To stay well-hydrated during exercise, the fluid, carbohydrates and electrolytes in a sports drink have to be rapidly absorbed to offset the loss of fluid and electrolytes in the sweat and the rapid oxidation of carbohydrate by muscle.
In order to stimulate absorption, a sports drink must be formulated to maximize gastric emptying and to speed the absorption of solutes (carbohydrates, electrolytes) and fluids into the bloodstream. Beverages that are carbonated or have high levels of carbohydrates generally empty slowly from the stomach and should be avoided during athletic activities because such drinks may cause bloating or a feeling of fullness, upset stomach, burping, and nausea.
Although the primary factors affecting the gastric emptying rate are the energy content and volume of the food or fluid that is ingested, dehydration and intense exercise can also slow gastric emptying.
Once fluid is emptied from the stomach, it enters the small intestine, where the characteristics of the sports drink become extremely important. The surface of the jejunum of the proximal small intestine is rich in receptors—special proteins that transport nutrients into the bloodstream. These transport proteins work in a manner very much like a revolving door, allowing sodium and glucose, two common and absolutely necessary ingredients in a sports drink, into the cell. As a result of this solute transport, water molecules quickly follow to maintain osmotic equilibrium. In other words, water absorption from the intestine is accelerated by the presence of sodium and glucose in the intestine.
The retardants fluid absorption acts if the osmolality of the beverage being consumed is too high. Too much carbohydrate, the wrong types of carbohydrate, or too high of an osmolality will slow fluid absorption.
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