No exercise-associated hyponatremia found in an observational field study of male ultra-marathoners participating in a 24-hour ultra-run
Journal Title (Medline/Pubmed accepted abbreviation): Phys Sportsmed
Year: 2010
Volume: 38
Number: 4
Page numbers: 94-100
doi: 10.3810/psm.2010.12.1831

Summary of Background and Research Design

Background: Exercise-associated hyponatremia (EAH) occurs when there is excessive water intake during exercise and is defined by a serum or plasma sodium concentration below the normal range of the testing laboratory, typically < 135 mmol/L. Among marathon runners, the overall prevalence of EAH is 22% and depends on the number and gender of participants as well as their fitness level. Ultra-marathon runners may be at an increased risk for EAH because they run at a slower pace for a longer time, allowing for more frequent fluid intake. Research investigating EAH in ultra-marathon runners is scarce. One factor that appears to influence incidence of EAH is the host country (EAH is more common in races held in the United States than in South Africa, Australia, New Zealand, or Switzerland), although the reason for this has not been established.

Hypothesis/purpose of study:To investigate the prevalence of EAH in male ultra-marathon runners participating in a 24-hour race in Basel, Switzerland. The hypothesis was that EAH prevalence would be higher compared with marathon runners and there would be an increase in body weight.

Subjects:22 experienced male runners (~ 35 runs each) volunteered to participate. 15 men completed the race without a break; 7 men did not complete the race.

Experimental design: Observational field study.

Treatments and protocol: Participants ran 1-km laps for 24 hours, with a refreshment stand at the end of the lap. Runners recorded their fluid intake during the race. Within 3 hours of race start and 1 hour of race end, body weight was measured and blood and urine samples were collected for assessment of hematocrit, plasma volume, sodium concentration, and urine specific gravity.

Summary of research findings:
  • Race completers ran a mean distance of 180.7 km at an average speed of 7.5 km/hour.
    • A total of 15.1 L of fluid was consumed by participants (0.62 L/hr).
  • Based on an alternative definition of EAH (= 5 mmol/L decrease in resting plasma sodium concentration), no participant developed EAH.
    • Plasma sodium concentration did not change from baseline (135.3 mmol/L) to race end (135.4 mmol/L).
  • Fluid intake negatively correlated with average running speed during the race (P < .0001).
    • Body weight change, sodium concentration, and urine specific gravity were not related to fluid intake.
  • Overall, there was a significant 2.2 kg decrease in body weight (P = .0009).
    • Body weight change was not associated with sodium concentration.
  • Urine specific gravity increased from baseline to race end (P = .0005).
    • Body weight change was not associated with change in urine specific gravity.
  • Plasma volume increased 4.9% from baseline to race end.
    • Sodium concentration was not associated with plasma volume change.
  • Hematocrit did not change from baseline to race end

Interpretation of findings/Key practice applications:

In contrast to results of a previous study, this study did not find any cases of EAH among ultra-marathon runners. Because body weight is an indicator of both fluid intake and retention, it was expected that ultra-marathon runners would gain weight. In this study, the ultra-marathon runners lost weight. Additionally, fluid intake (0.6 L/hr) in the participants did not exceed the recommendations for endurance athletes.
Inexperience and fitness level is also a factor for EAH, and the athletes in this study had extensive running experience, decreasing their risk of developing EAH. Interestingly, the runners were dehydrated according to the typical definition (decreased weight and increased urine specific gravity); however, they may have been overly hydrated as indicated by the increased plasma volumes and stable sodium concentrations. It could be possible that the decreased weight in this study was from a decrease in fat and skeletal muscle mass instead of fluid loss. Moreover, research has indicated that other factors such as vasopressin and aldosterone may be regulating fluid homeostasis in ultra-marathon runners, and that increased activity of these hormones is related to EAH in this population (not indicated by weight). However, this study did not measure vasopressin or aldosterone activity. Future research should include these hormonal activity assessments.


The intake of sodium and other nutrients was not measured in this study. It would have been interesting to see dietary intake data in relation to blood sodium levels. It is also notable that 7 of the 22 runners did not complete the race. The reasons for dropout included both exhaustion and heat stroke (number of runners with each not stated). Thus, it appears possible that at least a few of the runners did experience problems with hydration status. Physiologic data from those runners not completing the race would have aided in the interpretation of the results of this study.
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