Here's a look at how water works to keep your body running smoothly, with some tips on when, how much and what kinds of fluids to drink. Plus, I've provided a rundown of many of the sports beverages on the market.

You're all wet

On average, the human body is more than 50 percent water. Runners and other endurance athletes average around 60 percent. This equals about 120 soda cans' worth of water in a 160-pound runner! A runner's watery physique results from physiological adaptations brought about by running. For one, running builds lean muscle tissue and reduces body fat; lean tissue contains more water than fat tissue does. (Fat tissue contains the least water of all body tissues, even less than bones.)

Another reason for your waterlogged state is your expanded blood volume. This occurs as you become physically fit and serves to improve oxygen and nutrient delivery to working muscles. The extra blood also helps remove wastes produced by muscles during exercise.

Sweat it

During running, muscles generate heat--lots of it. A typical 5-mile run burns about 500 calories, and 70 percent of this heat must exit the body to keep muscle tissue from literally cooking. The body stays cool by producing sweat, the evaporation of which rids your body of unwanted heat--roughly 600 calories of heat for every quart of sweat that evaporates. And during an hour of running, you can easily lose more than 2 quarts of sweat.

How much you sweat depends upon several factors. Warm weather and high humidity both increase sweat production. And the faster you run, the more heat you generate, so the more you sweat. Sweat rate is also influenced by your fitness level: the sweat glands in a fit body enlarge and increase in number, so you sweat more. All these bodily adjustments create more efficient cooling while you run.

Running dry

So sweating keeps you cool, but losing all that fluid lessens the efficiency of the internal operations of your body. Most runners fall short on fluid replacement and only manage to replace about half their sweat losses. If you don't take in fluids as you sweat, your blood actually thickens. This makes your heart pump harder and slows oxygen and nutrient delivery to exercising muscles. Result: Your body suffers.

As you dehydrate and your pace slows, you may become dizzy, weak or nauseated. Eventually you may cramp up, get chills or even hallucinate. Some of these same symptoms may even occur at the office or at home, as your unmet fluid need doesn't always conveniently show up on your run.

Drink up

The old rule that you need eight glasses of water or fluid daily is just that--old. Your fluid needs depend on many factors, including body size, fitness level, training schedule and dietary factors such as caffeine and alcohol consumption, both of which increase fluid loss from the body. So how much fluid you need is an individual matter.

Your best bet is to monitor urine color and frequency of urination. Pale yellow urine is a good sign that plenty of fluid is on board for waste excretion. (But don't judge your urine color within a few hours after taking vitamin supplements, since the unused vitamins, particularly the B vitamin riboflavin, turn your urine a bright yellow.) Frequent urination is another good sign that you're getting enough fluid.

Spread out your fluid intake over the day to keep body water levels steady and to ward off the

threat of dehydration. And remember to drink past the feeling of thirst, since that sensation shuts off quickly once you begin drinking. In fact, it actually turns off before you've replenished lost fluids.

Water your options

Choices, choices. The beverage aisle in any grocery store overflows with drinks: bottled waters (spring, mineral and sparkling), bottled teas, juices and many other concoctions. And there's always . . . plain tap water. So what's best? Tap water is fine, and it's cheap. Plus, local municipal water supplies must follow strict safety regulations, so if the water out of your faucet tastes okay, by all means drink it.

Many consumers opt for bottled water, which generally tastes better than tap water because bottlers use ozone as a disinfectant instead of chlorine. And though the general perception is that bottled water is better for you than tap water, the fact is, most safety regulations are higher for municipal water than bottled.

Some bottled water may offer minerals such as calcium and magnesium, but if you live in an area that has hard water, your local water probably has more minerals than bottled does. For instance, a quart of tap water from my hometown supplies about 10 percent of my magnesium needs. (If you're not sure if you have hard water, just check your water faucets for mineral deposits.)

Bottled teas and juices are tasty, thirst-quenching options, but watch for caffeine, which can increase body water loss by increasing urine production. And you may be taking in unwanted calories, as many of these beverages have a high content of sugar or corn syrup.

Fluid on the run

During exercise, your body needs fluid--fast. And during longer runs, a supply of carbohydrates also becomes crucial for maintaining energy levels. As you run, both fat and carbohydrates are burned for energy, but glycogen, which is the form that carbohydrate takes when stored in the muscles, runs low after about 90 minutes of running. When this happens, you'll weaken and your pace may turn sluggish.

The solution is quite simple: drink a sports beverage. These are formulated to supply a steady stream of energizing carbohydrates and to maximize fluid absorption. Sports drinks also stimulate thirst, and help you to retain the fluid due to the electrolyte content (water can sometimes be excreted instead of retained, as it dilutes body fluids)

Sodas and juices don't work as well as fluid-replacement solutions during exercise because their relatively high carbohydrate concentrations of 10 to 14 percent slow fluid absorption in the intestinal tract. Most sports drinks contain half the carbohydrate content of these other beverages, or about 50 to 80 calories per 8 ounces. Small amounts of electrolytes (sodium) added to many sports drinks also boost fluid absorption.

Since your fluid losses amount to over a quart an hour, drink about 1/2 to 1 cup of sports drink every 15 minutes. In other words, aim for around 100 calories of carbohydrates every 30 minutes, which is ideal for keeping you energized. Even on runs lasting less than an hour, drinking helps prevent overheating, especially during tough, warm-weather workouts. If you haven't yet, give a sports drink a try during your next training session.

Sports drinks a plenty

Sports drinks have become a billion-dollar-a-year industry. Several new products have appeared on the market over the last few years, and they're filling the shelves in supermarkets and convenience stores, even popping out of soda machines. But what's best to buy? That's pretty much determined by your personal preference, but do select one that contains around 50 to 80 calories per 8-ounce serving; any more and the carbohydrate concentration will inhibit fluid absorption.

Test different brands during training, particularly on long runs, and see what works best for you. Some are slightly carbonated, which is fine if that's your preference. Whatever you choose, a sports beverage can be a valuable part of your refueling and rehydrating regimen.

Nanci S. Guest is a certified personal trainer & nutritionist, and is completing her Master of Science degree in nutrition this June. She owns "Power Play: Nutrition, Fitness, Performance" in Vancouver, BC, and for the past 8 years she has been providing individuals, sports teams & the community with nutritional consulting & personal training services, as well as research services, seminars and article writing for local & national publications.