FLUID AND ELECTROLYTE STATUS DURING A 100 MILE RUN IN EXTREME HEAT.

Glace BW, Murphy CA, McHugh MP

Nicholas Institute of Sports Medicine and Athletic Trauma, New York, NY

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.

Medicine and Science in Sports and Exercise. 32: S58, 2000.