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Hydration issues in the endurance athlete are centered on the two components of the extracellular fluid compartments: total body sodium balance and water balance. Exercise-associated hyponatremia (EAH) is defined by serum sodium ([Na+]) concentration <135 millimoles per liter (mmol/L) occurring during or up to 24 hours after physical activity and is severe if <125 mmol/L.1 It is clinically divided into asymptomatic or symptomatic, whereby the latter is often seen with [Na+] <128 mmol/L.2 Dehydration is defined as a deficit in total body water (TBW) with accompanying disruption of metabolic processes and typically results in hypernatremia. Changes in TBW can be best measured in a lab using deuterium oxide; however changes in body weight have been used as a practical surrogate measure with dehydration being defined as body weight loss >3%.3 This is estimated to be about 5% TBW.1 Dehydration must be distinguished from hypovolemia, which pertains to a reduction in plasma volume. Less common electrolyte abnormalities include hypernatremia and hypomagnesmia.1


The etiology of EAH has been described through two generalized models: dilutional and depletional.1,2 The dilutional model proposes that sustained overhydration with hypotonic fluids in the setting of impaired water clearance though inappropriate arginine vasopressin (AVP) secretion results in euvolemic or hypervolemic hyponatremia and is associated with weight gain. The depletional model proposes that excessive loss of under-replaced solute (sodium and potassium) occurs through sweat or impaired renal retention resulting in hypovolemic hyponatremia and is associated with weight loss. It is widely accepted that EAH is multifactorial and is predominately caused by dilutional mechanisms. Thus, when weight is gained during an endurance event, there is greater likelihood that an athlete will be hyponatremic.3 Excessive water loss through hypotonic sweat is the primary cause of dehydration in endurance athletes.

Epidemiology including risk factors and primary prevention

Guidelines in the past for ad libitum fluid intake were aimed at preventing rapid and severe dehydration in extreme conditions. It is now known that the single most important risk factor for EAH is sustained, excessive fluid (water, sports drinks, or other hypotonic fluid) intake in excess of water loss through sweat, respiratory and renal excretion resulting in a positive fluid balance.1 Prevention is primarily aimed at organized educational programs advising athletes to drink to thirst, monitor their body weight, and be cognizant of symptoms of EAH. Pre-event sodium supplementation has proven to have little to no role in prevention.2

Other risk factors include weight gain during exercise, exercise duration >4 hours (i.e., slow pace), event inexperience or inadequate training, high or low body mass index (BMI), and readily available fluids.1 The use of non-steroid anti-inflammatory drugs (NSAIDs) has also been a proposed risk factor presumably due to potentiation of renal water retention.4,5,6 Incidence is greater in females compared to males; however, when adjusting for BMI and racing time, the apparent sex difference is not significant.1 When organizing or supervising an endurance event, reducing the availability of fluids along the routes of exercises could be consider as a strategy to minimize the incidence of hyponatremia. 8

Symptomatic EAH is rare, with severe complications representing <1% of all EAH cases. There have been 14 reported deaths since 1981.1 Over the past decade, EAH deaths have been confirmed in the lay press in high school football players following practice, a soldier on the first day of Ranger training, a policeman participating in a 19 km bike ride, a college student performing calisthenics for a fraternity, a bushwalker, an ironman triathlete, and a canoeist during an ultradistance race. The literature also reports symptomatic cases of EAH after long distance swimming, mountain cycling, yoga, 2h of weightlifting plus tennis, and in an individual with cystic fibrosis after low-intensity lawn bowling.14 More recently, there have been 3 deaths in American football players encouraged to drink copious volumes of hypotonic fluids to relieve exercises-associated muscle cramps (EAMC). It is now recognized that EAMC reflects neurological fatigue rather than dehydration and electrolyte imbalances.1

Cases of asymptomatic EAH (diagnosed through routine screenings for research purposes) have recently been documented in 33% of 10 rugby players following an 80-min match, 70% of 30 elite junior rowers during an extended training period, 11% of 1,089 Ironman triathletes tested post-race, 6% of 33 endurance cyclists tested pre- and post-race, 67% of 15 ultramarathon runners testing during the race, 5% of 161 marathon and half-marathon runners tested pre-race, and 8% of 192 marathon and half-marathon runners tested post-race. Thus, despite increased awareness of the hazards of overdrinking; EAH fatalities, case reports, and incidence rates have spread into a wider variety of sporting activities.14


Dilutional hyponatremia results from total body water expansion relative to total body exchangeable sodium.1 Osmotic gradients result in water shifting into the intracellular compartment leading to cellular edema. Symptoms result from pathologic central nervous system (CNS) tissue expansion and become life threatening with elevated intracranial pressures.

