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.