Can Increased Sodium Intake Reduce Exercise-associated Hyponatremia?

Abstract

The primary risk factor for exercise-associated hyponatremia is overhydration, but other factors can play a role, including Na+ losses and intakes. PURPOSE: To test the efficacy of a CHO-containing (6%) beverage with 60 vs. 21 mmol/L on plasma [Na+] when consumed to nearly match mass loss, while cycling in the heat. METHODS: In a randomized, crossover study, 10 males (VO2max 60 ± 3 mL/min/kg, 75 ± 5 kg, 20-50 y) cycled (55% VO2max) in the heat (34 °C) for 3 h while consuming a beverage containing either 21 mmol/L Na+ (L) or 60 mmol/L Na+ (H). Beverage volume was based on body mass changes in 1 h pre-trials under similar conditions. Blood was sampled every 30 min. Tc, HR, and body mass were monitored. Linear mixed effects model analysis of variance was conducted with Subject as a random effect, and Diet and Time as fixed effects. T-tests were conducted on single measures. RESULTS: Not all subjects completed 3 h of exercise, with mean exercise time being 175 min on both L and H. Mean fluid intake was (3.3 ± 0.6 L, 3.4 ± 0.7 L, resp.; p=0.45). Body mass change was (-0.39 ± 0.31, -0.20 ± 0.36 kg, resp.; p=0.09). Plasma sodium decreased over time on L, whereas increased slightly on H (p< 0.001), with mean rate of change being (-0.6 ± 0.6 mmol/L/h, 0.6 ± 0.6 mmol/L/h, resp.; p =0.057). At ride completion, four subjects were hyponatremic (plasma [Na+] < 135 mmol/L) on L, and only one on H. No differences in end Tc (38.6 ± 0.8, 38.6 ± 0.8 °C, resp.; p=0.90) or end HR (147 ± 31, 157 ± 24 bpm, resp.; p=0.39) were observed. CONCLUSION: Consumption of a beverage with sodium concentration of 60 mmol/L has advantage over one with a concentration typically found in sports drinks (21 mmol/L), preventing plasma sodium reduction during long-lasting exercise in the heat.