The Effect of Sodium Loading on Plasma Volume and Running Performance in the Heat

Abstract

0574 PURPOSE: To determine if increased plasma volume achieved with ingestion of a highly concentrated sodium solution would enhance running performance in the heat METHODS: Eight male, endurance-trained runners (56.91 ± 4.29 ml/kg/min VO2max; 36.4 ± 13.6 yr) completed this randomised, cross-over experiment. Running trials (70% VO2max; 32°C, 50% RH) to exhaustion were separated by 1 to < 3 wk. Subjects ingested either a low (LoNA: 10 meq Na+/L) or high sodium (HiNA: 164 meq Na+/L) solution (10 ml/kg BW) divided into 7 portions, fed every 10 min. Ingestion took place at 0900h, 2 h after a standardised breakfast. Exercise commenced 25 min after the last bolus. Urine was taken and blood was drawn at rest (−105 min), after drinking (−25 min), prior to exercise (−5 min), and after exercise. RESULTS: Samples were analysed for haemoglobin (Hb), haematocrit (Hct), plasma [Na+], plasma osmolarity and urine osmolarity and specific gravity. Plasma volume (PV) change was calculated from Hct and Hb. Significance of effects of beverage on PV and [Na+] was established by a two-way, repeated measures ANOVA. Time to exhaustion was the performance measure and difference was established by paired t-test. HiNA increased PV over time (p = 0.038) and time to exhaustion (HiNA = 74 ± 32 min; LoNA = 60 ± 30 min; mean ± S.D.) (p = 0 .019). Urine specific density prior to exercise indicated subjects were hydratedFineither core nor skin temperature, plasma [Na+] nor urine osmolarity were different between trials. CONCLUSIONS: Hypervolaemia, achieved with sodium ingestion, prior to exercise enhances endurance running in the heat.