Electrolyte and Carbohydrate Replacement During Exercise
There is little physiological basis for the presence of sodium in an oral rehydration solution for enhancing intestinal water absorption as long as sodium is sufficiently available in the gut from the previous meal or in the pancreatic secretions. Inclusion of sodium < mmol L- in fluid replacement drinks during exercise has not shown consistent improvements in retention of ingested fluid in the vascular compartment. A primary rationale for electrolyte supplementation with fluid replacement drinks is, therefore, to replace electrolytes lost from sweating during exercise greater than -in duration. Normal plasma sodium concentration is mmol L-, making sweat ~ mmol L- hypotonic relative to plasma. At a sweat rate ofL h-, a total sodium deficit of mmol h- could occur during exercise. Drinking water can lower elevated plasma electrolyte concentrations back toward normal and restore sweating but complete restoration of the extracellular fluid compartment cannot be sustained without replacement of lost sodium. In most cases, this can be accomplished by normal dietary intake. If sodium enhances palatability, then its presence in a replacement solution may be justified because drinking can be maximized by improving taste qualities of the ingested fluid.
The addition of carbohydrates to a fluid replacement solution can enhance intestinal absorption of water. However, a primary role of ingesting carbohydrates in a fluid replacement beverage is to maintain blood glucose concentration and enhance carbohydrate oxidation during exercise that lasts longer than h, especially when muscle glycogen is low. As a result, fatigue can be delayed by carbohydrate ingestion during exercise of duration longer than h, which normally causes fatigue without carbohydrate ingestion. To maintain blood glucose levels during continuous moderate-to-high intensity exercise, carbohydrates should be ingested throughout exercise at a rate of h-. These amounts of carbohydrates can be obtained while also replacing relatively large amounts of fluid if the concentration of carbohydrates is kept below ml- of fluid. For example, if the desired volume of ingestion is – ml h-, then the carbohydrate requirements can be met by drinking fluids with concentrations in the range of -. With this procedure, both fluid and carbohydrate requirements can be met simultaneously during prolonged exercise. Solutions containing carbohydrate concentrations will cause a net movement of fluid into the intestinal lumen because of their high osmolality, when such solutions are ingested during exercise. This can result in an effective loss of water from the vascular compartment and can exacerbate the effects of dehydration.
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Few investigators have examined the benefits of adding carbohydrates to water during exercise events lasting less than Although preliminary data suggest a potential benefit for performance the mechanism is unclear. It would be premature to recommend drinking something other than water during exercise lasting less than Generally, the inclusion of glucose, sucrose, and other complex carbohydrates in fluid replacement solutions has equal effectiveness in increasing exogenous carbohydrate oxidation, delaying fatigue, and improving performance. However, fructose should not be the predominant carbohydrate because it is converted slowly to blood glucose not readily oxidized which does not improve performance. Furthermore, fructose may cause gastrointestinal distress.
Fluid Replacement and Exercise Performance
Although the impact of fluid deficits on cardiovascular function and thermoregulation is evident, the extent to which exercise performance is altered by fluid replacement remains unclear. Although some data indicate that drinking improves the ability to perform short duration athletic events in moderate climates, other data suggest that this may not be the case. It is likely that the effect of fluid replacement on performance may be most noticeable during exercise of duration greater than and/or at extreme ambient environments.
The addition of a small amount of sodium to rehydration fluids has little impact on time to exhaustion during mild prolonged exercise in the heat, ability to complete of moderate exercise, or capacity to perform during simulated time trials. A sodium deficit, in combination with ingestion and retention of a large volume of fluid with little or no electrolytes, has led to low plasma sodium levels in a very few marathon or ultra-marathon athletes. Hyponatremia blood sodium concentration between and mmol L- has been observed in ultra-endurance athletes at the end of competition and is associated with disorientation, confusion, and in most cases, grand mal seizures. One major rationale for inclusion of sodium in rehydration drinks is to avoid hyponatremia. To prevent development of this rare condition during prolonged exercise, electrolytes should be present in the fluid or food during and after exercise.
Maintenance of blood glucose concentrations is necessary for optimal exercise performance. To maintain blood glucose concentration during fatiguing exercise greater than above VOmax, carbohydrate ingestion is necessary. Late in prolonged exercise, ingested carbohydrates become the main source of carbohydrate energy and can delay the onset of fatigue. Data from field studies designed to test these concepts during athletic competition have not always demonstrated delayed onset of fatigue, but the inability to control critical factors such as environmental conditions, state of training, drinking volumes make confirmation difficult. Inclusion of carbohydrates in a rehydration solution becomes more important for optimal performance as the duration of intense exercise exceeds
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