Fluid and Electrolyte Balance in Ultra-Endurance Running

Abstract

1238 Severe hyponatremia (serum sodium levels <130 mEq/L) is a frequently observed phenomenon in ultra-endurance performance. However, the underlying mechanisms are poorly defined. PURPOSE: To evaluate fluid and electrolyte balance during the 2003 Spartathlon, a 246 km continuous running race. METHODS: A total of 53 runners, 48 male and 5 female (43.9 ± 1.2 y and 70.5 ± 1.1 kg), participated in the study. Blood was drawn before, at 100 km and immediately after the race for the measurement of serum electrolytes, osmolality and changes in plasma volume. Body weight was also recorded at the aforementioned time points and a food and fluid frequency questionnaire was administered pre-race to evaluate runners' plans for fluid and dietary consumption of electrolytes during the race. RESULTS: Twenty seven subjects managed to complete the race in 32.5 ± 0.6 h. Severe hyponatremia was observed in 5 finishers (18.5%) with serum sodium 128.0 ± 0.7 mEq/L vs. 133.5 ± 0.7 in non-hyponatremic runners, p<0.001. Finishing time was not different for hyponatremic runners, compared to nonhyponatremic runners. Five runners were found to be hyponatremic at 100 km of the race with three of them managing to complete the race. Percent changes in body weight at the end of the race was −2.9 ± 1.7 % for the hyponatremic runners, while exhibiting a dramatic increase in plasma volume of 30.4 ± 10.9% and a decrease in serum osmolality at the end of the race. Non-hyponatremic runners decreased their body weight more than hyponatremic runners (−5.5 ± 0.4%, p<0.001), while plasma volume showed a modest decrease. Sodium levels in non-hyponatremic runners tended to be preserved during the race. Potassium levels decreased during the race only in non-hyponatremic runners. Hyponatremic runners reported greater fluid consumption and lower sodium intake (12.6 ± 1.6 g vs. 23.7 ± 3.8 g) during the race. CONCLUSIONS: Data suggest that hyponatremia of ultra-endurance exercise may be related to lower sodium intake and greater fluid load. Additionally, ultra-endurance runners might be resistant to hyponatremia developed during the first 1/3 of the race (100 km) without hampering performance. Supported by the Gatorade Sports Science Institute