The excretory rate of the kidneys is between 800-1000 milliliters/hour (mL/h) in the normal resting adult and the athlete loses about 500 ml/h during exercise. Thus, fluid consumption at a rate of 1.5 liters/hour (L/h) theoretically should prevent overhydration.2

Osmoreceptors within the circumventricular organ of the brain lack a blood-brain barrier and are in communication with blood and baroreceptors (carotid and aortic arch), thus act as physiological sensors to regulate plasma osmolality and volume by coordinating thirst and AVP secretion.1

AVP is synthesized in the hypothalamus, stored in the posterior pituitary gland, and acts at the V2 receptor in the collecting ducts of the kidney to open aquaporin channels thus reabsorbing water. Under normal circumstances, AVP is suppressed in the presence of hypoosmolality. In some athletes, AVP is not appropriately suppressed (as typified by the finding of inappropriately elevated urine osmolality). This release of AVP leads to water retention in the distal tubule of the kidney and impaired water excretion. Coupled with excessive water intake, inappropriate water retention will lead to hyponatremia.14

Non-osmotic AVP secretion or Syndrome of Inappropriate Antidiuretic Hormone (SIADH) may occur during exercise due to stimuli including elevated body temperature, volume contraction, hypoglycemia, interleukin-6 (IL-6) release, and nausea or vomiting, and medication use (such as NSAIDs and selective serotonin reuptake inhibitors (SSRIs)).1,2,14

Osmotic inactivation and/or impaired mobilization of osmotically inactive sodium stores has also been theorized to play a role in the development of hyponatremia during endurance events.3 It has been hypothesized that athletes who develop EAH either cannot activate the exchangeable pool of sodium in response to sodium losses or alternatively sodium may move into non-osmotically active forms. The mechanisms that control the exchange of sodium between these compartments remain unknown.3,14

In addition, glycogen metabolism may be an important component in the cause of hyponatremia that occurs without weight gain because each kilogram of glycogen can contain upwards of 3 kg of associated metabolic water. 15 As glycogen is metabolized, water is released and if not excreted could lead to depression of the serum sodium.

Moreover, absorption of water retained in the gastrointestinal system at the end of a race or exercise has been suggested to cause acute drop in serum sodium concentration. This may explain a “transient lucid period” followed by an acute presentation of EAHE about 30 minutes after stopping physical activity.16

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

Serum [Na+] is not always a good predictor of symptoms. Asymptomatic EAH is a biochemical finding and athletes may only experience mild transient complaints that are typical of any endurance event. Mild symptomatic EAH may present with non-specific signs and symptoms such as light headedness, headaches, dizziness, nausea, bloating, puffiness, and/or increase in body weight.1,2,15 These athletes usually do not improve in the Trendelenburg position, and they do not have signs of encephalopathy. Serum [Na+] levels should be measured as they may acutely progress to severe EAH or life-threatening exercise-associated hyponatremic encephalopathy (EAHE). EAHE is characterized by headache, vomiting, altered sensorium, seizures, dyspnea or frothy sputum (non-cardiogenic pulmonary edema), decorticate posturing or mydriasis (signs of brainstem herniation), and/or coma as sequelae of cerebral edema and brainstem herniation.1

Dehydration levels greater than 8% may cause athletes to experience severe thirst and dry mouth. If the level of dehydration reaches 15-20%, serious health consequences such as tachycardia, hypotension, and even death may result.7

Specific secondary or associated conditions and complications

Athletes with asymptomatic EAH are at risk for developing delayed-onset symptomatic EAH up to 24 hours after the event.

A secondary complication of symptomatic EAH is CNS-triggered non-cardiogenic pulmonary edema, which may require supplemental oxygen or intubation if adequate oxygen saturation is not maintained.

Performance decrements and cardiovascular strain have been documented when baseline body fluid volume decreases are >2%.

