Exercise-Associated Hyponatraemia

Rehydration during Endurance Exercise: Challenges, Research, Options, Methods

During endurance exercise, two problems arise from disturbed fluid-electrolyte balance: dehydration and overhydration. The former involves water and sodium losses in sweat and urine that are incompletely replaced, whereas the latter involves excessive consumption and retention of dilute fluids. When experienced at low levels, both dehydration and overhydration have minor or no performance effects and symptoms of illness, but when experienced at moderate-to-severe levels they degrade exercise performance and/or may lead to hydration-related illnesses including hyponatremia (low serum sodium concentration). Therefore, the present review article presents (a) relevant research observations and consensus statements of professional organizations, (b) 5 rehydration methods in which pre-race planning ranges from no advanced action to determination of sweat rate during a field simulation, and (c) 9 rehydration recommendations that are relevant to endurance activities. With this information, each athlete can select the rehydration method that best allows her/him to achieve a hydration middle ground between dehydration and overhydration, to optimize physical performance, and reduce the risk of illness.

Wilderness Medical Society Clinical Practice Guidelines for the Management of Exercise-Associated Hyponatremia: 2019 Update

Exercise-associated hyponatremia (EAH) is defined by a serum or plasma sodium concentration below the normal reference range of 135 mmol·L-1 that occurs during or up to 24 h after prolonged physical activity. It is reported to occur in individual physical activities or during organized endurance events conducted in environments in which medical care is limited and often not available, and patient evacuation to definitive care is often greatly delayed. Rapid recognition and appropriate treatment are essential in the severe form to increase the likelihood of a positive outcome. To mitigate the risk of EAH mismanagement, care providers in the prehospital and in hospital settings must differentiate from other causes that present with similar signs and symptoms. EAH most commonly has overlapping signs and symptoms with heat exhaustion and exertional heat stroke. Failure in this regard is a recognized cause of worsened morbidity and mortality. In an effort to produce best practice guidelines for EAH management, the Wilderness Medical Society convened an expert panel in May 2018. The panel was charged with updating the WMS Practice Guidelines for Treatment of Exercise-Associated Hyponatremia published in 2014 using evidence-based guidelines for the prevention, recognition, and treatment of EAH. Recommendations are made based on presenting with symptomatic EAH, particularly when point-of-care blood sodium testing is unavailable in the field. These recommendations are graded on the basis of the quality of supporting evidence and balanced between the benefits and risks/burdens for each parameter according to the methodology stipulated by the American College of Chest Physicians.

Is Sodium Supplementation Necessary to Avoid Dehydration During Prolonged Exercise in the Heat?

The primary purpose of this work was to gain further insight into the need for sodium supplementation for maintenance of appropriate hydration during prolonged exercise under hot conditions. Participants of a 161-km ultramarathon (ambient temperature reaching 39° C) underwent body weight measurements immediately before, during, and after the race, and completed a postrace questionnaire about supplemental sodium intake and drinking strategies during 4 race segments. The postrace questionnaire was completed by 233 (78.7%) race finishers. Significant direct relationships were found for percentage weight change during the race with intake rate (r = 0.18, p = 0.0058) and total amount (r = 0.24, p = 0.0002) of sodium in supplements. Comparing those using no sodium supplements throughout the race (n = 15) with those using sodium supplements each race segment (n = 138), body weight change across the course showed significant group (p = 0.022), course location (p < 0.0001), and interaction (p = 0.0098) effects. Posttests revealed greater weight loss at 90 km (p = 0.016, -3.2 ± 1.6% vs. -2.2 ± 1.5%, mean ± SD) and the finish (p = 0.014, -3.2 ± 1.5% vs. -1.9 ± 1.9%) for those using no sodium supplements compared with those using sodium supplements each segment. Six runners who used no sodium supplements, drank to thirst, and only drank water or a mixture of mostly water with some electrolyte-containing drink finished with mean weight change of -3.4%. Although the use of supplemental sodium enhanced body weight maintenance, those not using sodium supplements maintained a more appropriate weight than those consistently using sodium supplements. Therefore, we conclude that the supplemental sodium is unnecessary to maintain appropriate hydration during prolonged exercise in the heat.

