Water and electrolyte requirements during exercise.

These studies demonstrate the body's capacity to minimize electrolyte losses during acute and repeated bouts of exercise and dehydration. Although there are marked shifts in water and selected ions in the exercising muscle, only during prolonged exertion is the ratio of intramuscular to extramuscular potassium significantly altered, suggesting that some modifications of the muscle cell membrane may occur. Muscle tissue not engaged in the exercise seems unaffected by the sweat loss during prolonged activity but relinquishes intracellular water shortly after work is terminated. Blood, muscle, sweat, and urine measurements before and following varied levels of dehydration demonstrate that body water loss during exercise in the heat is accomplished at the expense of larger water losses from extracellular and intracellular compartments. Moreover, the loss of ions in sweat and urine have little effect on the potassium content of either plasma or muscle. With repeated days of dehydration and heavy exercise, plasma volume increases in proportion to an increase in body sodium storage. Since red blood cells and hemoglobin are confined to the vascular space, both may decrease significantly as a function of the hemodilution induced by repeated days of exercise and dehydration. This may, in part, explain the apparent anemia reported by sports physicians among athletes undergoing intensive training. It is also possible that such hemodilution may produce low concentrations of plasma potassium, which might be falsely interpreted as suggestive of hypokalemia. In any event, some caution should be used in the clinical interpretation of plasma concentrations of various constituents among endurance-trained athletes. In general, it seems that the large sweat losses incurred during training and competition are adequately tolerated by the athlete, with concomitant adjustments in the water and electrolyte distribution of their fluid compartments. Despite the sizable excretion of ions and sweat, the athlete's large caloric intake and renal conservation of sodium minimize the threat of chronic dehydration or electrolyte deficits, or both.