Three independent biological mechanisms cause exercise-associated hyponatremia: Evidence from 2,135 weighed competitive athletic performances

Abstract

To evaluate the role of fluid and Na ؉ balance in the development of exercise-associated hyponatremia (EAH), changes in serum Na ؉ concentrations ([Na ؉ ]) and in body weight were analyzed in 2,135 athletes in endurance events. Eighty-nine percent of athletes completed these events either euhydrated (39%) or with weight loss (50%) and with normal (80%) or elevated (13%) serum [Na ؉ ]. Of 231 (11%) athletes who gained weight during exercise, 70% were normonatremic or hypernatremic, 19% had a serum [Na ؉ ] between 129 -135 mmol͞liter, and 11% a serum [Na ؉ ] of <129 mmol͞liter. Serum [Na ؉ ] after racing was a linear function with a negative slope of the body weight change during exercise. The final serum [Na ؉ ] in a subset of 18 subjects was predicted from the amount of Na ؉ that remained osmotically inactive at the completion of the trial. Weight gain consequent to excessive fluid consumption was the principal cause of a reduced serum [Na ؉ ] after exercise, yet most (70%) subjects who gained weight maintained or increased serum [Na ؉ ], requiring the addition of significant amounts of Na ؉ (>500 mmol) into an expanded volume of total body water. This Na ؉ likely originated from osmotically inactive, exchangeable stores. Thus, EAH occurs in athletes who (i) drink to excess during exercise, (ii) retain excess fluid because of inadequate suppression of antidiuretic hormone secretion, and (iii) osmotically inactivate circulating Na ؉ or fail to mobilize osmotically inactive sodium from internal stores. EAH can be prevented by insuring that athletes do not drink to excess during exercise, which has been known since 1985.endurance ͉ exchangeable Na ϩ stores ͉ fluid overload ͉ overdrinking ͉ syndrome of inappropriate ADH secretion

Exercise-Associated Hyponatremia

“…The authors estimated that athletes who gained Ͼ4% body weight during exercise had a 45% probability of developing hyponatremia. Importantly, 70% of individuals who gained weight during exercise did not develop hyponatremia, pointing to other important factors in the pathogenesis, as discussed next (34).…”

“…In the study of Noakes et al (34), hyponatremia did not develop in 70% of the athletes who overconsumed fluids and had an increase in TBW. This indicates that other important factors must be operational in the pathogenesis of EAH.…”

“…Therefore, either defects in renal water excretion and/or significant sodium losses or failure to mobilize exchangeable sodium stores may occur in athletes who develop EAH. Furthermore, some athletes develop hyponatremia without appreciable gains in total body weight (34). As discussed next, these athletes may have significant sodium losses or also may have gained net body free water as a result of the metabolism of glycogen and triglycerides and not as a result of ingestion.…”

“…As described previously, this seems to occur by the ingestion of hypotonic fluids (water or sports drinks) in excess of sweat, urine, and insensible (mainly respiratory and gastrointestinal) losses. In a seminal study, Noakes et al (34) described a linear relationship with a negative slope between the serum sodium after racing and the degree of weight change in 2135 athletes ( Figure 2). The primary cause of this weight gain during exercise must be the consumption of fluids during exercise.…”

Extrarenal Na + Balance, Volume, and Blood Pressure Homeostasis in Intact and Ovariectomized Deoxycorticosterone-Acetate Salt Rats

“…Finally, we believe that our notion of water-free Na ϩ retention receives support from a recent study in marathon runners and elite athletes. 28 Perturbations of the serum Na ϩ concentration in these athletes could be largely explained by osmotically inactive Na ϩ storage or failure to remove Na ϩ from osmotically inactive Na ϩ reservoirs.…”