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Clinics (Sao Paulo). 2009;64(4):351-6.

Risk of hypothermia in a new Olympic event: the 10-km marathon swim.

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Exercise Physiology Laboratory, Ministry of Health, National Institute of Traumatology and Orthopedics, Rio de Janeiro, RJ, Brazil.



There are no available data addressing the potential clinical risks of open-water swimming competitions.


Address the risks of hypothermia and hypoglycemia during a 10-km open-water swimming competition in order to alert physicians to the potential dangers of this recently-introduced Olympic event.


This was an observational cross-sectional study, conducted during a 10-km open-water event (water temperature 21 degrees C). The highest ranked elite open-water swimmers in Brazil (7 men, 5 women; ages 21+/-7 years old) were submitted to anthropometrical measurements on the day before competition. All but one athlete took maltodextrine ad libitum during the competition. Core temperature and capillary glycemia data were obtained before and immediately after the race.


Most athletes (83%) finished the race with mild to moderate hypothermia (core temperature <35 degrees C). The body temperature drop was more pronounced in female athletes (4.2+/-0.7 degrees C vs. male: 2.7+/-0.8 degrees C; p=0.040). When data from the athlete who did not take maltodextrine was excluded, capillary glycemia increased among athletes (pre 86.6+/-8.9 mg/dL; post 105.5+/-26.9 mg/dL; p=0.014). Time to complete the race was inversely related to pre- competition body temperature in men (r=-0.802; p=0.030), while it was inversely correlated with the change in capillary glycemia in women (r=-0.898; p=0.038).


Hypothermia may occur during open-water swimming events even in elite athletes competing in relatively warm water. Thus, core temperature must be a chief concern of any physician during an open-water swim event. Capillary glycemia may have positive effects on performance. Further studies that include more athletes in a controlled setting are warranted.

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