By weighing themselves before and after exercise, athletes can determine the amount of water lost during a strenuous session. It is not uncommon for someone exercising in a hot and humid environment to lose 8 to 12 pounds after four hours of exercise. That's the equivalent of one to 1.5 gallons of sweat. Even a marathon runner may metabolize about three-quarters of a pound of fat, but lose up to 10 pounds of water weight. The best choices for replacing water are sports drinks that contain sodium and carbohydrates, both of which increase fluid uptake from the intestines into the bloodstream.
Research shows that the body absorbs more fluid when electrolytes such as sodium are added.
In one study, six volunteers underwent two exposures to heat and then engaged in an exercise regimen that caused mild dehydration resulting in a 2 to 3 percent decrease in body weight. Each volunteer then rehydrated with either water or a water-sodium solution. During the three-hour rehydration period, subjects rehydrating with water alone restored 68 percent of the fluid they lost, whereas subjects rehydrating with the sodium solution replaced 82 percent of their lost fluids. According to the American College of Sports Medicine, carbohydrates should also be included in rehydration drinks. Research shows that carbohydrates increase water absorption by interacting with sodium in the intestinal wall.