To examine the relationship between gender and the development of hyponatremia in marathon runners.
A retrospective analysis of prerace and postrace data collected on 117 runners completing the Houston Marathon from 2000 to 2003.
The Houston Marathon.
A total of 117 marathon runners (63 male and 54 female) who consented to participate in hyponatremia research.
Prerace and postrace body weight and serum sodium ([Na+]) concentrations were measured. Total fluid intake was self-reported immediately following the race.
Of the runners tested, 28% developed hyponatremia ([Na+] ≤ 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).
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.
From the 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.
Received for publication June 2004; accepted January 2005.
Reprints: Dr. Tamara D. Hew, MRC/UCT Research Unit for Exercise Science and Sports Medicine, Sports Science Institute of South Africa, P. O. Box 115, Newlands 7725 South Africa (e-mail: firstname.lastname@example.org).