Essentials of Assessment


Signs and symptoms can overlap between EAH, dehydration, heat illness, or acute altitude illnesses. Correct diagnosis of EAH versus dehydration guides appropriate management and athlete outcome, therefore EAH should be considered in differential diagnosis of an individual that has been or is currently participating in strenuous activity or prolonged exercise.16

Evaluation should include history of present illness, past medical history (with attention to hypertension, diabetes, hyperlipidemia, heart failure, chronic liver or kidney disease, and/or neurologic insults), supplements or medications (with attention to diuretics, antihistamines, anti-hypertensives, lithium, SSRIs and NSAIDs), pre- and post-race weight, amount of fluid and food ingested during the event and what type, urine production (clear or dark and amount), vomiting or diarrhea, shortness of breath, history of problems in past races, and amount of training prior to current event.

Physical examination

Physical exam focuses on the central nervous system, pulmonary system, and cardiovascular system. Assessment of vital signs is essential. Tachycardia and hypotension are associated with dehydration, whereas vital signs (including body temperature) are usually not grossly abnormal in hyponatremia. A rectal temperature should be obtained to most accurately measure core body temperature in cases of suspected concomitant exertional heat-related illness as this may impact initial treatment protocol.8 Basic cardiopulmonary exams may provide important information. Unconscious patients should be assessed for abnormal posturing and pupillary responses. In the conscious athlete, a brief mental status exam can be performed if there is question for altered sensorium. Evaluate for peripheral and pulmonary edema. Weight gain or loss can be used to assess water balance and may provide indication of serum [Na+]. Physical signs of dehydration include dry mucous membranes, poor skin turgor, sunken eyes, and delayed capillary refill.

Increased vulnerability to orthostatism is a common phenomenon after ultra-endurance races that does not necessarily correlate with dehydration status, although the mechanism is not completely understood. 17

Functional assessment

Presentation of EAH varies depending on severity, ranging from asymptomatic to seizures to coma, and death. Dehydration also has a wide range in presentation including mental status changes (confusion, behavioral changes including increased aggression).9

It is still controversial whether dehydration causes impairment in executive function and “mental readiness”, reason why different tests have been used for research, with controversial results. Differences have been noted in endurance versus track-and-field events, with no changes in endurance athletes after the race17, and reported perceived tiredness, alertness, confusion, fatigue, anger, or depression in track-and field18.

Laboratory studies

The Second International Exercise Associated Hyponatremia Consensus Development Conference was that “medical directors should ensure the availability of on-site serum [Na+]  analysis”.19 On-site serum [Na+] can be rapidly obtained within minutes using the i-STAT® handheld blood analysis system. Medical teams at most endurance events are equipped with i-STAT® analyzers and accompanying testing cartridges. Depending on which cartridges are available, renal function labs may also be obtained which can provide information on hydration status. There may be, however, instances where patients are suspected of having EAH and serum [Na+] cannot be obtained (see treatment section for management in this circumstance), especially in wilderness activities.16

Pseudohyponatremia should be considered. Glucometers can rapidly determine blood glucose levels providing important information considering serum [Na+] is artificially lowered (i.e., pseudohyponatremia) by hyperglycemic states and must be corrected using correction factors.10 Other causes of pseudohyponatremia include hyperlipidemia and hyperproteinemia, both of which need to be determined through laboratory blood panels.10

In the hospital setting, serum and urine osmolality should be obtained. As stated earlier, deuterium oxide is used to measure changes in TBW but is not practical for on-site testing. Further laboratory work-up can include Copeptin, AVP, aldosterone levels for electrolyte regulation, and urine lactate levels for hydration correlation. These laboratory tests require more studies to determine their diagnostic and treatment value (refer to the “Cutting Edge/emerging and Unique Concepts and Practice” section for further information).