Wilderness Medical Society practice guidelines for treatment of exercise-associated hyponatremia: 2014 update

Exercise-associated hyponatremia (EAH) is defined by a serum or plasma sodium concentration below the normal reference range of 135 mmol/L that occurs during or up to 24 hours after prolonged physical activity. It is reported to occur in individual physical activities or during organized endurance events conducted in austere environments in which medical care is limited and often not available, and patient evacuation to definitive care is often greatly delayed. Rapid recognition and appropriate treatment are essential in the severe form to ensure a positive outcome. Failure in this regard is a recognized cause of event-related fatality. In an effort to produce best practice guidelines for EAH in the austere environment, the Wilderness Medical Society convened an expert panel. The panel was charged with the development of evidence-based guidelines for management of EAH. Recommendations are made regarding the situations when sodium concentration can be assessed in the field and when these values are not known. These recommendations are graded on the basis of the quality of supporting evidence and balance between the benefits and risks/burdens for each parameter according to the methodology stipulated by the American College of Chest Physicians. This is an updated version of the original WMS Practice Guidelines for Treatment of Exercise-Associated Hyponatremia published in Wilderness & Environmental Medicine 2013;24(3):228-240.

Are we being drowned in hydration advice? Thirsty for more?

Hydration pertains simplistically to body water volume. Functionally, however, hydration is one aspect of fluid regulation that is far more complex, as it involves the homeostatic regulation of total body fluid volume, composition and distribution. Deliberate or pathological alteration of these regulated factors can be disabling or fatal, whereas they are impacted by exercise and by all environmental stressors (e.g. heat, immersion, gravity) both acutely and chronically. For example, dehydration during exercising and environmental heat stress reduces water volume more than electrolyte content, causing hyperosmotic hypohydration. If exercise continues for many hours with access to food and water, composition returns to normal but extracellular volume increases well above baseline (if exercising upright and at low altitude). Repeating bouts of exercise or heat stress does likewise. Dehydration due to physical activity or environmental heat is a routine fluid-regulatory stress. How to gauge such dehydration and - more importantly-what to do about it, are contested heavily within sports medicine and nutrition. Drinking to limit changes in body mass is commonly advocated (to maintain ≤2% reduction), rather than relying on behavioural cues (mainly thirst) because the latter has been deemed too insensitive. This review, as part of the series on moving in extreme environments, critiques the validity, problems and merits of externally versus autonomously controlled fluid-regulatory behaviours, both acutely and chronically. Our contention is that externally advocated hydration policies (especially based on change in body mass with exercise in healthy individuals) have limited merit and are extrapolated and imposed too widely upon society, at the expense of autonomy. More research is warranted to examine whether ad libitum versus avid drinking is beneficial, detrimental or neither in: acute settings; adapting for obligatory dehydration (e.g. elite endurance competition in the heat), and; development of chronic diseases that are associated with an extreme lack of environmental stress.

Effective body water and body mass changes during summer ultra-endurance road cycling

Because body mass change (ΔMb) does not represent all water losses and gains, the present field investigation determined if (a) ΔMb equalled the net effective body water change during ultra-endurance exercise and (b) ground speed and exercise duration influenced these variables. Thirty-two male cyclists (age range, 35-52 years) completed a 164-km event in a hot environment, were retrospectively triplet matched and placed into one of three groups based on exercise duration (4.8, 6.3, 9.6 h). Net effective body water loss was computed from measurements (body mass, total fluid intake and urine excreted) and calculations (water evolved and mass loss due to substrate oxidation, solid food mass and sweat loss), including (ΔEBWgly) and excluding (ΔEBW) water bound to glycogen. With all cyclists combined, the mean ΔMb (i.e. loss) was greater than that of ΔEBWgly by 1200 ± 200 g (P = 1.4 × 10(-18)), was similar to ΔEBW (difference, 0 ± 200 g; P = .21) and was strongly correlated with both (R(2) = .98). Analysis of equivalence indicated that ΔMb was not equivalent to ΔEBWgly, but was equivalent to ΔEBW. Due to measurement complexity, we concluded that (a) athletes will not calculate the effective body water calculations routinely and (b) body mass change remains a useful field-expedient estimate of net effective body water change.

