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 (Delta BM) 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% Delta BM), 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%Delta BM trials (-0.2 +/- 0.2 mmol/l) but were significantly different during the +2%Delta BM 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%Delta BM 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) Delta BM 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.