Moderate Dehydration Effects On Running Economy Quantification Methods

Any mass-specific quantification of prolonged running economy deterioration may be underestimated by the presence of dehydration. This may be especially true for small runners, for whom an equivalent loss of mass equates to a relatively larger percentage of body mass. PURPOSE: This study sought to compare the mass-specific oxygen cost (OC) and energy cost (EC) of running with, and without adjustments for dehydration. METHODS: Eight experienced female marathon runners (37 ± 5 yrs; 158 ± 9 cm; 53.3 ± 7.8 kg) completed a 30 km treadmill run (166 ± 24 min) at 1% grade in a warm environment (29.9 °C, 55%rh) with 2 m/s airflow. Ad libitum fluid intake was recorded at 5 km intervals and subtracted from the measured total mass loss, interpolated linearly over the same intervals. The mass specific OC (ml/kg/km) and EC (kcal/kg/km) per unit distance of running were obtained from 2-minutes of steady-state VCO2 and VO2 measures at each 5 km interval and compared to uncorrected mass (i.e., initial mass). EC and OC values corrected for interpolated fluid losses were compared to non-corrected values. Dehydration was further characterized via venous blood taken before and after the 30 km run. RESULTS: Sweat rate (1.12 ± 0.28 L/h) exceeded fluid intake (0.70 ± 0.31 L/h), producing moderate dehydration (- 2.8 ± 1.1% body mass). Plasma volume was reduced (- 4.3 ± 2.6%) and plasma osmolality increased (12 ± 7 mOsm/kg) commensurately. Main effects of dehydration (2-way RM ANOVA) were found for OC (+3.3 ml/kg/km; p = 0.008) and EC (0.016 kcal/kg/km; p = 0.008), though both were of small magnitude (~1.5%). No interaction was found, but the largest effect magnitude was observed at 30 km (OC, 6 ml/kg/km; EC, 0.03 kcal/kg/km, ~3%). CONCLUSIONS: Small but significant changes in OC and EC of running occur when exercise in the heat is prolonged, and dehydration is moderate. In warm to hot environments with moderate or greater fluid losses, OC and EC should use interpolated body mass changes to avoid over correction. Under shorter, less stressful conditions, corrections may not be necessary.