title: Exertional Hyponatremia and Serum Sodium Change During Ultraendurance Cycling.
abstract: BACKGROUND
  Exertional hyponatremia (EH) during prolonged exercise involves all avenues of fluid-electrolyte gain and loss. Although previous research implicates retention of excess fluid, EH may involve either loss, gain, or no change of body mass. Thus, the etiology, predisposing factors, and recommendations for prevention are vague-except for advice to avoid excessive drinking.
  
  
  OBJECTIVE
  This retrospective field study presents case reports of two unacquainted recreational cyclists (LC, 31y and AM, 39 years) who began exercise with normal serum electrolytes but finished a summer 164-km ride (ambient, 34±5°C) with a serum [Na+] of 130 mmol/L.
  
  
  METHODS
  To clarify the etiology of EH, their pre- and post-exercise measurements were compared to a control group (CON) of 31 normonatremic cyclists (mean ± SD; 37±6 years; 141±3 mmol Na+/L).
  
  
  RESULTS
  Anthropomorphic characteristics, exercise time, and post-exercise ratings of thermal sensation, perceived exertion and muscle cramp were similar for LC, AM and CON. These two hyponatremic cyclists consumed a large and similar volume of fluid (191 and 189 ml/kg), experienced an 11 mmol/L decrease of serum [Na+], reported low thirst sensations; however, LC gained 3.1 kg (+4.3% of body mass) during 8.9 hr of exercise and AM maintained body mass (+0.1kg, +0.1%, 10.6h). In the entire cohort (n = 33), post-event serum [Na+] was strongly correlated with total fluid intake (R2= 0.45, p < .0001), and correlated moderately with dietary sodium intake (R2=0.28, p = .004) and body mass change (R2= 0.22, p = .02). Linear regression analyses predicted the threshold of EH onset (<135 mmol Na+/L) as 168 ml fluid/kg.
  
  
  CONCLUSIONS
  The wide range of serum [Na+] changes (+6 to -11 mmol/L) led us to recommend an individualized rehydration plan to athletes because the interactions of factors were complex and idiosyncratic.
authors.forename: Lawrence E
authors.initials: LE
authors.lastname: Armstrong
authors.forename: Elaine C
authors.initials: EC
authors.lastname: Lee
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Casa
authors.forename: Evan C
authors.initials: EC
authors.lastname: Johnson
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Ganio
authors.forename: Brendon P
authors.initials: BP
authors.lastname: McDermott
authors.forename: Jakob L
authors.initials: JL
authors.lastname: Vingren
authors.forename: Hyun M
authors.initials: HM
authors.lastname: Oh
authors.forename: Keith H
authors.initials: KH
authors.lastname: Williamson
mesh_terms: D000328:Adult; D056352:Athletes; D054874:Athletic Performance; D001642:Bicycling; D015331:Cohort Studies; D003163:Competitive Behavior; D004326:Drinking; D015444:Exercise; D006358:Hot Temperature / Q000009:adverse effects; D006801:Humans; D007010:Hyponatremia / Q000097:blood / Q000209:etiology* / Q000517:prevention & control; D008297:Male; D009120:Muscle Cramp / Q000209:etiology / Q000517:prevention & control; D010807:Physical Endurance; D005082:Physical Exertion; D012189:Retrospective Studies; D012964:Sodium / Q000097:blood; D012982:Sodium, Dietary / Q000627:therapeutic use; D064133:Sports Nutritional Physiological Phenomena; D013781:Texas; D013894:Thirst; D014887:Weather
chemical_list: D012982:Sodium, Dietary; D012964:Sodium