Laboratory studies indicate that low dietary sodium intakes are safe during successive days of prolonged exercise-heat exposure. The protocols of such studies involved controlled low-sodium diets -NaCl/day, hot environments C, F, prolonged daily exercise .- h/day, and large sweat losses – L/day that altered sodium balance. The findings of these studies were as follows: a sodium homeostasis was accomplished successfully while consuming a low-sodium diet after – days of exercise in heat; b the risk of heat illness heat exhaustion was greater on days -, but no cases of heat illness were observed once sodium balance had been reached;hourly replacement of fluid losses due to sweat and urine with an equal volume of pure water was important in the development of heat acclimatization and the maintenance of exercise performance; and d the following diets resulted in similar physiological responses, versus g, versus g, andg versusg NaCl/day.
When the kidneys regulate sodium reabsorption, potassium is exchanged for sodium and lost in urine. This process was previously proposed as a mechanism for the potassium depletion observed in permanent residents of hot environments and in some soldiers undergoing basic training. The clinical signs and symptoms of intracellular potassium depletion include impaired nerve conduction, muscular weakness, and cardiac rhythm abnormalities. Although potassium depletion may result from chronic exposure to a hot environment, it is unlikely in the vast majority of athletic and industrial situations.
A low extracellular sodium concentration may be observed in a small percentage of endurance athletes. This condition, known as hyponatremia, involves a plasma sodium concentration below mEq Na/L. Although athletes typically encounter this illness subsequent to prolonged competition lasting more than h, hyponatremia has been observed during exercise of only -duration. Four theoretical causes exist: sodium losses and plasma sodium levels are normal, but a large volume of pure water or hypotonic fluid is consumed and retained, thereby reducing the concentration of extracellular sodium; sweat volume and sodium concentration are very great, resulting in sodium depletion; a large, inappropriate secretion of the hormone arginine vasopressin induces excessive free water retention; and etiologies – combine to regulate extracellular fluid tonicity and volume inappropriately. Although proponents may be found for each of these mechanisms, recent evidence supports the first etiology because the ingestion of excessive volumes of hypotonic fluid L in during exercise has led to hyponatremia, and the following signs and symptoms: increased body mass, extreme fatigue, nausea, and disorientation. Severe cases of water intoxication also have involved combinations of grand mal seizure, pulmonary edema, increased intracranial pressure, and respiratory arrest. It is probable that an overemphasis on drinking requirements leads to this syndrome. In most training or competitive situations, a weight gain indicates that excess fluid has been retained in the body. Because fluid replacement is necessary for health and maintenance of performance in the vast majority of exercise scenarios, athletes must learn to ingest reasonable, not excessive, quantities of fluid.
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As noted above, the average daily diet of modern cultures provides more sodium than exercise-induced sweating usually demands; the excess sodium is excreted by the kidney. It is only when humans dramatically depart from normal routines athletes participating in ultraendurance events, extended heat exposure while not acclimatized that deficiencies occur. Therefore, the key to avoiding all sodium imbalances that impart decrements in performance or illness is to match sodium and water consumption with acute sodium and water losses. You can accomplish this in three ways. First, you should eat normal meals and neither restrict nor greatly increase sodium intake before and during training or competition. Consumption of electrolytes in beverages may be prudent during exercise that lasts more than if sweat sodium and water loss are large. Sweat contains approximately NaCl/L and NaCl/L in acclimatized and nonacclimatized individuals, respectively. Second, you should not assume that you can ingest unlimited quantities of fluid during exercise and expect it to be absorbed and distributed to the extracellular and intracellular spaces in a rapid and uniform pattern. To avoid consuming too much or too little fluid, you can simply measure body weight before and after exercise. In fact, some ultraendurance events the Western States Mile Race now require contestants to weigh themselves at regular intervals; competitors are instructed to increase and decrease water consumption accordingly. During prolonged, low-intensity exercise, you usually will lose kg of body water L of sweat per hour. Therefore, each kilogram of body weight change represents L and each pound represents a pint of fluid.
Third, you can estimate the amount of salt NaCl you lose during training or competition by using the method shown in table Multiply the duration of exercise hours by your sweat rate liters per hour; see figureon page to get the total water loss liters. Then, if you are very fit and well acclimatized to the heat, estimate your sweat salt concentration asg NaCl/L; if you are unfit and not acclimatized, estimate this value asg NaCl/L of sweat. Next, multiply this sweat salt concentration grams per liter by your total water loss to derive the total grams of NaCl lost in sweat. Also, remember that longer events will involve urine NaCl losses. To put this salt loss into perspective, remember that the average adult in America consumes -NaCl/day. Clearly, a serious salt deficit is possible in an ultraendurance event.
To select foods and beverages for postexercise dietary replacement, tablesuggests that most commonly used fluids replace very little NaCl, per oz serving. This includes the fluid-electrolyte replacement beverages. Liberal salting of food also provides a simple source of NaCl in the case of large deficits. One teaspoon of table salt equals approximately of NaCl. Finally, the following recipe may be used to provide sodium and carbohydrate in a large volume for field use: add one packet of unsweetened Kool Aid beverage powder.
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