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Hyponatremia in distance athletes: How to avoid water intoxication

By Jon Englund, MD, Sports Fellowship Trained Family Practice Physician

Hyponatremia is a condition where the concentration of sodium (i.e., Na) in the blood becomes dangerously low (<135 mmol/L). In the distance athlete, it most commonly occurs by overzealous water consumption resulting in the dilution of blood and a decreased sodium concentration. Rarely, it can be caused by sodium depletion due to excessive sweating in conjunction with fluid replacement from a liquid that has a lower sodium concentration than the athlete's own sweat.

Although most hyponatremia is mild, its occurrence is relatively common. The incidence of hyponatremia has been measured at 5-18% of participants in U.S. marathons over the last several years. This number appears to be decreasing as education about hyponatremia improves.

Who's at risk for hyponatremia?

  • Distance athletes participating in events longer than 3-4 hours
  • Slower athletes having more time to consume fluids assume an even greater risk
  • Inexperienced participants, often educated to "drink plenty of fluids to stay hydrated" exhibit a higher risk
  • Participants in races with a high availability of drinking fluids are at increased risk
  • Additionally, low body weight, female sex, and use of non-steroidal anti-inflammatories have been found to be risk factors.

Unfortunately, symptoms of hyponatremia are similar to those of other medical conditions that affect distance athletes (i.e., heat illness, dehydration and exercise-associated collapse). Symptoms of hyponatremia include fatigue, light-headedness, weakness, cramping, nausea and headaches. Symptoms of severe hyponatremia include confusion/disorientation, seizures and coma.

The diagnosis of hyponatremia involves an exam to screen for the other conditions listed above. Patients with a normal rectal temperature and without signs of dehydration need a rapid blood test to check their sodium concentration. Rapid blood tests can be done at race sites with a turn around time typically less than 10 minutes.

Mild hyponatremia can be treated with the avoidance of water intake and the liberal use of salt in the post-race setting. Moderate to severe hyponatremia is treated by the intravenous administration of hypertonic saline (i.e., fluid with a high concentration of sodium).

The key to prevention of hyponatremia is to avoid overdrinking. Since sweat rates and sweat sodium concentration vary greatly among individuals and are affected by environmental conditions, it is difficult to make recommendations about how much fluid one should consume during competition.

There are 2 strategies generally recommended to determine how much fluid one needs to replace. The 1st strategy is to drink to thirst. This is a reasonable approach assuming that fluids can be easily accessed throughout the competition. The 2nd strategy is to calculate your individual optimal hydration rate. This can be done using a method described on the USA track and field Web site.

Healthy tips

  • Consumption of salty snacks before and during the race can help protect against hyponatremia.
  • Drinking sports drinks does not eliminate the risk of hyponatremia. Although sports drinks have some sodium, the concentration is still less than the sodium concentration of blood. "Overdrinking" of sports drinks can still cause hyponatremia. However, they do have some electrolytes, including sodium and carbohydrates, and are recommended as the drink of choice (vs. plain water) for distance exercise lasting longer than 1 hour.

For questions on hyponatremia or other sports medicine topics, call the Aurora Sports Medicine Hotline at 414-219-7776 or 800-219-7776.