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Hyponatremia in Endurance Athletes

In the 1998 New Zealand Ironman triathlon, 18 percent of finishers were diagnosed with hyponatremia. This trend is becoming more and more common. The reason: environment + salty sweaters + high sweat rate + improper nutrition.

Hot and humid environments can contribute greatly to the success of an athlete finishing the triathlon 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 is followed.

What Is It?

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

Physiological Ranges of Plasma Sodium

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:

By knowing these three things ahead of time, the fitness professional can provide strategies that will help to reduce the risk of the athlete 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. With excessive fluid intake, there is an increased risk of developing hyponatremia because urine production is usually decreased. This reduces the body’s ability to excrete excess water. In addition, sodium loss via sweat is increased, which makes it easier to dilute the body’s sodium content.

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 athletes who are at-risk and educating these athletes about how to properly plan to try to prevent hyponatremia.

Identification of at-risk athletes should be the first step. That is, you need to determine an athlete's sweat rate and sweat sodium content. If these are high and salty, then the athlete is predisposed to hyponatremia during long exercise sessions, especially in the heat and humidity.

Once an athlete is identified as at-risk or not, education should be the focus. The following strategies should be discussed with the athlete and should be part of his nutrition plan:


Knowing the inherent risks and symptoms of hyponatremia is important when working with athletes. Identifying athletes who are at-risk for becoming hyponatremic is extremely important and proper education strategies can be developed based on whether or not they are a heavy and/or salty sweater. Sports drinks should be favored over water during exercise in order to provide the athlete the necessary fluid, carbohydrates and sodium. The flavor of sports drinks is formulated so that the athlete wants to drink more and therefore constantly re-hydrates and replenishes the sodium lost through sweat.


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