Your body is mostly (50 to 70 percent) water. Exactly how much water depends on how old you are and how much muscle and fat you have. Muscle tissue has more water than fat tissue. Because the average male body has proportionately more muscle than the average female body, it also has more water.

For the same reason — more muscle — a young body has more water than an older one. You definitely won’t enjoy the experience, but if you have to, you can live without food for weeks at a time, getting subsistence levels of nutrients by digesting your own muscle and fat.

But water’s different. Without it, you’ll die in a matter of days — more quickly in a place warm enough to make you perspire and lose water more quickly. 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 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 (digesting food, producing energy, and building tissue) occur.

• Send electrical messages between cells so that your muscles can 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.

As much as three-quarters of the water in your body is in intracellular fluid, the liquid inside body cells. The rest is in 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.
• Urine.

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 shrivels and dies. If there’s too much water, the cell bursts.

Your body maintains its fluid balance through the action of substances called electrolytes, which are mineral compounds that, when dissolved in water, become 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 don’t.

Water molecules and small mineral molecules flow through freely, unlike larger molecules such as proteins. The process by which sodium flows out and potassium flows in to keep things on an even keel 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 flows 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.

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 don’t 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, that increases urine production. ADH does just the opposite, helping your body conserve water rather than eliminate it. If you don’t heed these signals, your tissues will begin to dry out. In other words, you’re dehydrating, and if you don’t — or can’t — get water, you won’t survive.

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.

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.

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. 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.

In 2004, the Adequate Intake (AI) for sodium, potassium, and chloride were set at one-size-fits-all averages for a healthy adult age 19–50 weighing 70 kilograms:

• Sodium: 1,500 milligrams.
• Potassium: 4,700 milligrams.
• Chloride: 2,300 milligrams.

Most Americans get much more as a matter of course, and sometimes you actually need extra water and electrolytes.

• You’re sick to your stomach - Repeated vomiting or diarrhea drains your body of water and electrolytes. Similarly, you also need extra water to replace the liquid lost in perspiration when you have a high fever. When you lose enough water to be dangerously dehydrated.

You also lose the electrolytes you need to maintain fluid balance, regulate body temperature, and trigger dozens of biochemical reactions. Plain water doesn’t replace those electrolytes. Check with your doctor for a drink that will hydrate your body without upsetting your tummy.

• You’re exercising or working hard in a hot environment - When you’re warm, your body perspires. The moisture evaporates and cools your skin so that blood circulating up from the center of your body to the surface is cooled. The cooled blood returns to the center of your body, lowering the temperature (your core temperature) there, too.

If you don’t cool your body down, you continue losing water. If you don’t replace the lost water, things can get dicey because not only are you losing water, you’re also losing electrolytes. The most common cause of temporary sodium, potassium, and chloride depletion is heavy, uncontrolled perspiration.

Deprived of water and electrolytes, your muscles cramp, you’re dizzy and weak, and perspiration, now uncontrolled, no longer cools you. Your core body temperature begins rising, and without relief — air conditioning or a cool shower, plus water, ginger ale, or fruit juice — you may progress from heat cramps to heat exhaustion to heat stroke.

The latter is potentially fatal. But — and it’s a big one — drinking too much water while exercising can also be hazardous to your health. Flooding your body with liquid dilutes the sodium in your bloodstream and may make your brain and other body tissues swell, a condition known as hyponaturemia or “water intoxication.”

The New Rule from the American College of Sports Medicine is to drink just enough water to maintain your body weight while working out. How much is that? Step on a scale before exercising.

Exercise for an hour. Step back on the scale. You need 16 ounces of water to replace every pound lost in your one hour’s exercise. Lose one pound, drink 16 ounces. Lose 1⁄2 pound, drink 8 ounces. That was easy!

• You’re on a high-protein diet - You need extra water to eliminate the nitrogen compounds in protein. This is true of infants on high-protein formulas and adults on high-protein weightreducing diets.

• You’re taking certain medications - Because some medications interact with water and electrolytes, always ask whether you need extra water and electrolytes whenever your doctor prescribes:

• Diuretics: They increase the loss of sodium, potassium, and chloride.
• Neomycin (an antibiotic): It binds sodium into insoluble compounds, making it less available to your body.
• Colchicine (an antigout drug): It lowers your absorption of sodium.

• You have high blood pressure - In 1997, when researchers at Johns Hopkins analyzed the results of more than 30 studies dealing with high blood pressure, they found that people taking daily supplements of 2,500 mg (2.5 grams) of potassium were likely to have blood pressure several points lower than people not taking the supplements.

Ask your doctor about this one, and remember: Food is also a good source of potassium. One whole banana has up to 470 milligrams of potassium, one cup of dates — 1,160 milligrams, and one cup of raisins — 1,239 milligrams.