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Hyponatremia in Endurance Athletes

By Bob Seebohar

water The sometimes hot and humid environments that endurance events are held at can contribute greatly to the success of an athlete finishing the race or not.  This type of environmental stress combined with an athlete’s quantity and quality of sweat, can spell trouble for the athlete and is certain to result in hyponatremia unless a sound nutrition plan centered on proper fluid and sodium intake are followed.

What Is It?

Hyponatremia is a disorder in fluid-electrolyte balance that results in an abnormally low plasma sodium concentration.

Symptoms of Hyponatremia

  • Gastrointestinal discomfort
  • Nausea and vomiting
  • Throbbing headache
  • Restlessness
  • Lethargy
  • Confusion
  • Respiratory distress
  • Seizures
  • Brainstem herniation
  • Death

Physiological Ranges of Plasma Sodium

  • Normal: 136-142 mmol/L
  • Mild: 125-135 mmol/L
  • Severe: <125 mmol/L

The risk of developing complications from hyponatremia depend somewhat on the measured level of plasma sodium in the body. There have been many studies that have measured these ranges during and after exercise and although the physiological ranges of plasma sodium will provide the fitness professional an idea of the severity of hyponatremia, the numbers do not always tell the whole story. 

For example, athletes have survived hyponatremia when their plasma sodium concentration was in the “severe” category and others have died when their levels were above the “severe” category.  There is no rhyme or reason to this, however, understanding the condition and the athlete will help.  When working with an at-risk athlete, it is important to factor these variables into the overall nutrition plan:

  • Length of training or race
  • Sweat rate (i.e.-heavy or light sweater)
  • Sweat sodium content (i.e.-salty or non-salty sweater)

By knowing these three things ahead of time, the endurance athlete can implement strategies that will help reduce the risk of developing hyponatremia.  Hyponatremia cannot be prevented during exercise but the risk can be reduced by proper planning. 


There are many factors that can cause hyponatremia but the most popular is excessive fluid intake.  While some researchers believe hyponatremia is associated with fluid overload, others believe it is associated with dehydration.  What is important to realize is that prolonged sweating can cause significant sodium losses and interdepartmental fluid shifts, as seen with fluid overload and dehydration, may predispose an athlete to hyponatremia.  The balance of fluid intake and timing becomes of utmost importance, which is what many researchers can agree upon.


Prevention of hyponatremia must include a combination of identifying if an athlete is at-risk and if so, educating them about how to properly plan to try to prevent hyponatremia. 

Determining sweat rate and sweat sodium content is a good first place to start.  If these are high and salty, then the athlete can be predisposed to hyponatremia during long exercise sessions, especially in the heat and humidity. 

Once an athlete is identified as at-risk or not, the shift should then be to education.  The following strategies can be used as part of an athlete’s nutrition plan:

  • Consume enough sodium during exercise.  Many athletes do not consume enough sodium during exercise.  This is not a prescription for a high sodium diet.  Rather, consuming enough sodium during training or racing can be of benefit.
  • Drink accordingly based on sweat rate.  If an athlete does not know their sweat rate, general recommendations include drinking 3-8 ounces of a sports drink every 15-20 minutes.  If an athlete has had their sweat rate measured, then a more specific fluid and sodium intake plan can be made.
  • Avoid overdrinking.  Athletes must adhere to their fluid and sodium intake plan that has been predetermined and they must understand the importance of drinking too much.  Gaining weight during exercise is a sure warning sign of excessive drinking. 
  • Limit pre-hydration with just water.  This can lower blood sodium before the event begins and already puts the athlete behind in their sodium intake plan.  Eat high salt foods or drinks with higher amounts of sodium just before training or racing.
  • Don’t overdrink after training or competition. At-risk athletes should get into the habit of weighing before and after exercise to determine their fluid needs.  Aim to drink 24 ounces of sodium rich fluid or foods high in sodium along with water for every pound lost during the event. 
Knowing the inherent risks and symptoms of hyponatremia is important for endurance athletes. Identifying if you are at-risk for becoming hyponatremic is extremely important so proper implementation strategies can be developed based on sweat rate and sodium loss.

Bob Seebohar, MS, RD, CSSD, CSCS is a sport dietitian and elite triathlon coach. He traveled to the 2008 Summer Olympics as the U.S. Olympic Committee Sport Dietitian and the personal Sport Dietitian for the 2008 Olympic Triathlon Team. He has served as head coach for Sarah Haskins, 2008 Olympian, was a performance team member (sport dietitian and strength coach) for Susan Williams, 2004 Olympic Triathlon bronze medalist. He is the current coach of Jasmine Oeinck, 2009 Elite National Champion.

Bob's new book, Metabolic Efficiency Training: Teaching the Body to Burn More Fat, will teach athletes how to structure their nutrition and training program throughout the year to maximize their body's ability to use fat as energy and improve body composition.  For more information and to order the book, visit or contact Bob at