Exercise-associated hyponatremia in an ultra-endurance mountain biker: a case report

Symptomatic exercise-associated hyponatremia (EAH), which is relatively common among marathon runners, is an uncommon event among ultra-endurance athletes. A 44-year-old man presented to the emergency department with increased thirst after successfully completing a 100-mile mountain bike race in Leadville, Colorado. Initial laboratory tests revealed a blood sodium level of 116 mEq/L. The primary etiologic factor in EAH is fluid consumption in excess of fluid losses in prolonged exertion. Early diagnosis and management is crucial to prevent cerebral and pulmonary edema.

A faster running speed is associated with a greater body weight loss in 100-km ultra-marathoners

In 219 recreational male runners, we investigated changes in body mass, total body water, haematocrit, plasma sodium concentration ([Na(+)]), and urine specific gravity as well as fluid intake during a 100-km ultra-marathon. The athletes lost 1.9 kg (s = 1.4) of body mass, equal to 2.5% (s = 1.8) of body mass (P < 0.001), 0.7 kg (s = 1.0) of predicted skeletal muscle mass (P < 0.001), 0.2 kg (s = 1.3) of predicted fat mass (P < 0.05), and 0.9 L (s = 1.6) of predicted total body water (P < 0.001). Haematocrit decreased (P < 0.001), urine specific gravity (P < 0.001), plasma volume (P < 0.05), and plasma [Na(+)] (P < 0.05) all increased. Change in body mass was related to running speed (r = -0.16, P < 0.05), change in plasma volume was associated with change in plasma [Na(+)] (r = -0.28, P < 0.0001), and change in body mass was related to both change in plasma [Na(+)] (r = -0.36) and change in plasma volume (r = 0.31) (P < 0.0001). The athletes consumed 0.65 L (s = 0.27) fluid per hour. Fluid intake was related to both running speed (r = 0.42, P < 0.0001) and change in body mass (r = 0.23, P = 0.0006), but not post-race plasma [Na(+)] or change in plasma [Na(+)] (P > 0.05). In conclusion, faster runners lost more body mass, runners lost more body mass when they drank less fluid, and faster runners drank more fluid than slower runners.

Higher prevalence of exercise-associated hyponatremia in female than in male open-water ultra-endurance swimmers: the 'Marathon-Swim' in Lake Zurich

We investigated the prevalence of exercise-associated hyponatremia (EAH) in 25 male and 11 female open-water ultra-endurance swimmers participating in the 'Marathon-Swim' in Lake Zurich, Switzerland, covering a distance of 26.4 km. Changes in body mass, fat mass, skeletal muscle mass, total body water, urine specific gravity, plasma sodium concentration [Na(+)] and haematocrit were determined. Two males (8%) and four females (36%) developed EAH where one female was symptomatic with plasma sodium [Na(+)] of 127 mmol/L. Body mass and plasma [Na(+)] decreased (p < 0.05). The changes in body mass correlated in both male and female swimmers to post-race plasma [Na(+)] (r = -0.67, p = 0.0002 and r = -0.80, p = 0.0034, respectively) and changes in plasma [Na(+)] (r = -0.68, p = 0.0002 and r = -0.79, p = 0.0039, respectively). Fluid intake was neither associated with changes in body mass, post-race plasma [Na(+)] or the change in plasma [Na(+)]. Sodium intake showed no association with either the changes in plasma [Na(+)] or post-race plasma [Na(+)]. We concluded that the prevalence of EAH was greater in female than in male open-water ultra-endurance swimmers.

Exercise-associated hyponatremia during winter sports

Exercise-associated hyponatremia (EAH) is hyponatremia that occurs <or= 24 hours after prolonged physical activity. It is a potentially serious complication of marathons, triathlons, and ultradistance events, and can occur in hot and cold environments. Clear evidence indicates that EAH is a dilutional hyponatremia caused by excessive fluid consumption and the inappropriate release of arginine vasopressin. Cerebral and pulmonary edema can cause serious signs and symptoms, including altered mental status, respiratory distress, seizures, coma, and death. Rapid diagnosis and urgent treatment with hypertonic saline is necessary to prevent severe complications or death. Prevention is based on educating athletes to avoid excessive drinking before, during, and after exercise.

