First reported cases of exercise-associated hyponatremia in Asia

Journal Title (Medline/Pubmed accepted abbreviation): Int. J. Sports Med.
Year: 2011
Volume: 32
Page numbers: 297-302

Summary of Background and Research Design

Background: Exercising for long periods of time (hrs), especially in hot, humid environments, puts athletes at risk for low sodium concentrations in the blood (hyponatremia). Symptoms of hyponatremia include confusion, agitation, and other neural problems and can be life-threatening. Not intuitively, one of the main causes of exercise-associated hyponatremia (EAH) is over-ingestion of water. The fluids can over-replace the water that has been lost to sweat and dilute the body’s total salt concentration. It is expected that most athletes will lose some weight during exercise because athletes typically don’t ingest enough fluid to fully replace fluid losses. If an athlete gains weight, it indicates too much pure water was consumed. Such a situation can occur in events in which athletes avidly hydrate themselves, but have sweat losses that don’t require such emphasis on hydration (e.g., prolonged low intensity exercise/slow running time at relatively cool temperatures).

Research questions: During the Adidas Sundown Marathon 2009 in Singapore, how many athletes that were admitted to the medical tent suffer from symptoms of EAH? What was the change in body weight of athletes that finished both the 42-km and 84-km races?

Subjects: Out of the almost 5,000 athletes who completed the race, 417 provided pre- and post-race body weights. From those that are represented here, 303 (12.5% females) ran the 42-km race and 114 (7.0% females) ran the 84-km race.

Protocol: The temperature during the race was 28.3-30.2°C (82.9-86.4ºF) with a relative humidity of 86-93% (high). Athletes ran an outdoor loop that was 42 km long beginning at midnight. Those running the 84 km race ran the 42 km loop twice and began at 7:00pm, 5 hrs before the 42 km racers. There were 20 stations along the route that provided sugar/electrolyte sports drinks and water. (Therefore, there were 40 opportunities for fluids for the 84 km racers.) Athletes were weighed within 1 hr of commencing the race and immediately after they finished. Blood samples were acquired from athletes who were admitted into the medical tent and were assessed for sodium, potassium, chloride, and blood urea nitrogen.

Summary of research findings
  • Out of the 8 athletes that were treated at the medical tent, 3 had blood sodium concentrations less than 135 mM and were therefore diagnosed with EAH.
  • For the 42-km racers, the body mass change was -1.6 ± 1.2% and was similar for men and women. A small percentage of racers (7%) gained weight, indicating risk of hyponatremia.
  • For the 84-km racers, the change in body mass was -2.3 ± 1.7% while 8% of the runners gained weight. The results were similar when comparing men to women.
  • There was no correlation found between finishing time and amount of weight lost.
  • One EAH victim placed 5th place out of 602 contenders before being admitted to the medical tent, indicating that EAH is not necessarily obvious right away.

Interpretation of findings/Key practice applications

It is important for endurance athletes to know the high risk and severity of EAH, take preventative measures (by consuming fluids and electrolytes during exercise), and to recognize symptoms (ex. slow running) early to prevent exacerbation of the condition. Many athletes are unaware that drinking too much water (in excess of urinary and sweat losses) can be dangerous. The authors state that it is safe to lose up to 2% body weight during exercise, but it has not yet been shown that drinking sports drinks can prevent EAH.
                           The authors recommend that race organizers should provide scales for the athletes to be used before, after, and even during the race so that they can monitor their change in body weight and prevent weight gain during exercise.


A more complete analysis of the hydration and sodium status of these athletes would have included: data from all of the racers, the amount of fluid consumed during the race, the athletes’ hydration status before and after the race, and blood sodium concentrations for all of the participants. More data should be acquired during other similar races in order to determine effects of temperature, time of day, wind, etc. on the risk of EAH.
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