title: Can changes in body mass and total body water accurately predict hyponatremia after a 161-km running race?
abstract: OBJECTIVE
  To relate changes in body mass, total body water (TBW), extracellular fluid (ECF), and serum sodium concentration ([Na]) from a 161-km ultramarathon to finish time and incidence of hyponatremia.
  
  
  METHODS
  Observational.
  
  
  METHODS
  : The 2008 Rio Del Lago 100-Mile (161-km) Endurance Run in Granite Bay, California.
  
  
  METHODS
  Forty-five runners.
  
  
  METHODS
  Pre-race and post-race body mass, TBW, ECF, and serum [Na].
  
  
  RESULTS
  Body mass and serum [Na] significantly decreased 2% to 3% (P < 0.001) from pre-race to post-race, but TBW and ECF were unchanged. Significant relationships were observed between finish time and percentage change in body mass (r = 0.36; P = 0.01), TBW (r = 0.50; P = 0.007), and ECF (r = 0.61; P = 0.003). No associations were found between post-race serum [Na] and percentage change in body mass (r = -0.04; P = 0.94) or finish time (r = 0.5; P = 0.77). Hyponatremia (serum [Na] < 135 mmol/L) was present among 51.2% of finishers. Logistic regression prediction equation including pre-race TBW and percentage changes in TBW and ECF had an 87.5% concordance with the classification of hyponatremia.
  
  
  CONCLUSIONS
  Hyponatremia occurred in over half of the 161-km ultramarathon finishers but was not predicted by change in body mass. The combination of pre-race TBW and percentage changes in TBW and ECF explained 87.5% of the variation in the incidence of hyponatremia.
  
  
  CONCLUSIONS
  Exercise-associated hyponatremia can occur simultaneously with dehydration and cannot be predicted by weight checks at races.
authors.affiliation: Sports Medicine Program, University of California Davis Medical Center, Sacramento, California, USA. dklebus@gmail.com
authors.forename: Daniel K
authors.initials: DK
authors.lastname: Lebus
authors.forename: Gretchen A
authors.initials: GA
authors.lastname: Casazza
authors.forename: Martin D
authors.initials: MD
authors.lastname: Hoffman
authors.forename: Marta D
authors.initials: MD
authors.lastname: Van Loan
mesh_terms: D000886:Anthropometry; D015992:Body Mass Index; D001834:Body Water / Q000502:physiology*; D002140:California; D045604:Extracellular Fluid; D005544:Forecasting; D006801:Humans; D007010:Hyponatremia / Q000209:etiology*; D019370:Observation; D005082:Physical Exertion / Q000502:physiology*; D012420:Running / Q000502:physiology*; D012964:Sodium / Q000097:blood
chemical_list: D012964:Sodium