of water have become as ubiquitous as cell phones in meetings, on commuter trains and on sidewalks. The nation is, by all appearances, parched, just a sip ahead of dehydration.

But last week, a panel of some of the most esteemed nutrition and health experts in the United States and Canada said there's no need to try to get eight glasses a day: People should just drink when they're thirsty.

American women already average 9 cups of water a day from all beverages combined, and American men, 13 cups, said the panel, which was convened by the Institute of Medicine, a nonprofit group that provides health policy advice to the National Academy of Sciences.

That's enough, the panel concluded, when you throw in the 2 to 3 extra cups of fluid contributed by the moisture content of the foods we eat. Consider that a turkey sandwich made with Swiss cheese, lettuce, and tomato on whole-wheat bread contains almost a half-cup of water; a tossed salad with vinaigrette dressing, close to a full cup.

The panel, charged with setting official guidelines for consumption of water as well as the minerals potassium and sodium, also found that, on average, Americans eat much more salt than they should and only half the potassium, which helps keep down blood pressure, staves off bone loss and reduces the risk of kidney stones. The Food and Drug Administration typically uses the institute's guidelines to calculate the recommended daily values it puts on food labels, though any change would likely take years to incorporate.

The International Bottled Water Association, an industry trade group, continues to support the idea that everyone needs to drink at least eight glasses of water per day, but the chairman of the Institute of Medicine's panel, Dr. Lawrence Appel, in a press release, stated: "We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water."

The panel also said it is a myth that coffee and alcoholic beverages are dehydrating (although the bottled water association, on its website, continues to argue they are). "Caffeinated beverages contribute to daily total water intake, similarly to noncaffeinated beverages," said Michael N. Sawka, PhD, a panel member who conducts fluid balance studies as part of his research on how humans adapt to environmental extremes at the US Army Research Institute of Environmental Medicine in Natick.

"You don't have to make up for coffee with water," he said, particularly if you're a regular coffee drinker. The body has an adaptation mechanism so that the more coffee you habitually consume, "the less increase there is in urine output."

Similarly, Sawka said, moderate alcohol consumption -- up to two drinks a day for men, one for women -- "is not robbing your body of water. It's fair to say that it can contribute to hydration."

There are people who may sometimes need to make a special effort to get enough liquids -- athletes and people who work in boiler rooms, for instance, Sawka said. "But even they will rehydrate" without much conscious effort before long, he said. And although the thirst mechanism is blunted in older people, "on a day-to-day basis, they're well hydrated."

"You don't have to count cups of fluid," Sawka said. "You can let your normal consumption behaviors be your guide."

So how did the eight-glasses-of-water-a-day myth get started in the first place? "That's a good question," Sawka said. It certainly "evolved based on something not having anything to do with scientific evidence."

On sodium, a key component of salt, the Institute of Medicine panel concluded that the daily adequate intake should be 1,500 milligrams daily for young and middle-aged adults; 1,300 milligrams for those over 50; and 1,200 milligrams for people older than 70. The official upper level for all adults, which shouldn't be exceeded, is 2,300 milligrams to reduce the risk for high blood pressure.

Virtually all men and nearly all women of every age consume far too much sodium, said Appel, a member of the departments of Medicine, Epidemiology, and International Health at Johns Hopkins University in Baltimore. Average sodium consumption figures for men range from 3,100 to 4,700 milligrams a day and for women from 2,300 to 3,100 milligrams, according to the report.

"My basic message," Appel said, "is the lower the better when it comes to sodium intake." He said 1,500 milligrams "would be a wonderful goal if we could all reach it, but the food supply is getting in our way." For instance, a half-can of Campbell's chicken noodle soup contains 1,145 milligrams; a half-cup of Prego spaghetti sauce, 610 milligrams. Even breakfast cereals may have 200 to 300 milligrams of sodium per serving.

The Grocery Manufacturers Association, a trade group of food, beverage and consumer product companies, called the sodium recommendations "unrealistic for ordinary Americans." The trade group said Appel's panel should have emphasized a diet rich in fruits and vegetables, rather than one low in sodium, but said group members are doing what they can to help improve the health of Americans. "We are . . . committed to finding new flavorings that are suitable substitutes for the critical flavor and texture provided by salt," Alison Kretser, the group's director of nutrition and scientific policy, said in a prepared statement.

For potassium, which can help reduce blood pressure, the panel set the adequate intake at 4,600 milligrams -- about twice as much as Americans currently average. "Dairy products are a pretty decent source" of potassium, Appel said, "but the mainstays are fruits, vegetables, and juices." The only people who need to confer with their doctors before substantially increasing their potassium intake are those with type 1 diabetes and those taking ACE inhibitors drugs, certain diuretics, or nonsteroidal anti-inflammatory drugs, or NSAIDS, including COX2-inhibitors, he said.

Of course, fresh foods from the produce aisle are also quite low in sodium, so incorporating more of them into the diet every day provides a double benefit. The more processed a food, the more sodium it tends to contain. For instance, a half-cup of homemade chocolate pudding contains 137 milligrams, while a half-cup made from an instant mix has 403 milligrams. That's one of the reasons fresh fruits and vegetables, along with dairy foods, make such good dietary mainstays. "The more we learn about nutrition," Appel said, "the more we see that a plant-based diet is pretty good for you."