Exercise-associated hyponatremia

Disorders of serum sodium concentration occur commonly in athletes participating in endurance sports. While hypernatremia is the most commonly seen disorder, hyponatremia can occur in 2% to 7% of participants. Hyponatremia is due to a combination of excessive water or hypotonic fluid intake as well as high levels of arginine vasopressin (or anti diuretic hormone), which limits the ability of the kidney to excrete water. Most of these cases are associated with either no or minimal side effects and do not require specific therapy other than close monitoring and fluid restriction. However, a small number of athletes may present with severe and life-threatening hyponatremia associated with cerebral edema and possibly noncardiogenic pulmonary edema. Rapid diagnosis and appropriate therapy of these symptomatic athletes with hypertonic saline is required to prevent severe complications or death. The ability to have rapid on-site measurement of serum sodium concentration greatly facilitates accurate diagnosis and therapy. Prevention is based on widespread education regarding the risks of overhydration and judicious intake of fluids during endurance events.

KEYWORDS

hyponatremia; cerebral edema; therapy; hypertonic saline.

Quantitative analysis of serum sodium concentration after prolonged running in the heat

This study compared measured serum [Na(+)] (S([Na+]); brackets denote concentration) with that predicted by the Nguyen-Kurtz equation after manipulating ingested [Na(+)] and changes in body mass (DeltaBM) during prolonged running in the heat. Athletes (4 men, 4 women; 22-36 yr) ran for 2 h, followed by a run to exhaustion and 1-h recovery. During exercise and recovery, subjects drank a 6% carbohydrate solution without Na(+) (Na(+)0), 6% carbohydrate solution with 18 mmol/l Na(+) (Na(+)18), or 6% carbohydrate solution with 30 mmol/l Na(+) (Na(+)30) to maintain BM (0%DeltaBM), increase BM by 2%, or decrease BM by 2% or 4% in 12 separate trials. Net fluid, Na(+), and K(+) balance were measured to calculate the Nguyen-Kurtz predicted S([Na+]) for each trial. For all beverages, predicted and measured S([Na+]) were not significantly different during the 0%, -2%, and -4%DeltaBM trials (-0.2 +/- 0.2 mmol/l) but were significantly different during the +2%DeltaBM trials (-2.6 +/- 0.5 mmol/l). Overall, Na(+) consumption attenuated the decline in S([Na+]) (-2.0 +/- 0.5, -0.9 +/- 0.5, -0.5 +/- 0.5 mmol/l from pre- to postexperiment of the 0%DeltaBM trials for Na(+)30, Na(+)18, and Na(+)0, respectively) but the differences among beverages were not statistically significant. Beverage [Na(+)] did not affect performance; however, time to exhaustion was significantly shorter during the -4% (8 +/- 3 min) and -2% (14 +/- 3 min) vs. 0% (22 +/- 5 min) and +2% (26 +/- 6 min) DeltaBM trials. In conclusion, when athletes maintain or lose BM, changes in S([Na+]) can be accurately predicted by changes in the mass balance of fluid, Na(+), and K(+) during prolonged running in the heat.

Drinking guidelines for exercise: what evidence is there that athletes should drink "as much as tolerable", "to replace the weight lost during exercise" or "ad libitum"?

The most recent (1996) drinking guidelines of the American College of Sports Medicine (ACSM) propose that athletes should drink "as much as tolerable" during exercise. Since some individuals can tolerate rates of free water ingestion that exceed their rates of free water loss during exercise, this advice has caused some to overdrink leading to water retention, weight gain and, in a few, death from exercise-associated hyponatraemic encephalopathy. The new drinking guidelines of the International Olympic Committee (IOC), recently re-published in this Journal, continue to argue that athletes must drink enough to replace all their weight lost during exercise and to ingest sodium chloride since sodium is "the electrolyte most critical to performance and health". In this rebuttal to that Consensus Document, I argue that these new guidelines, like their predecessors, lack an adequate, scientifically proven evidence base. Nor have they been properly evaluated in appropriately controlled, randomized, prospective clinical trials. In particular, these new guidelines provide erroneous recommendations on five topics. If novel universal guidelines for fluid ingestion during exercise are to be promulgated by important international bodies including the IOC, they should first be properly evaluated in appropriately controlled, randomized, prospective clinical trials conducted under environmental and other conditions that match those found in "out-of-doors" exercise. This, and the potential influence of commercial interests on scientific independence and objectivity, are the two most important lessons to be learned from the premature adoption of those 1996 ACSM drinking guidelines that are not evidence-based. These concerns need to be addressed before the novel IOC guidelines are accepted uncritically. Otherwise the predictable consequences of the premature adoption of the 1996 ACSM guidelines will be repeated.

