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Curly haired woman drinking glass of water

Good old-fashioned H20 is one of your body’s most basic and vital needs. “It’s in our cells and in our blood and in all of the tissues in-between,” says Barbara Rolls, Ph.D., professor of Nutritional Sciences at Pennsylvania State University and a thirst expert.

Proper hydration helps our digestive system run smoothly, keeps our cells young and helps our immune system fight infection. So it would make sense that the more you guzzle, the healthier you’ll be, inside and out, right? Not exactly, says Rolls. “For most of us, fluid balance is not a problem." In other words, you’re getting enough water already, sans that fancy water bottle glued to your hand. It’s time to get real about water—read on to get the facts, not fiction.

Define…Water

Experts stress that we need to broaden what we think of as water.  “Many people think that the only way to hydrate is with plain drinking water, but that’s just not true,” says Keith Ayoob, R.D., associate clinical professor of pediatrics at Albert Einstein College of Medicine in New York.

Currently, the National Institute of Medicine recommends that women get 2.7 liters (about nine cups) of water per day. But the suggested sources go beyond glasses of clear water. All beverages, including water, as well as moisture in foods like fruit, vegetables, lean meat and soups count.

“When you think about it, both coffee and soup—even soda—are mostly water,” says Ayoob. “Any type of fluid goes toward your daily quota.”

8x8 Reality

Wavy haired woman drinking a bottle of water

But what about the 8 x 8 rule (drink eight glasses of eight ounces of plain water per day, on top of everything else)? There’s no scientific evidence backing that recommendation.

A Dartmouth physiologist named Heinz Valtin conducted a comprehensive search for the origin of this widely repeated statement and published his findings in the American Journal of Physiology in 2002.

Not only did he conclude that there was no evidence to suggest that healthy adults needed to drink large amounts of water, but he also found a possible reason for the long purported myth: a misinterpreted National Resource Council suggestion from 1945.

The original text stated: “A suitable allowance of water for adults is 2.5 liters daily in most circumstances. An ordinary standard for diverse persons is one milliliter for each calorie of food. Most of this quantity is contained in prepared foods.”

He suggested that the last sentence was possibly ignored and the statement as a whole interpreted as you must drink eight glasses (2.5 liters) of water each day.

Thirst ≠ Dehydration

Contrary to the popular mythjust because you’re thirsty doesn’t mean you’re already dehydrated. Dehydration occurs when the body loses more fluid than it takes in, but many people feel thirsty before they reach this deficit, says Rolls.

Two possible reasons for this disconnect: dry mouth and salty foods (more water is needed to dilute salt in the blood, so your thirst center sends an alert possibly causing you to feel thirsty). One small study followed young men around for a day, recording hydration markers in the blood and when they felt thirsty.  Researchers found that the men felt thirsty before any actual fluid deficits occurred in their bodies.

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