Fluid Fluency

Everything you need to know about hydrating in the heat

by: Liz Applegate Ph.D.

Running during these scorching summer months can leave you parched as a lizard in the Mojave Desert. And, yes, you probably know you need to drink a lot of liquid during these times, but you may not realize just how vital it is. Taking in too little fluid can be disastrous for your running and your health. Drink the right amount of the right beverages, and you'll feel great and run fast.

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.

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