Dehydration reduces the water in the blood and subsequently the quantity of blood. As blood volume decreases, heart rate and body temperature climb, hurting athletic performance. When athletes lose large quantities of water from their bodies during competition, they risk heat illness or heat stroke. Although drinking fluids during competition helps prevent dehydration, water absorption from the stomach often cannot keep pace with the high sweat rates.

Plain water is the standard against which all sport drinks are compared. However, carbohydrates-electrolyte beverages may help athletes in some situations.

The athlete should experiment with beverages to determine tolerance. Sports drinks that contain 4 percent to 8 percent carbohydrate by weight (9.5 to 16 grams of carbohydrate per 8 ounce serving) are optimal for fluid replacement. Athletes should drink 500 - 600 milliliters ( 2-3, 8-ounce cups) of these fluids ½ hour prior to competition to ensure that fluid delivery to the blood has started by the time of competition.

The timing of this pre-event drink is critical. If taken too early, it can lead to a full bladder and discomfort. If taken too late, it will not be ready to help the body by the start of competition. Each athlete will require different amounts of fluid to stay well hydrated, but a suitable starting amount is to drink 750 - 1250 milliliters per hour (3-5, 8-ounce cups) of a 4 percent to 9 percent carbohydrate sports drink. The amount of fluid consumed can be varied based on each individual.

Levels of carbohydrate greater than 10 percent in sports drinks may slow the emptying of fluids from the stomach. But recent research shows that the utilization rate of carbohydrate from these beverages is very similar for drinks containing 4.5 percent to 17 percent carbohydrate. In other words, you won't necessarily be reducing the percentage of carbohydrate by utilization by increasing the percent of carbohydrate in the drink, but you may slow fluid emptying from the stomach.

Carbonation, believed to slow gastric emptying and perhaps cause stomach upset, may not be the culprit we once thought. Recent laboratory studies show little influence of carbonation on the rate of stomach emptying. But the flavor of these beverages, for reasons not well understood, did slow stomach emptying compared to water. However, flavoring is an important stimulus to voluntary beverage consumption and should not necessarily be avoided since the total amount of fluid one drinks would be more important to hydration than any slowing of absorption due to flavoring.