title: Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners.
abstract: OBJECTIVE
  This work combines and reanalyzes 5 yr of exercise-associated hyponatremia (EAH) research at 161-km ultramarathons in northern California with primary purposes to define the relationship between postrace blood sodium concentration ([Na]) and change in body weight; to examine the interactions among EAH incidence, ambient temperature, and hydration state; and to explore the effect of hydration status on performance.
  
  
  METHODS
  Prerace and postrace body weight and finish time data were obtained on 887 finishers, and postrace [Na] was also obtained on a subset of 669 finishers.
  
  
  RESULTS
  EAH incidence was 15.1% overall (range, 4.6%-51.0% by year) and had a significant positive relationship with ambient temperature. Of the runners with EAH, 23.8% were classified as overhydrated (weight change, ≥0), 40.6% were euhydrated (weight change, <0% to -3%), and 35.6% were dehydrated (weight change, <-3%) at the finish. There was a weak significant relationship (r = 0.17, P < 0.0001) between postrace [Na] and change in body weight such that a lower [Na] was more common with increased weight loss. Considering all finishers examined, 18.5% were dehydrated and 34.9% were overhydrated at the finish. There was a weak significant relationship (r = 0.092, P = 0.006) between change in body weight and performance in that faster runners tended to lose more weight. Top finishers varied in body weight change from ∼1% gain to ∼6% loss.
  
  
  CONCLUSIONS
  EAH incidence can be high in 161-km ultramarathons in northern California. In this environment, EAH is more common with dehydration than overhydration and is more common in hotter ambient temperature conditions. Because weight loss >3% does not seem to have an adverse effect on performance, excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst are ill advised.
authors.affiliation: Department of Veterans Affairs, Northern California Health Care System, Mather, CA 95655-1200, USA. martin.hoffman@va.gov
authors.forename: Martin D
authors.initials: MD
authors.lastname: Hoffman
authors.forename: Tamara
authors.initials: T
authors.lastname: Hew-Butler
authors.forename: Kristin J
authors.initials: KJ
authors.lastname: Stuempfle
mesh_terms: D001834:Body Water / Q000502:physiology; D002140:California; D003681:Dehydration / Q000209:etiology*; D006801:Humans; D007010:Hyponatremia / Q000209:etiology*; D010807:Physical Endurance / Q000502:physiology; D012420:Running / Q000502:physiology*; D013696:Temperature; D013997:Time Factors; D014883:Water-Electrolyte Imbalance / Q000209:etiology; D015431:Weight Loss / Q000502:physiology