HYPONATREMIA IN ASYMPTOMATIC MARTHONERS

Abstract

PURPOSE: A prospective study of marathoners in the 2001 Houston Marathone undertaken to investigate the influence of relative and absolute fluid consumption, gender, and NSAID use on the development of hyponatremia. METHODS: Runners who finished with [Na]≤135(HypoNa) were compared with those > 135 (NormNA) for changes in [Na], clinical symptoms, body weight (BW), Hct, cups of fluid consumed (TCUPS), TCUPS per hour (TCUP/hr), and TCUPS per pre-race weight (TCUP/kg). Groups were also analyzed by gender and NSAID use. BW and vital signs were obtained and [Na] were measured using a portable blood analyzer immediately before and after the race for each subject. TCUPS data and NSAID use was determined by a post race questionnaire. RESULTS: The wet bulb globe temperature was 58°F at the start and 69°F at the finish. 45 runners, 22 women and 23 men, participated in the study. All subjects had normal pre-race [Na] (mean 142.6). 40/45(89%) subjects had lower post race [Na] (mean 138.1). 10/45 (22%) were HypoNa (Na≤135). All runners were asymptomatic. 9/10 HypoNa were women. Mean [Na] change for HypoNa was −9.6 and −0.37 for NormNa (p < 0.001). TCUP for HypoNa was greater (mean 27.7) than NormNa (mean 10) (p < 0.05). Estimates of relative fluid consumption and rate of consumption were also greater for HypoNa (TCUP/kg mean 0.39 vs. 0.22; p < 0.05 and TCUP/hr mean 6 vs. 4.2; p < 0.05). HypoNa had less weight loss (mean 0.19kg vs. 2.35kg; p < 0.05). Overall, women had less weight loss (mean −0.46kg vs. −1.98kg; p < 0.001), a greater change in post race [Na] (mean −5.6 vs. −3.4; p < 0.05), and lower post race Hct. (mean 43.2 vs. 47.0; p < 0.05). There was no gender differences in TCUP (mean 21.4 vs. 22.5; NS). NSAIDs were used by 51% (50% of females and 52% of males) of subjects. 6/10 of HypoNa used NSAIDs. There were NS differences for any variable when comparing NSAID users vs. non-users. CONCLUSION: 89% of subjects had lower [Na] levels after completing a marathon in relatively mild weather conditions. 10 of these subjects had [Na] ≤135, considered clinically as hyponatremic; but were asymptomatic. Clearly, hyponatremic subjects consume more fluid and lose less weight. Women appear to be more likely to have greater [Na] loss and to develop hyponatremia during a marathon regardless of TCUPS. NSAID use was not found to be correlated with hyponatremia.