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Manufacturers claim sports drinks effectively replace lost sweat. Perspiration, as you know, is the body's way of cooling itself. In hot weather, you could lose as much as a quart or two of water in an hour.

As you sweat, the heart works harder, your body temperature drops, and your performance declines. Unless this fluid is replaced, you could end up dehydrated even before you're thirsty and suffer from headaches and dizziness, lapse into a coma and die.

Since sports drinks contain sodium and glucose, they are said to replace fluids more effectively than water and are quickly absorbed from the small intestine into the bloodstream. That claim is trumpeted by Gatorade which supposedly 'works 30 percent faster than water.' The basis for this catchy phrase is a study published in the American Journal of Physiology.

In that study, exercise physiologist Carl V. Gisolfi and his colleagues at the University of Iowa tested how well seven volunteers absorbed a six percent carbohydrate solution with electrolytes and distilled water. They found that the carbo solution (which was similar to Gatorade) was absorbed 30 percent faster.

That should have put everything in place except for one thing: the solution was infused directly into the small intestine of the volunteers. It never passed the stomach. Unless you happen to be a Martian, whatever you drink first passes through the stomach before going to the small intestine.

The stomach can greatly influence how quickly a fluid is absorbed and reaches the bloodstream. Unfortunately, none of the various manufacturers of sports drinks appear to be interested in knowing how well their products are absorbed by the stomach. And even if their products are quickly absorbed, there is no evidence that the rate of fluid absorption affects athletic performance.

What does this teaches us? Simply that 'the best way to avoid dehydration is to drink enough fluids to offset your sweat loss - a pint for every pound loss during a workout. You should drink before, during, and after a workout. Several kinds of fluids will do the job. Plain water, which is easily absorbed by the body, is perfectly adequate - and is in fact the best beverage to drink before you exercise,' said the editors of Consumer Reports.

The sodium in sports drinks supposedly helps the body retain fluids after intense, prolonged exercise better than water. Promoters of performance drinks say this was shown in a study published in the International Journal of Sports Medicine.

However, Edward F. Coyle, director of the Human Performance Laboratory of the University of Texas and co-author of the study, said the differences between the two fluids were minimal. And if you eat a meal within three hours, you can get enough sodium without wasting money on any sports drink.

Another selling point of sports drinks is that they effectively replace electrolytes that are lost when you sweat. Again, this is a triumph of hope over reason. While sports drinks do contain sodium and potassium that are lost when you perspire, there's no need to consume them since experts say you can easily get these minerals from a good meal.

'The electrolytes in sports drinks have one benefit: manufacturers claim the electrolytes help keep you thirsty and encourage you to keep drinking until you get enough fluid. But if you follow the experts' advice, that potential benefit is irrelevant. Sports physiologists recommend that you start drinking before you feel thirsty, and keep drinking even after your thirst is quenched,' said the editors of Consumers Reports.