Neuroimaging such as computerized tomography (CT) of the brain may be obtained once an athlete with EAHE is at an advanced medical care facility, however this must not interrupt acute treatment.1

Supplemental assessment tools

Having scales available for monitoring body weight should be considered when organizing an endurance event. Athletes should obtain their baseline event-day weight as it acts as a surrogate measurement of body fluid balance. If pre-race weight is not obtained, the history of the athlete’s normal weight is a good substitute. Lack of weight loss or weight gain during an endurance event is a positive indicator of fluid overload and possible hyponatremia, and fluid intake should be reduced.1 Some weight loss (1.5-2.5 kilograms) is expected during an endurance race, but should not exceed 2% of body weight as this increases the risk for dehydration.11 However, some athletes have shown to have better results despite more than 2% body weight loss in endurance races.17,20,21,22 Recent studies suggest tailoring recommendations to individual performance, but more studies need to be done for better understanding of safety and performance guidelines.

Early predictions of outcomes

Severity of initial presenting symptoms and not absolute serum [Na+] can predict outcome and should guide therapy.1 Any athlete suspected for EAH should have rapid determination of serum [Na+]. With appropriate recognition and management, the vast majority of asymptomatic or mildly symptomatic EAH or dehydration cases resolve without long-term sequelae. For both EAH and dehydration, loss of consciousness or disorientation suggests more severe abnormalities and should be acted upon emergently.

Social role and social support system

Athletes with asymptomatic EAH or those treated for mild symptomatic EAH should be discharged from the event’s medical assessment area with a companion to monitor for the development of neurological symptoms that would prompt immediate medical attention.1


Extreme environmental temperatures can affect the likelihood of developing either dehydration or EAH. Cold may elevate the osmotic set-point for secretion of AVP, especially when age >65 years.12 Non-acclimatized athletes may require hydration beyond thirst drive in temperatures >38ºC (100.4ºF) to improve performance and prevent dehydration.12 Prolonged exercise in warmer climates has suggested excessive-sodium losses as the primary mechanism for EAH, however evidence to date is limited.2

When organizing or supervising an endurance event, reducing the availability of fluids along the routes of exercises could be consider as a strategy to minimize the incidence of hyponatremia.23

Professional Issues

Medical personnel at the sports events should be aware of proper treatment of EAH and differential diagnoses, as incorrect treatment can further compromise serum [Na+] and further deteriorate the athlete condition. Medical directors should consider education via pre-race briefings/webinars and by suggesting reading material or educational podcasts/videos.16

Good Samaritan laws vary among states, however, generally only cover doctors who are also bystander fans. Physicians might be participating and can help out in an emergency on the course if Good Samaritan laws are in place. If a doctor is an official race event volunteer, then additional malpractice insurance should be obtained. Recently insurance policies have been developed specifically for volunteer medical teams.

Rehabilitation Management and Treatments

Available or current treatment guidelines at different disease stages

Treatment should be determined by the degree of neurological impairment and not simply by serum [Na+], because both the magnitude and rate of development of hyponatremia influence brain edema.1 On-site treatment guidelines for hyponatremia are now organized by whether serum [Na+] has or has not been confirmed by measurement and whether the athlete is asymptomatic, symptomatic, or in severe EAHE.1,2

If EAH is confirmed by [Na+] measurement, and there are no neurological symptoms beyond headache, limiting fluid intake until onset of urination is typically sufficient. A salty snack or anoral hypertonic fluid such salty soup or bouillon, especially in those with [Na+] <130 mmol/L, may also be used to resolve hyponatremia. Observe for 60 minutes until symptoms resolve and educate on neurologic signs and symptoms of EAH.  Discharge athletes from the medical area with a companion and advise to seek immediate medical attention, should neurologic signs and symptoms develop. In the case of severe EAH or EAHE, emergent treatment with IV hypertonic saline (100 mL 3% bolus) to decrease intracranial pressure. is needed. Up to three 100-mL of 3% hypertonic saline boluses at 10-minute intervals may be given.

On-site treatment of suspected EAH but not confirmed by serum [Na+] measurement is challenging as symptoms are similar to that of volume depletion and heat illness. Consider the differential diagnosis and weigh risk versus benefit of fluid restriction. Hypotonic fluids should be restricted in suspected EAH with consideration of the potential harm that could result from fluid restriction if the diagnosis is incorrect. If there are symptoms of severe EAH or EAHE, and the clinical suspicion is high, empiric treatment with rapid infusion of IV hypertonic saline should be initiated emergently to prevent further neurological deterioration. As EAH is an acute and not chronic process, there is no risk of osmotic demyelination. If EAHE is wrongly assumed, a bolus of hypertonic saline has minimal negative consequences, even in the case of hypovolemia or hypernatremia, and may provide benefit as a volume expander. When serum [Na+] cannot be determined and field treatment has not been successful, emergency transport to a definitive care facility should be expedited.

