title: Women hydrate more than men during a marathon race: hyponatremia in the Houston marathon: a report on 60 cases.
abstract: OBJECTIVE
  To examine the relationship between gender and the development of hyponatremia in marathon runners.
  
  
  METHODS
  A retrospective analysis of prerace and postrace data collected on 117 runners completing the Houston Marathon from 2000 to 2003.
  
  
  METHODS
  The Houston Marathon.
  
  
  METHODS
  A total of 117 marathon runners (63 male and 54 female) who consented to participate in hyponatremia research.
  
  
  METHODS
  Prerace and postrace body weight and serum sodium ([Na+]) concentrations were measured. Total fluid intake was self-reported immediately following the race.
  
  
  RESULTS
  : Of the runners tested, 28% developed hyponatremia ([Na+] < or = 135 mmol/L). Hyponatremic runners (n = 33) drank significantly more fluid (31.70 versus 18.90 cups; P < 0.001), lost the least weight (-0.14 versus -1.61 kg; P < 0.001), and dropped serum [Na+] levels further (-7.48 versus -1.92; P < 0.001) compared with nonhyponatremic runners. Female runners (n = 54) were significantly lighter (62.46 versus 80.73 kg; P < 0.001), ran slower (303.02 versus 269.06 minutes; P < 0.001), lost the least weight (-0.62 versus -1.68 kg; P < 0.001), dropped serum [Na+] levels further (-4.44 versus -2.67; P < 0.01), and had lower postrace serum [Na+] values (136.87 versus 138.50; P < 0.01) compared with male runners while consuming the same total amount of fluid during the race (22.87 versus 22.30 cups; P = 0.83, NS). There were significant inverse relationships between serum [Na+] change versus body weight change (r = -0.65; P < 0.001) and between postrace [Na+] versus body weight change (r = -0.60; P < 0.001), with significant sex differences noted only between nonhyponatremic female and male runners (-0.91 versus -0.2.05 kg; P < 0.001) and between hyponatremic and nonhyponatremic male runners (-0.11 versus -2.05 kg; P < 0.001).
  
  
  CONCLUSIONS
  Female marathon runners drink more fluid than male runners in proportion to body size. A loss of 3 kg body weight corresponds to a 0 change in serum [Na+] from prerace to postrace, suggesting that a loss of 3 kg during a marathon represents euhydration and not dehydration. All cases of hyponatremia reported in this study are a result of overhydration based on this convention.
authors.affiliation: MRC/UCT Research Unit for Exercise Science and Sports Medicine, Department of Human Biology, University of Cape Town Sport Science Institute of South Africa, Newlands, South Africa. thew@sports.uct.ac.za
authors.forename: Tamara D
authors.initials: TD
authors.lastname: Hew
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D000886:Anthropometry; D004326:Drinking; D005260:Female; D005440:Fluid Therapy / Q000379:methods*; D005500:Follow-Up Studies; D006801:Humans; D007010:Hyponatremia / Q000453:epidemiology* / Q000209:etiology; D015994:Incidence; D008297:Male; D008875:Middle Aged; D010807:Physical Endurance; D011336:Probability; D012189:Retrospective Studies; D018570:Risk Assessment; D012420:Running / Q000502:physiology*; D012494:Sampling Studies; D012737:Sex Factors; D013781:Texas; D014883:Water-Electrolyte Imbalance / Q000453:epidemiology / Q000209:etiology