The influence of beverage composition and gastrointestinal function on fluid and nutrient availability during exercise

In order to maximize fluid and/or carbohydrate availability during exercise, in which performance may be limited by one or the other, the composition and ingestion pattern of a beverage can be adjusted accordingly. As carbohydrate concentration increases, the rate of gastric emptying decreases. With more highly concentrated carbohydrate solutions, the net fluid absorption of water in the intestine is also reduced, although lower concentrations of glucose containing carbohydrates (~3-7%) actually can stimulate net intestinal absorption. Increasing the osmolality of a beverage can reduce the gastric emptying rate, but it has a more profound effect on gastric and intestinal secretions. Hyperosmolality increases secretions and as such decreases the rate of net absorption. Sodium inclusion in a beverage is warranted not only to offset losses (primarily from sweat) but, moreover, to promote fluid absorption in the intestine and to increase fluid retention. Recommendations include maintenance of fluid balance by ingestion of sufficient fluids, immediately prior to and during exercise, to restrict body weight losses to =1%. Somewhat more concentrated carbohydrate solutions may be ingested when the risk of hypohydration is low and performance is limited by carbohydrate availability. With increasing carbohydrate concentrations, at least up to 18%, there is a greater rate of carbohydrate passage through the gastrointestinal tract. When carbohydrate concentrations exceed 5%, the type of carbohydrate (mono- or disaccharides, long chain polymers, etc.) is important as the osmolality increases with increasing concentration and decreasing chain length. Thus, when concentrations of >8% are warranted, based on energy demands, there is an advantage to using glucose polymer solutions, owing to a decrease in gastrointestinal secretions and, hence, an increase in rate of net fluid absorption.