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

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.