From "Nutrition for Dummies" Third Edition, by Carol Ann Rinzler. p 177-184.
Investigating the Many Ways Your Body Uses Water
Water is a solvent. It dissolves other substances and carries nutrients and other material (such as blood cells) around the body, making it possible for every other organ to do its job. You need water to
-Digest food, dissolving nutrients so that they can pass through the intestinal cell walls into your bloodstream, and move food along through your intestinal tract
-Carry waste products out of your body
-Provide a medium in which biochemical reactions such as metabolism occur
-Send electrical messages between cells so that your muscles will move, your eyes can see, your brain can think, and so on
-Regulate body temperature – cooling your body with moisture (perspiration) that evaporates on your skin
-Lubricate your moving parts.
Maintaining the Right Amount of Water in Your Body
As much as three-quarters of the water in your body is intracellular fluid, the liquid inside body cells. The rest is extracellular fluid, which is all the other body liquids, such as
-Interstitial fluid (the fluid between cells)
-Blood plasma (the clear liquid in blood)
-Lymph (a clear, slightly yellow fluid collected from body tissues that flows through your lymph nodes and eventually into your blood vessels)
-Bodily secretions such as sweat, seminal fluid, and vaginal fluids
A healthy body has just the right amount of fluid inside and outside each cell, a situation medical folk call fluid balance. Maintaining your fluid balance is essential to life. If too little water is inside a cell, it will shrivel and die. If there is too much water, the cell will burst.
A balancing act: The role of electrolytes
Your body maintains its fluid balance through the action of substances called electrolytes, which are mineral compounds that dissolve into electrically charged particles called ions.
Many minerals, including calcium, phosphorus, and magnesium, form compounds that dissolve into charged particles, but nutritionists generally use the term electrolyte to describe sodium, potassium, and chlorine. The most familiar electrolyte is the one found on every dinner table: sodium chloride – plain old table salt. (In water, its molecules dissolve into two ions: one sodium ion and one chloride ion.)
Under normal circumstances, the fluid inside your cells has more potassium than sodium and chloride. The fluid outside is just the opposite: more sodium and chloride than potassium. The cell wall is a semipermeable membrane; some things pass through, but others do not. Water molecules and small mineral molecules flow through freely, but larger molecules such as proteins do not.
The process by which sodium flows out and potassium flows in to keep things on an even keep is called the sodium pump. If this process were to cease, sodium ions would build up inside your cells. Sodium attracts water; the more sodium there is inside the cell, the more water will flow in. Eventually, the cell would burst and die. The sodium pump, regular as a clock, prevents this imbalance from happening so you can move along, blissfully unaware of those efficient, electric ions.
Dehydrating without enough water and electrolytes
Drink more water than you need, and your healthy body simply shrugs its shoulders, so to speak, urinates more copiously, and readjusts the water level. It’s hard for a healthy person on a normal diet to drink himself or herself to death on water.
But if you do not get enough water, your body lets you know pretty quickly.
The first sign is thirst, that unpleasant dryness in your mouth caused by the loss of water from cells in your gums, tongue, and cheeks. The second sign is reduced urination.
Reduced urination is a protective mechanism triggered by ADH, a hormone secreted by the hypothalamus, a gland at the base of your brain. The initials are short for antidiuretic hormone. Remember, a diuretic is a substance such as caffeine, which increases urine production. ADH does just the opposite, helping your body conserve water rather than eliminate it.
If you do not pay heed to these signals, your tissues will begin to dry out. In other words, if you’re dehydrating, and if you don’t – or can’t – get water, you won’t survive.
What else do those electrolytes do?
In addition to keeping fluid levels balance, sodium, potassium, and chloride (the form of chlorine found in food) ions create electrical impulses that enable cells to send messages back and forth between themselves, so you can think, see, move, and perform all the bioelectrical functions you take for granted.
Sodium, potassium, and chloride are also major minerals and essential nutrients. Like other nutrients, they are useful in these bodily processes:
-Sodium helps digest proteins and carbohydrates and keeps your blood from becoming too acidic or too alkaline.
-Potassium is used in digestion to synthesize proteins and starch and as a major constituent of muscle tissue
-Chloride is a constituent of hydrochloric acid, which breaks down food in your stomach. It also is used by white blood cells to make hypochlorite, a natural antiseptic.
Getting the Water You Need
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:
-850 to 1,200 milliliters (28 to 40 ounces) is lost in breath and perspiration.
-600 to 1,600 milliliters (20 to 53 ounces) is lost in urine.
-50 to 200 milliliters (1.6 to 6.6 ounces) is lost in feces.
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 need eight full glasses of water a day. Or, at least they though 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 one milliliter 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 points out that some of the water you require is right there in your food. For example, fruits and vegetables are full of water. Lettuce, for example, is 90% water. Furthermore, you get water from foods that you’d never think of as water sources: hamburger (more than 50%), cheese (the softer the cheese, the higher the water content – Swiss cheese is 38% water; skim milk ricotta, 74%), a plain, hard bagel (29% water), milk powder (2%), and even butter and margarine (10%). Only oils have no 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.
You also get water from liquids, such as milk, coffee, tea, soft drinks, and fruit juices, but here’s an interesting face: Not all liquids are equally liquefying. The caffeine in coffee and tea and the alcohol in beer, wine, and spirits are diuretics. Although caffeinated and alcohol beverages provide water, they also increase its elimination from your body.
How does water know where to go?
Osmosis is the principle that governs how water flows through a semipermeable membrane 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 the least dense to the side where it is 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 from 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 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 moth 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!
Taking in Extra Water and Electrolytes as Needed
In the United States, most people regularly consume much more sodium than they need. In fact, some people who are sodium-sensitive may end up with high blood pressure that can be lowered if they reduce their sodium intake.
Potassium and chloride are found in so many foods that, here, too, a dietary deficiency is a rarity. In fact, the only recorded case of chloride deficiency was among infants given a formula liquid from which the chloride was inadvertently omitted.
The estimated minimum daily requirements for sodium, potassium, and chloride are one-size-fits-averages for a healthy adult weighing 70 kg (154 lbs), including:
-Sodium: 500 milligrams
-Potassium: 2,000 milligrams
-Chloride: 750 milligrams
Most Americans get much more as a matter of course.
When ginger ale won’t cut it
Serious dehydration calls for serious medicine such as the World Health Organization’s handy-dandy, two-tumbler electrolyte replacement formula.
Here’s what you need:
Glass No. 1
8 ounces orange juice
A pinch of salt
½ teaspoon sweetener (honey, corn syrup)
Glass No. 2
8 ounces boiled or bottled or distilled water
¼ teaspoon baking soda
Take a sip from one glass, then the other, and continue until finished.