title: Hyponatremia among runners in the Zurich Marathon.
abstract: 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.
  
  
  METHODS
  Observational cohort study.
  
  
  METHODS
  2006 Zurich Marathon (cool and rainy weather, time limit of 5 hours).
  
  
  METHODS
  167 marathon runners were recruited the month before the race.
  
  
  METHODS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Agricultural and Food Sciences, ETH Zurich, Zurich, Switzerland. samuel.mettler@inw.agrl.ethz.ch
authors.forename: Samuel
authors.initials: S
authors.lastname: Mettler
authors.forename: Carmen
authors.initials: C
authors.lastname: Rusch
authors.forename: Walter O
authors.initials: WO
authors.lastname: Frey
authors.forename: Lukas
authors.initials: L
authors.lastname: Bestmann
authors.forename: Caspar
authors.initials: C
authors.lastname: Wenk
authors.forename: Paolo C
authors.initials: PC
authors.lastname: Colombani
mesh_terms: D000328:Adult; D015331:Cohort Studies; D004326:Drinking; D005260:Female; D006801:Humans; D007010:Hyponatremia / Q000175:diagnosis / Q000453:epidemiology / Q000503:physiopathology*; D015994:Incidence; D008297:Male; D008875:Middle Aged; D012420:Running / Q000502:physiology*; D013557:Switzerland / Q000453:epidemiology; D014882:Water-Electrolyte Balance / Q000502:physiology*