Sallis, R. E., Longacre, M., & Morris, L (2006). Hyponatremia of exercise in Hawaiian Ironman triathletes. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1042.

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"Hyponatremia of exercise is a frequent condition among Ironman Triathletes and is a common cause of collapse. Defined as a plasma sodium concentration less than 135 mmol/liter, it is most often caused by drinking hypotonic fluid in excess of sweat losses. Slower runners and females have been thought to be most at risk, presumably because they have a greater opportunity to drink excessively".

This study examined the race finish time and gender of athletes presenting to the medical tent with hyponatremia (sodium levels less than 135 mmol/l) during the 2004 Hawaiian Ironman triathlon. Entrants (N = 1727) were grouped into 6 classes based on their race finish times, which were labeled as class I (<10 hours; 7.1% of male entrants and .5% of female entrants), class II (<11 hours; 28% of males and 6.7% of females), class III (<13 hours; 37.6% of males and 47.2% of females), class IV (<15 hours; 12% of males and 26% of females), class V (<17 hours; 7.2% of males and 9.3% of females) and class VI (did not finish; 8.1% of males and 10.2% of females). Hyponatremia levels were defined as mild (130-134), moderate (125-129) and severe (120-124).

Men (N = 40) and women (N = 15) registered symptoms of Hyponatremia. Eighteen (M = 14; F = 4) were found to have plasma sodium levels <135 mmol/l. The 14 males represented 1.08% of the 1297 male entrants, while the four females represented .93% of the 430 female entrants. Of the 14 males with hyponatremia, 10 were mild, 2 were moderate and 2 were severe. Of the 4 females, 3 were mild and 1 was moderate. Correlating the hyponatremia with race finish times, we found that of the 10 males with mild hyponatremia, five were class II, four were class III and one was class IV. Of the two males with moderate hyponatremia, one was class II and one was class III and in the two males with severe hyponatremia, one was class III and one was class VI. Of the three females with mild hyponatremia, all were class III, while the one female with moderate hyponatremia was class VI.

Implication. There is no gender difference in the incidence of Hyponatremia in Ironman entrants. Slower runners seemed to be at no greater risk than fast runners.

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