Abstract
Exercise associated hyponatremia (EAH) is a blood electrolyte disorder characterised by a low plasma sodium concentration (<135mmol/L) during or following a bout of exercise. Its symptoms not only impair exercise performance and well-being, but can result in serious health consequences in severe cases. The incidence of EAH has increased considerably since its first reported cases in the 1980’s, corresponding with a rising popularity of high-risk endurance-style races and increased messages that encourage an over-consumption of fluids. Previous studies have investigated the effects of sodium supplements in preventing EAH and improving endurance performance; however with discrepancies in the methodology the results are inconclusive. This study aimed to build on the previous research, investigating the effect of sodium supplements on exercise performance and plasma sodium in a 72-km road cycling time-trial. Secondary aims were to assess the effect of sodium supplements on fluid balance and behavioural responses such as thirst.
Nine well-trained cyclists (5 male, 4 female) participated in a randomised, double-blind, crossover study. Participants cycled a hilly 72-km course in temperate conditions, consuming either a 700mg/h salt capsule, or a corn flour placebo. Time-trials were repeated after 7 – 14 days rest with the other intervention. Measurements were taken pre, post, and 30 minutes following the time trials, analysing blood, sweat and urine indices, as well as subjective feelings of thirst and salt cravings.
Sodium supplements had no effect on time-trial performance compared to the placebo (overall time = 171min vs.172min; P=0.46). There was also no effect on plasma sodium concentration during the exercise (relative plasma [Na+] change (pre-post): salt = 0.56%, placebo = 0.47%; P=0.60). The most notable difference was a significantly larger rise in plasma volume (P=0.02), which corresponded with an increased thirst (P=0.17) among those supplemented with sodium. Sodium supplements therefore play no role in preventing EAH or improving performance during exercise of approximately 3h duration in temperate conditions. In these conditions, EAH is most effectively prevented through avoiding the over-consumption of fluids. However, in hotter environments, where sweat sodium losses are greater, sodium supplements may play a more important role.