Fluid Fluency

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.

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 aplenty
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.

Liz Applegate, Ph.D., Nutrition columnist and Editor Runner's World and Faculty at University of California at Davis. Author of "Eat Your Way to a Healthy Heart, Chocolate and 99 other Foods to Help Your Heart."