BOSTON, MASSACHUSETTS. Hyponatremia is an abnormally low concentration of sodium in body fluids. It is a major cause of death, life-threatening seizures or coma among long-distance runners. Previous studies have implicated excessive fluid intake, but have been small or restricted to runners presenting for medical attention.
Harvard Medical School researchers have undertaken the largest study to date of hyponatremia in marathon runners to identify its incidence and principal risk factors. They approached runners in the 2002 Boston marathon before the race and recruited 766 elite and non-elite runners. Demographic information and level of training for the race were determined, and they were weighed at the start and finish. Blood samples were taken after the race, along with data on fluid intake. Analysis showed that thirteen per cent of the runners had hyponatremia (a serum sodium concentration of 135 mmol or less per liter) at the finish line. Of these, 0.6 per cent had critical hyponatremia (120 mmol or less per liter). Excessive fluid intake was found to be the most important factor contributing to hyponatremia. Runners who lost weight during the race were at much lower risk. A race time of over 4 hours was also significant - it led to over seven times the risk of hyponatremia compared with a time under 3.5 hours. A low body mass index (less than 20) also increased the risk. Contrary to earlier studies, women did not develop hyponatremia significantly more than men. Also, the type of drink consumed (water or sports drink) was not a factor.
The researchers conclude that hyponatremia occurs in a substantial portion of non-elite marathon runners (racing time over 4 hours), and estimate that it affected approximately 1,900 runners in the Boston marathon. They suggest that the public be better informed about the risk factors that are linked to this potentially life-threatening condition.
Experts from the University of Texas Southwestern Medical Center add that marathon running is relatively
safe, with less than one death in 50,000, usually due to congenital heart disease. But they believe that the
global recommendation for fluid replacement during racing may not be optimal for all runners, and individual
fluid-replacement schedules are to be encouraged.