Coordination of care

It is critical that medical personnel in emergency transport services are aware of the athlete’s diagnosis, prior treatments, and the importance of avoiding isotonic or hypotonic fluid resuscitation which would worsen hyponatremia. The patient may need management in an intensive care unit (ICU) if they do not improve.

Measurement of treatment outcomes

There are no current guidelines necessitating ongoing serum  [Na+] measurement during management for EAH as treatment is dictated primarily by symptoms, however this may be practiced in asymptomatic or mild symptomatic EAH athletes before discharge to ensure that the serum [Na+] is at least >130 mmol/L.

Patient & family education

Prevention through education is the main goal and requires broad programs targeted to athletes, coaches, trainers, and parents emphasizing the importance of appropriate hydration practices, recognition of EAH signs and symptoms, and urgency of therapy.1 They must understand that excessive fluid intake will not prevent muscle cramps and exertional heat stroke. This education, along with management protocols, must also reach onsite, emergency, and hospital medical personnel.

Some practical recommendations from the International Marathon Director’s Association include:12

  1. Drink to thirst. Obey the body’s natural physiological cues.
  2. Water, sodium, and glucose should be available at fluid replacement stations spaced 1.6 km (minimum) to 5 km (maximum) apart.
  3. Calibrated scales along a marathon course should be at the discretion of the medical team and weight loss of >4% or any weight gain constitutes justification for medical consultation.
  4. Sports drinks contain less sodium than body fluids and can worsen EAH.13

Translation into practice: practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills

It is important to be aware of factors that contribute to over hydration, as the most common cause of EAH is sustained excessive fluid intake greater than volume lost. These risk factors include exercise duration >4 hours (i.e., slow pace), event inexperience or inadequate training, and readily available fluids.1 Athletes should be counseled to “drink to thirst” in order to avoid overhydration. Finely tuned osmoreceptors and baroreceptors regulate both plasma osmolality and circulating volume through thirst and AVP secretion. Hence using thirst as guide is most likely to avoid overhydration. During exercise, the recommendation is to drink no more than 500-1200 mL/h, depending on training level and environmental factors, but thirst is the best gauge for hydration. Sports drinks should not be used to maintain serum [Na+], and can worsen EAH,13 as all sports drinks are hypotonic to plasma.1

Cutting Edge/ Emerging and Unique Concepts and Practice

The increasing usage of i-STAT® handheld blood analysis systems has advanced on-site care of EAH by improving its early recognition. Recent evidence suggests that some athletes mobilize sodium stores more than others, yet this is not well understood.3 A small prospective observational study by Whatmough et al. 24 demonstrated that marathon runners who used an NSAID before or during the marathon had decrease in serum [Na+] while the serum [Na+] increased in those who did not use an NSAID.

Gaps in the Evidence- Based Knowledge

Current controversy exists over the hypovolemic variant of EAH, particularly the contribution of sodium loss through hypotonic sweat and role of brain natriuretic peptide in urinary sodium losses. Current guidelines are to treat these individuals with IV hypertonic saline bolus, followed by IV normal saline to replete volume.1 Other areas of future investigation include the role of diet and sodium supplementation, success of the “drink to thirst” strategy, recurrence rates of EAH, long-term health implications, possible genetic markers, the role of NSAIDs, investigating alternative treatments for mild EAH, and the variability in serum [Na+] and body weight in the days leading up to an event and at event start.1


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Original Version of the Topic

Robert Irwin, MD, Michelle D. Francavilla, MD. Hydration Issues in the Athlete and Exercise Associated Hyponatremia. 12/28/2012.

Previous Revision(s) of the Topic

Richard G. Chang, MD, Jameel J Khan, MD. Hydration Issues in the Athlete and Exercise Associated Hyponatremia. 8/25/2016.

Author Disclosures

Julio Vazquez-Galliano, MD
Nothing to Disclose

Daniela Mehech, MD
Nothing to Disclose

Cecilia Cordova Vallejos, MD
Nothing to Disclose

Jasal Patel, MD
Nothing to Disclose