What is hyponatremia?
Hyponatremia, or low blood sodium, is a growing concern among medical staff at endurance events such as marathons and long-distance triathlons. While most cases of hyponatremia are mild and may have no symptoms at all, more serious cases can lead to significant medical problems such as seizures, swelling or the brain or even death.
A study done at the 2002 Boston Marathon found that about 13% of athletes participating in the study had mild hyponatremia after the race based on analysis of a blood sample taken at the finish line. Most of these runners had no symptoms but there was one runner died from hyponatremia.
Who is at risk?
Hyponatremia typically occurs in runners and triathletes that drink too much fluid before and during the race. It usually occurs in slower participants because they are able to drink more fluid than they lose in sweat and urine. Several cases have made national headlines including a woman the the 2002 Boston Marathon and more recently, a young mother participating in a radio contest that involved drinking large amounts of water in a short period of time.
Drinking too much water or sports drinks can dilute the level of sodium in the blood. In most cases, the kidneys handle the excess fluid intake by producing more urine. However, in athletes that develop hyponatremia, the kidneys are not able to process the extra fluid.
Hyponatremia Risk Factors
Female runners
Slower runners (> 4 hours for marathon)
Lower body-mass or weight
Drinking more than 3 liters during race
Drinking at every aid station
From Christopher SD, et al “Hyponatremia among Marathon Runners in the Boston Marathon” , NEJM
Avoiding Hyponatremia
Hyponatremia can be prevented by avoiding excessive fluid intake during the race. The goal is to try to match your fluid loss through sweating. Trying to take in fluids that have a higher sodium content may help prevent hyponatremia, although we have seen some cases of hyponatremia in triathletes that only drank sports drinks, and not water, during the race.
