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2000 Publications

FLUID AND ELECTROLYTE STATUS DURING A 100 MILE RUN IN EXTREME HEAT.by Glace BW, Murphy CA, McHugh MPNicholas Institute of Sports Medicine and Athletic Trauma, New York, NY - last modified 2013-02-10 00:00
Medicine and Science in Sports and Exercise. 32: S58, 2000.

 

Ultramarathoners are at risk for fluid and electrolyte disturbances, and are reported to experience a high incidence of hyponatremia. We monitored intake and serum chemistries of 26 runners competing in an 100 mile foot race in temperatures which peaked >35°C. Food intake was determined by interview with runners approximately every 8 miles. Anthropometrics were determined 12 hrs pre-race, at mile 44 and at the finish; blood was drawn pre-race, at mile 55 and immediately post-race.

Twenty-three runners completed at least 55 miles, and 13 finished 100 miles in 26.2 +/- 0.7 hrs. Finishers ingested 19.4 +/- 1.6 L fluid and 16.4 +/- 1.9 gms sodium [Na]. Fluid and electrolyte intake during the first half of the race was similar between those that finished the race and those that did not. Body weight was unchanged over time [ANOVA, p=0.52]. Serum Na tended to fall from 143 to 140 mEq/L during the race [p=0.06], and was inversely correlated with weight loss [p=0.009].

Most of the runners experienced nausea or vomiting; these symptoms were not related to serum sodium levels. Hyponatremia [<135 mEq/L] was seen in 1 runner at mile 55, but resolved by mile 100. Urinary sodium decreased [p=0.002] as serum aldosterone increased pre- to post- race[ p<0.001. Plasma volume [Strauss equation] decreased during the first half by 4% but increased by 14% over the entire race.

Despite extraordinary heat stress, runners consumed adequate fluid to maintain body weight although dietary sodium fell far short of the recommended 1 gm/hr. Expansion of plasma volume and a trend to decrease serum Na were consistent with excessive consumption of free water and inadequate intake of sodium during a running event lasting greater than 24 hours in hot and humid conditions.