Hyponatremia among runners in the Zurich Marathon

Clin J Sport Med. 2008 Jul;18(4):344-9. doi: 10.1097/JSM.0b013e31817e3515.


Objective: Hyperhydration and exercise-associated hyponatremia (EAH) are critical issues during endurance events. We studied a cohort of marathon runners to examine EAH's prevalence in a marathon with a short time limit and to investigate underlying mechanisms that may be responsible for its development.

Design: Observational cohort study.

Setting: 2006 Zurich Marathon (cool and rainy weather, time limit of 5 hours).

Participants: 167 marathon runners were recruited the month before the race.

Main outcome measures: Body mass, plasma sodium, and osmolality were measured just before the start and immediately after the race. Fluid intake during the race was ascertained by a recall questionnaire.

Results: Five subjects (3 %) developed asymptomatic EAH, and no symptomatic EAH was found. Body mass change during the race correlated similarly with postrace sodium levels (r = -0.72, P < 0.0001) and with sodium change during the race (r = -0.66, P < 0.0001). Postrace sodium levels correlated significantly with sodium change during the race (r = 0.74, P < 0.0001). Fluid intake correlated significantly (r = -0.43, P < 0.0001) with plasma sodium change between the start and finish of the race. Postrace sodium levels and postrace osmolality were significantly correlated (r = 0.68, P < 0.0001).

Conclusion: In this study we observed a relatively low incidence of EAH in subjects running the marathon in around 2.5 to 5 hours and in a cool environment. Plasma sodium change during the race and postrace sodium levels correlated with body mass change. There was also a direct correlation between fluid intake and plasma sodium change during the race.

MeSH terms

  • Adult
  • Cohort Studies
  • Drinking
  • Female
  • Humans
  • Hyponatremia / diagnosis
  • Hyponatremia / epidemiology
  • Hyponatremia / physiopathology*
  • Incidence
  • Male
  • Middle Aged
  • Running / physiology*
  • Switzerland / epidemiology
  • Water-Electrolyte Balance / physiology*