Hyponatremia in marathon runners due to inappropriate arginine vasopressin secretion

PURPOSE

Exercise-associated hyponatremia (EAH), as defined by a blood sodium concentration [Na+] less than 135 mmol/L, may lead to hypotonic encephalopathy with fatal cerebral edema. Understanding the pathogenetic role of antidiuresis may lead to improved strategies for prevention and treatment.

METHODS

Normonatremic marathon runners were tested pre- and post-race for creatine kinase, interleukin-6, cortisol, prolactin, and arginine vasopressin. Similar testing also was carried out in runners with encephalopathy caused by EAH, including 2 cases with fatal cerebral edema.

RESULTS

Normonatremic runners (n = 33; 2001) with a mean 3% decrease in body weight showed a 40-fold increase in interleukin-6 (66.6 +/- 11.9 pg/mL from 1.6 +/- 0.5 pg/mL, P = .001), which was significantly correlated with increases in creatine kinase (r = 0.88, P = <.0001), cortisol (r = 0.70, P = .0003), and prolactin (r = 0.67, P <.007), but not arginine vasopressin (r = 0.44, P = .07). Collapsed runners with EAH (n = 22; 2004) showed a mean blood urea nitrogen less than 15 mg/dL with measurable plasma levels of arginine vasopressin (>0.5 pg/mL) in 43% of cases. Two marathon runners with fatal cerebral edema additionally showed less than maximally dilute urines (>100 mmol/kg/H2O) and urine [Na+] greater than 25 mEq/L.

CONCLUSIONS

Cases of EAH fulfill the essential diagnostic criteria for the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Runners with hypotonic encephalopathy at subsequent races were treated with intravenous hypertonic (3%) saline on the basis of this paradigm, which resulted in rapid clinical improvement without adverse effects. Release of muscle-derived interleukin-6 may play a role in the nonosmotic secretion of arginine vasopressin, thereby linking rhabdomyolysis to the pathogenesis of EAH.

Evidence-Based Approach to Lingering Hydration Questions

Studies related to fundamental hydration issues have required clinicians to re-examine certain practices and concepts. The ingestion of substances such as creatine, caffeine, and glycerol has been questioned in regards to safety and hydration status. Reports of overdrinking (hyponatremia) also have brought into question the practices of drinking appropriate fluid amounts and the role that fluid-electrolyte balance has in the etiology of heat illnesses such as heat cramps. This article offers a fresh perspective on timely topics related to hydration, fluid balance, and exercise in the heat.

Exercise-associated hyponatremia

Exercise-associated hyponatremia has been described after sustained physical exertion during marathons, triathlons, and other endurance athletic events. As these events have become more popular, the incidence of serious hyponatremia has increased and associated fatalities have occurred. The pathogenesis of this condition remains incompletely understood but largely depends on excessive water intake. Furthermore, hormonal (especially abnormalities in arginine vasopressin secretion) and renal abnormalities in water handling that predispose individuals to the development of severe, life-threatening hyponatremia may be present. This review focuses on the epidemiology, pathogenesis, and therapy of exercise-associated hyponatremia.

