Gastric emptying plays an important role in sports nutrition. The emptying of the stomach not only affects well-being, which has a significant influence on performance, especially in competition. The availability of food and the associated continuous supply of energy depend on it. Last but not least, gastric emptying is also crucial for hydration, as the absorption of ingested water and most of the substances dissolved in it takes place in the small intestine. Therefore, the rate at which drinks are emptied from the stomach is an important factor in determining the rate of water absorption. The rate at which the stomach empties can be influenced by a variety of factors. These include the calorie content, volume, osmolality, temperature and pH of the liquid ingested, diurnal and inter-individual variations, metabolic state (rest/movement) and ambient temperature. The calorie content of the fluid ingested is one – if not the most important variable – that determines the rate of gastric emptying.Gastric emptying is increasingly slowed down the higher the calorie content of the liquid ingested. At rest, gastric emptying is slowed by caloric solutions regardless of the nutrient source (i.e. carbohydrate, fat or protein). Consequently, pure water is known to empty the stomach of resting individuals faster than caloric solutions. During moderate-intensity exercise (less than 70-75% VO2max), gastric emptying occurs at a similar rate as at rest. More intense exercise appears to generally delay gastric emptying. If fluids are ingested at regular intervals during prolonged exercise of more than 2 hours, analysis of the stomach contents after exercise shows that solutions containing up to 10 percent carbohydrates are emptied from the stomach at a similar rate to pure water. Carbonated drinks have no effect on gastric emptying or energy supply. However, they do change the distribution of the stomach contents. Both solid and liquid food components are increasingly shifted into the upper stomach. This makes belching into the oesophagus more likely.
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Fluid absorption in the body is not only dependent on gastric emptying, but also on the absorption of fluids in the small intestine. The terms isotonic, hypertonic and hypotonic are frequently used in the context of fluid intake in sport. In isotonic drinks, the number of dissolved particles, such as electrolytes and sugar, is the same as the number of dissolved particles in the intestinal cells that line the intestinal wall. Pure water is hypotonic and contains fewer particles than the intestinal cells. An imbalance occurs in the body due to a difference in the number of particles between the inside and outside of cells. This type of imbalance is called an osmotic gradient and is balanced by the process of osmosis. Water flows through a passive transport process from the area with a lower particle concentration to areas of higher concentration. Pure water is absorbed faster by the small intestine than many electrolyte drinks. In the event of severe fluid loss during exercise, it can therefore be a strategy during long competitions to supply pure water, at least briefly, between the intake of higher-dose electrolyte drinks in order to avoid dehydration. However, salt and sugar should not be avoided during a competition. In competition practice, the stomach is never completely empty. Pure water mixes at least partially with particles of the stomach contents. In the small intestine, the osmotic gradient is altered by the active and passive transport of sugar and electrolytes from the intestinal contents into the cells and (net) water absorption is promoted. The activation of these transporters also increases the permeability of the mucosa, which promotes further absorption. Although hypertonic fluids can contain numerous minerals and sugars as energy carriers, they slow down fluid absorption. In terms of energy density, there is evidence of a negative relationship between carbohydrate content and gastric emptying in slightly hypertonic solutions. The amount of glucose required to stimulate water absorption is relatively low. For rehydration after exercise, ingesting an adequate amount of a dilute solution is more beneficial than drinking a smaller amount of a more concentrated beverage. The simultaneous intake of fructose and glucose leads to faster gastric emptying than a pure glucose solution with the same nutritional value. This applies to a combination of glucose/maltodextrin with fructose as well as in combination with sucrose. The faster gastric emptying of fructose and sucrose occurs both at rest and under stress and is probably due to faster absorption in the intestine, which causes reduced feedback inhibition in the stomach. It has also been shown that exposure to fructose for several days increases the subsequent gastric emptying of fructose-containing solutions. Thus, fructose consumption appears to promote carbohydrate delivery to the intestinal mucosa both acutely and chronically. Last but not least, the taste of sports drinks plays an important role in preventing dehydration. It is known that non-scheduled, “voluntary” fluid intake during exercise in the heat is higher with sweetened, flavored beverages than with plain water. Short-chain carbohydrates are usually perceived as unpleasantly sweet when consumed in large quantities. Fructose has the highest sweetening power. Maltodextrin, on the other hand, is quite tasteless and only slightly sweet. Maltodextrin provides energy just as quickly and in the same amount as a corresponding dose of glucose. A drink with medium sweetening power by mixing maltodextrin with other simple sugars could therefore offer advantages.
The body loses water and electrolytes with sweat. The rate of perspiration can exceed the maximum gastric emptying rate of the fluids ingested. A certain degree of dehydration is therefore frequently observed, especially in competition. Excessive replacement of sweat losses with plain water or low sodium fluids can lead to hyponatremia (low blood sodium levels), which can be associated with struggling, dizziness and disorientation, among other symptoms. Sodium replacement is essential for rehydration after exercise. Diluted hypotonic glucose-sodium solutions are highly effective rehydration solutions for oral intake. Fluid absorption in the small intestine is stimulated by glucose and sodium (and to a lesser extent by fructose and other electrolytes). Glucose and sodium are absorbed into the small intestinal mucosa via a common membrane carrier (SLGT1 transporter). Glucose potentiates sodium absorption and creates an osmotic gradient for fluid absorption.
The optimum frequency, quantity and composition of drinks depends heavily on the intensity and duration of exercise, the environmental conditions and the physiology of the individual. The composition of sweat is individual and can vary greatly. The amount of sweat is also determined by the metabolic rate as well as the ambient temperature and humidity. The most reliable method of measuring sweat loss is to weigh an athlete immediately before and after exercise. The reference method for determining sodium loss in sweat is the WBW whole-body washdown. The WBW method is considered to be the most accurate measurement of electrolyte loss through sweat from the whole body, as the entire sweat outflow is recorded and taken into account and the normal evaporation process is not affected. However, the WBW method requires a controlled laboratory environment. Even with well-controlled sweat test methods, some variability can be observed among subjects. The day-to-day variability in WBW for sodium is approximately 11-17 percent. This variability in sweat response should be taken into account when interpreting results.Athletes are well advised to record their weight loss and fluid intake as well as their choice of drink in terms of energy and sodium content over a longer period of time and under different conditions (temperature, intensity and duration of exercise).
Conclusion: The carbohydrate content in sports drinks increases the amount of fuel that can be supplied to the body, but at higher doses reduces the rate at which water is absorbed. If the provision of fluids is the top priority, e.g. in hot weather, the carbohydrate content of drinks should be rather low. In competition, however, this limits the speed of energy intake. It is important to weigh this up and, if necessary, adjust or vary the composition of the fluids consumed during a longer competition. In addition to the ambient temperature, humidity is also an important factor, as are the physiological and biochemical characteristics of the individual athlete. The continuous intake of a hypotonic carbohydrate-electrolyte drink during training or competition tends to offer the greatest benefit for fluid intake. The appropriate drink should be chosen depending on the training and competition conditions.
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