Effect of sodium chloride supplementation on serum sodium concentration, cardiovascular function, and physical and cognitive performance
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These studies determined the effects of sodium chloride supplementation on serum and sweat sodium concentration, cardiovascular function, and physical and cognitive performance. Sweat sodium losses, alone, caused a significant decline in serum sodium concentration (-6.4±1.6 mEq/L, p=0.001) during 3h cycling in the heat in endurance-trained athletes with high sweat sodium losses. However, sodium chloride supplementation matching sweat sodium losses (NA; 5.9±1.5g NaCl/h) maintained serum sodium concentration. Post-exercise maximal cycling power declined and was significantly lower than pre-exercise in placebo (PL; p=0.012), but power was not significantly different in NA (p=0.057). Pre- to post-exercise response time during a Stroop Test improved in NA (p=0.009), while there was no change in PL (p=0.597). Post-exercise postural sway was less in NA vs. PL (p=0.044). Three days of sodium chloride supplementation (~15 g NaCl/d) resulted in a significant increase in plasma volume in healthy untrained males at rest (5.9±7.6 %) and during exercise at 60%VO₂peak (8.6±5.2 %) compared to PL. During NA, stroke volume was 10% higher during exercise vs. PL (139±27 vs. 126±24 ml/beat, respectively, p=0.004). Cardiac output was 8% higher in NA during exercise vs. PL (21.0±3.1 vs. 19.4±2.6 L/min, respectively, p=0.013). Mean arterial pressure during exercise was not different in NA vs. PL (p=0.548) as total peripheral resistance decreased (p=0.027) with the increased cardiac output. Sweat sodium concentration was 9% higher in NA vs. PL during exercise in the heat (70.4±19.5 vs. 64.5±21.7 mEq/L, p=0.044). In summary, serum sodium concentration declines when high sweat sodium losses are not replaced while hydration status is maintained. Acute sodium chloride supplementation during exercise which matches sodium losses maintains serum sodium concentration. This maintenance of serum sodium concentration results in both physical and cognitive benefits compared to when serum sodium concentration declines. Chronic intake of sodium chloride for 3 days increases plasma volume in healthy untrained men and improves cardiovascular function, as both stroke volume and cardiac output are increased, while oxygen consumption and blood pressure are unchanged. Therefore, acute and chronic sodium supplementation positively alters fluid and sodium balance which results in beneficial effects on physical and cognitive performance and cardiovascular function during exercise.