Because you don't store water, you need to take in a new supply every day, enough to replace what you lose when you breathe, perspire, urinate, and defecate. On average, this needed amount adds up to 1,500 to 3,000 milliliters (50 to 100 ounces; 6 to 12.5 cups) a day. Here's where the water goes:
1 850 to 1,200 milliliters (28 to 40 ounces) is lost in breath and perspiration.
1 600 to 1,600 milliliters (20 to 53 ounces) is lost in urine.
1 50 to 200 milliliters (1.6 to 6.6 ounces) is lost in feces.
Toss in some extra ounces for a safe margin, and you get the current recommendations that women age 19 and up consume about 11 cups of water a day and men age 19 and up, about 15. But not all that water must come in a cup from the tap. About 15 percent of the water that you need is created when you digest and metabolize food. The end products of digestion and metabolism are carbon dioxide (a waste product that you breathe out of your body) and water composed of hydrogen from food and oxygen from the air that you breathe. The rest of your daily water comes directly from what you eat and drink. You can get water from, well, plain water. Eight 10-ounce glasses give you 2,400 milliliters, approximately enough to replace what your body loses every day, so everyone from athletes to couch potatoes knows that a healthy body needs eight full glasses of water a day. Or at least they thought they knew, but then Dartmouth Medical School kidney specialist Heinz Valtin turned off the tap.
Yes, the National Research Council's Food and Nutrition Board says each of us needs about 1 milliliter (ml) of water for each calorie of food we consume. On a 2,000-calorie-a-day diet, that's about 74 fluid ounces, or slightly more than nine 8-ounce glasses a day. Fair enough, Valtin said, but who says that it all has to come from, well, water? His report in the American Journal of Physiology (2003) points out that some of the water you require is right there in your food. Fruits and vegetables are full of water. Lettuce, for example, is 90 percent water. Furthermore, you get water from foods that you'd never think of as water sources: hamburger (more than 50 percent), cheese (the softer the cheese, the higher the water content — Swiss cheese is 38 percent water; skim milk ricotta, 74 percent), a plain, hard bagel (29 percent water), milk powder (2 percent), and even butter and margarine (10 percent). Only oils have no water.
How does water know where to go?
Osmosis is the principle that governs how water flows through a semipermeable membrane (one that lets only certain substances pass through) such as the one surrounding a body cell.
Here's the principle: Water flows through a semipermeable membrane from the side where the liquid solution is least dense to the side where it's denser. In other words, the water, acting as if it has a mind of its own, tries to equalize the densities of the liquids on both sides of the membrane.
How does the water know which side is more dense? Now that one's easy: Wherever the sodium content is higher. When more sodium is inside the cell, more water flows in to dilute it. When more sodium is in the fluid outside the cell, water flows out of the cell to dilute the liquid on the outside.
Osmosis explains why drinking seawater doesn't hydrate your body. When you drink seawater, liquid flows out of your cells to dilute the salty solution in your intestinal tract. The more you drink, the more water you lose. When you drink seawater, you're literally drinking yourself into dehydration.
Of course, the same thing happens — though certainly to a lesser degree — when you eat salted pretzels or nuts. The salt in your mouth makes your saliva saltier. This draws liquid out of the cells in your cheeks and tongue, which feel uncomfortably dry. You need . . . a drink of water!
In other words (actually in Valtin's words), a healthy adult in a temperate climate who isn't perspiring heavily can get enough water simply by drinking only when he or she is thirsty. Gulp. Or by drinking water when he or she is also drinking lots of coffee, tea, soft drinks, or alcohol.
Not all liquids are equally liquefying. The caffeine in coffee and tea and the alcohol in beer, wine, and spirits are diuretics, chemicals that make you urinate more copiously. Although caffeinated and alcohol beverages provide water, they also increase its elimination from your body — which is why you feel thirsty the morning after you've had a glass or two of wine. And when you feel thirsty, what do you do? Drink some water.
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