Quantitative analysis of serum sodium concentration after prolonged running in the heat

Abstract

This study compared measured serum [Na+] (S[Na+]; brackets denote concentration) with that predicted by the Nguyen-Kurtz equation after manipulating ingested [Na+] and changes in body mass (ΔBM) during prolonged running in the heat. Athletes (4 men, 4 women; 22–36 yr) ran for 2 h, followed by a run to exhaustion and 1-h recovery. During exercise and recovery, subjects drank a 6% carbohydrate solution without Na+(Na+0), 6% carbohydrate solution with 18 mmol/l Na+(Na+18), or 6% carbohydrate solution with 30 mmol/l Na+(Na+30) to maintain BM (0%ΔBM), increase BM by 2%, or decrease BM by 2% or 4% in 12 separate trials. Net fluid, Na+, and K+balance were measured to calculate the Nguyen-Kurtz predicted S[Na+]for each trial. For all beverages, predicted and measured S[Na+]were not significantly different during the 0%, −2%, and −4%ΔBM trials (−0.2 ± 0.2 mmol/l) but were significantly different during the +2%ΔBM trials (−2.6 ± 0.5 mmol/l). Overall, Na+consumption attenuated the decline in S[Na+](−2.0 ± 0.5, −0.9 ± 0.5, −0.5 ± 0.5 mmol/l from pre- to postexperiment of the 0%ΔBM trials for Na+30, Na+18, and Na+0, respectively) but the differences among beverages were not statistically significant. Beverage [Na+] did not affect performance; however, time to exhaustion was significantly shorter during the −4% (8 ± 3 min) and −2% (14 ± 3 min) vs. 0% (22 ± 5 min) and +2% (26 ± 6 min) ΔBM trials. In conclusion, when athletes maintain or lose BM, changes in S[Na+]can be accurately predicted by changes in the mass balance of fluid, Na+, and K+during prolonged running in the heat.