Fluid Replacement During Exercise
Without adequate fluid replacement during prolonged exercise, rectal temperature and heart rate will become more elevated compared with a well-hydrated condition. The most serious effect of dehydration resulting from the failure to replace fluids during exercise is impaired heat dissipation, which can elevate body core temperature to dangerously high levels C.Exercise-induced dehy- dration causes hypertonicity of body fluids and impairs skin blood flow and has been associated with reduced sweat rate thus limiting evaporative heat loss, which accounts for more than of heat loss in a hot-dry environment. Dehydration body weight loss can also elicit significant reduction in cardiac output during exercise since a reduction in stroke volume can be greater than the increase in heart rate. Since a net result of electrolyte and water imbalance associated with failure to adequately replace fluids during exercise is an increased rate of heat storage, dehydration induced by exercise presents a potential for the development of heat-related disorders, including potentially life-threatening heat stroke. It is therefore reasonable to surmise that fluid replacement that offsets dehydration and excessive elevation in body heat during exercise may be instrumental in reducing the risk of thermal injury.
To minimize the potential for thermal injury, it is advocated that water losses due to sweating during exercise be replaced at a rate equal to the sweat rate. Inadequate water intake can lead to premature exhaustion. During exercise, humans do not typically drink as much water as they sweat and, at best, voluntary drinking only replaces about two-thirds of the body water lost as sweat. It is common for individuals to dehydrate by – of their body weight during exercise in the heat despite the availability of adequate amounts of fluid. In many athletic events, the volume and frequency of fluid consumption may be limited by the rules of competition number of rest periods or time outs or their availability spacing of aid stations along a race course. While large volumes of ingested fluids are tolerated by exercising individuals in laboratory studies, field observations indicate that most participants drink sparingly during competition. For example, it is not uncommon for elite runners to ingest less than ml of fluid during distance events in a cool environment lasting more than Actual rates of fluid ingestion are seldom more than ml h-, and most athletes allow themselves to become dehydrated by – kg of body weight in sports such as running, cycling, and the triathlon. It is clear that perception of thirst, an imperfect index of the magnitude of fluid deficit, cannot be used to provide complete restoration of water lost by sweating. As such, individuals participating in prolonged intense exercise must rely on strategies such as monitoring body weight loss and ingesting volumes of fluid during exercise at a rate equal to that lost from sweating, body weight reduction, to ensure complete fluid replacement. This can be accomplished by ingesting beverages that enhance drinking at a rate of one pint of fluid per pound of body weight reduction. While gastrointestinal discomfort has been reported by individuals who have attempted to drink at rates equal to their sweat rates, especially in excess of L h- this response appears to be individual and there is no clear association between the volume of ingested fluid and symptoms of gastrointestinal distress. Further, failure to maintain hydration during exercise by drinking appropriate amounts of fluid may contribute to gastrointestinal symptoms. Therefore, individuals should be encouraged to consume the maximal amount of fluids during exercise that can be tolerated without gastrointestinal discomfort up to a rate equal to that lost from sweating.
Enhancing palatability of an ingested fluid is one way of improving the match between fluid intake and sweat output. Water palatability is influenced by several factors including temperature and flavoring. While most individuals prefer cool water, the preferred water temperature is influenced by cultural and learned behaviors. The most pleasurable water temperature during recovery from exercise was C, although when water was ingested in large quantities, a temperature of-was preferred. Experiments have also demonstrated that voluntary fluid intake is enhanced if the fluid is flavored, and/or sweetened. It is therefore reasonable to expect that the effect of flavoring and water temperature should increase fluid consumption during exercise, although there is insufficient evidence to support this hypothesis. In general, fluid replacement beverages that are sweetened artificially or with sugars, flavored, and cooled to between andshould stimulate fluid intake.
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The rate at which fluid and electrolyte balance will be restored is also determined by the rate at which ingested fluid empties from the stomach and is absorbed from the intestine into the blood. The rate at which fluid leaves the stomach is dependent on a complex interaction of several factors, such as volume, temperature, and composition of the ingested fluid, and exercise intensity. The most important factor influencing gastric emptying is the fluid volume in the stomach. However, the rate of gastric emptying of fluid is slowed proportionately with increasing glucose concentration above When gastric fluid volume is maintained at ml or more, most individuals can still empty more than ml h- when the fluids contain a – carbohydrate concentration. Therefore, to promote gastric emptying, especially with the presence of – carbohydrate in the fluid, it is advantageous to maintain the largest volume of fluid that can be tolerated in the stomach during exercise – ml. Mild to moderate exercise appears to have little or no effect on gastric emptying while heavy exercise at intensities greater than of maximal capacity may slow gastric emptying Laboratory and field studies suggest that during prolonged exercise, frequent every – min consumption of moderate ml to large ml volumes of fluid is possible. Despite the apparent advantage of high gastric fluid volume for promoting gastric emptying, there should be some caution associated with maintaining high gastric fluid volume. People differ in their gastric emptying rates as well as their tolerance to gastric volumes, and it has not been determined if the ability to tolerate high gastric volumes can be improved by drinking during training. It is also unclear whether complaints of gastrointestinal symptoms by athletes during competition are a function of an unfamiliarity of exercising with a full stomach or because of delays in gastric emptying. It is therefore recommended that individuals learn their tolerance limits for maintaining a high gastric fluid volume for various exercise intensities and durations.
Once ingested fluid moves into the intestine, water moves out of the intestine into the blood. Intestinal absorptive capacity is generally adequate to cope with even the most extreme demands; and at intensities of exercise that can be sustained for more than min, there appears to be little effect of exercise on intestinal function. In fact, dehydration consequent to failure to replace fluids lost during exercise reduces the rate of gastric emptying supporting the rationale for early and continued drinking throughout exercise.