Exercise-associated hyponatremia: role of cytokines

Exercise-associated hyponatremia (EAH) has emerged in recent years as a life-threatening complication of endurance sports that may lead to fatal cerebral and pulmonary edema. Defined as a serum sodium concentration <135 mEq/L (1 mEq/L = 1 mmol/L), symptomatic EAH is a dilutional hyponatremia with abnormal fluid retention mediated by decreased urine production, which is a variant of the syndrome of inappropriate antidiuretic hormone secretion. Strategies for prevention and treatment must take into account the pathophysiology underlying this dominant clinical paradigm. Beyond educating runners to drink moderately, monitoring changes in body weight during endurance sports may facilitate the early detection of positive fluid balance characteristic of symptomatic cases. Rapid diagnosis by point-of-care testing indicates the need for fluid restriction in mild cases and emergent treatment with hypertonic (3%) NaCl to reverse acute hypotonic encephalopathy. The efficacy of arginine vasopressin V(2) receptor antagonists warrants study as an alternative treatment to loop diuretics for volume overload in these patients. Nonosmotic stimulation of arginine vasopressin secretion may be mediated in part by enhanced release of muscle-derived interleukin-6 during glycogen depletion, linking exertional rhabdomyolysis to the pathogenesis of EAH.

Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon

CONTEXT

Critical assessment of recommendations that athletes consume additional sodium during athletic events.

OBJECTIVE

To evaluate if sodium supplementation is necessary to maintain serum sodium concentrations during prolonged endurance activity and prevent the development of hyponatraemia.

DESIGN

Prospective randomised trial of athletes receiving sodium (620 mg table salt), placebo (596 mg starch), or no supplementation during a triathlon. The sodium and placebo tablets were taken ad libitum, with the suggested range of 1-4 per hour.

SETTING

The 2001 Cape Town Ironman triathlon (3.8 km swim, 180 km cycle, 42.2 km run).

SUBJECTS

A total of 413 triathletes completing the Ironman race.

MAIN OUTCOME MEASURES

Sodium supplementation was not necessary to maintain serum sodium concentrations in athletes completing an Ironman triathlon nor required to prevent hyponatraemia from occurring in athletes who did not ingest supplemental sodium during the race.

RESULTS

Subjects in the sodium supplementation group ingested an additional 3.6 (2.0) g (156 (88) mmol) sodium during the race (all values are mean (SD)). There were no significant differences between the sodium, placebo, and no supplementation groups with regard to age, finishing time, serum sodium concentration before and after the race, weight before the race, weight change during the race, and rectal temperature, systolic and diastolic blood pressure after the race. The sodium supplementation group consumed 14.7 (8.3) tablets, and the placebo group took 15.8 (10.1) tablets (p = 0.55; NS).

CONCLUSIONS

Ad libitum sodium supplementation was not necessary to preserve serum sodium concentrations in athletes competing for about 12 hours in an Ironman triathlon. The Institute of Medicine's recommended daily adequate intake of sodium (1.5 g/65 mmol) seems sufficient for a healthy person without further need to supplement during athletic activity.

What can be concluded regarding water versus sports drinks from the Vrijens-Reher experiments?

This study assessed whether replacing sweat losses with sodium-free fluid can lower the plasma sodium concentration and thereby precipitate the development of hyponatremia. Ten male endurance athletes participated in one 1-h exercise pretrial to estimate fluid needs and two 3-h experimental trials on a cycle ergometer at 55% of maximum O2 consumption at 34°C and 65% relative humidity. In the experimental trials, fluid loss was replaced by distilled water (W) or a sodium-containing (18 mmol/l) sports drink, Gatorade (G). Six subjects did not complete 3 h in trial W, and four did not complete 3 h in trial G. The rate of change in plasma sodium concentration in all subjects, regardless of exercise time completed, was greater with W than with G (−2.48 ± 2.25 vs. −0.86 ± 1.61 mmol·l−1·h−1, P = 0.0198). One subject developed hyponatremia (plasma sodium 128 mmol/l) at exhaustion (2.5 h) in the W trial. A decrease in sodium concentration was correlated with decreased exercise time ( R = 0.674; P = 0.022). A lower rate of urine production correlated with a greater rate of sodium decrease ( R = −0.478; P = 0.0447). Sweat production was not significantly correlated with plasma sodium reduction. The results show that decreased plasma sodium concentration can result from replacement of sweat losses with plain W, when sweat losses are large, and can precipitate the development of hyponatremia, particularly in individuals who have a decreased urine production during exercise. Exercise performance is also reduced with a decrease in plasma sodium concentration. We, therefore, recommend consumption of a sodium-containing beverage to compensate for large sweat losses incurred during exercise.

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

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 mmol/liter, and 11% a serum [Na+] of <129 mmol/liter. 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.