Fluid-Electrolyte Balance During a Summer Endurance Cycling Event: Two Exertional Hyponatremia Case Reports

Abstract

PURPOSE: The purpose of this field study was to observe the fluid and electrolyte turnover of trained cyclists during a mass participation event in the heat (38±2 °C). We hypothesized that cyclists would not maintain fluid-electrolyte balance. METHODS: Forty-three male and 8 female subjects (mean±SD age, 38±6y) were monitored during the Hotter'N Hell Hundred event in Wichita Falls, Texas. Two of these cyclists became the cases in focus and were compared to the remaining cyclists (controls, CON, n=49). Subjects kept detailed diet records throughout the event, which were later analyzed using dietary analysis software (Nutritionist Pro™). Body mass was measured at the start, 52 km, 97 km, and the finish line (161 km). Pre- and post-race measures of plasma osmolality and sodium also were obtained. Urine specific gravity was monitored and recorded at the start, 52 km, 97 km, and 161 km. RESULTS: All values in the table below (except CON sweat rate, measured from 0 to 57 km, n=16) were recorded at the finish line medical tent.TABLECONCLUSIONS: CON maintained plasma sodium balance but finished the event mildly dehydrated, despite consuming 5.43 L of fluid. However, even though A and B exhibited high sweat rates, their excessive fluid intakes and smaller total body water (i.e., smaller body mass than CON) resulted in borderline, asymptomatic exertional hyponatremia (EHA) with marked hypotonicity. This occurred in subject B despite a large dietary sodium intake, and in A & B despite faster finish times than CON. Although published cases of symptomatic exertional hyponatremia (EHS) are rare, our findings support the existence of EHA during endurance cycling. Because EHA is a precursor to EHS, our data support the need for monitoring body weight changes.