Sodium-free fluid ingestion decreases plasma sodium during exercise in the heat

Abstract

This study assessed whether replacing sweat losses with sodium-free fluid can lower the plasma sodium concentration and thereby precipitate the development of hyponatremia. Ten male endurance athletes participated in one 1-h exercise pretrial to estimate fluid needs and two 3-h experimental trials on a cycle ergometer at 55% of maximum O2 consumption at 34 degrees C and 65% relative humidity. In the experimental trials, fluid loss was replaced by distilled water (W) or a sodium-containing (18 mmol/l) sports drink, Gatorade (G). Six subjects did not complete 3 h in trial W, and four did not complete 3 h in trial G. The rate of change in plasma sodium concentration in all subjects, regardless of exercise time completed, was greater with W than with G (-2.48 +/- 2.25 vs. -0.86 +/- 1.61 mmol. l-1. h-1, P = 0.0198). One subject developed hyponatremia (plasma sodium 128 mmol/l) at exhaustion (2.5 h) in the W trial. A decrease in sodium concentration was correlated with decreased exercise time (R = 0.674; P = 0.022). A lower rate of urine production correlated with a greater rate of sodium decrease (R = -0. 478; P = 0.0447). Sweat production was not significantly correlated with plasma sodium reduction. The results show that decreased plasma sodium concentration can result from replacement of sweat losses with plain W, when sweat losses are large, and can precipitate the development of hyponatremia, particularly in individuals who have a decreased urine production during exercise. Exercise performance is also reduced with a decrease in plasma sodium concentration. We, therefore, recommend consumption of a sodium-containing beverage to compensate for large sweat losses incurred during exercise.

Exercise-associated hyponatremia: the influence of pre-exercise carbohydrate status combined with high volume fluid intake on sodium concentrations and fluid balance

“…It is difficult to determine, therefore, precisely how our results would extrapolate to an exercise duration two to three times longer, but it is conceivable that if the 2 mEq/l decline that we observed with euhydration over 90 min continued, severe EAH (\130 mEq/L) could be reached relatively quickly. The results of Vrijens and Rehrer (1999) support the idea that the downward trend observed over 90 min would continue since their subjects demonstrated a similar decrease in plasma sodium over 180 min with water ingestion. The analysis of body mass changes during exercise showed a 2.77 and 2.66% dehydration for the LCDH and HCDH conditions; these levels of dehydration are not extreme.…”

“…Of interest, however, is the fact that not only did euhydration prevent the rise in sodium levels, but it also actually initiated a decrease in sodium levels that occurred without hyperhydration. Others have observed a similar response when subjects replaced the majority of fluid lost or hyperhydrated subjects during prolonged exercise in a warm environment (Baker et al 2008;Barr et al 1991;McConnell et al 1997;Vrijens and Rehrer 1999;Twerenbold et al 2003). The relatively large decrease in plasma volume in the dehydration conditions was due to the exercise and dehydration-induced hemoconcentration; however, the smaller decline observed in the euhydrated conditions was primarily due to the exerciseinduced plasma volume shift.…”

Sodium replacement and fluid shifts during prolonged exercise in humans

“…In contrast, Vrijens and Rehrer (1999) found more rapid (@2.5 vs 0.9 mEqál ±1 áh ±1 ) declines in plasma [Na + ] when their subjects drank water rather than a commercial carbohydrate-electrolyte solution at rates of @1.2 láh )1 during 3 h of exercise in a 34°C environment (Vrijens and Rehrer 1999). Their subjects produced very little urine, and low (@0.05±0.1 láh ±1 ) rates of urine production correlated with dangerous falls in plasma [Na + ] with water ingestion.…”

Renal handling of salt and water in humans during exercise with or without hydration

“…Because of this change in body water, whether or not associated with an electrolyte loss, the tonicity of the whole body can also change (Barr and Costill 1989;Carigan 1999;Edelman et al 1958;Noakes et al 1985;Putterman et al 1993;Rivera-Brown et al 1999;Vrijens and Rehrer 1999). Body tonicity comprises the tonicity of ECF and ICF.…”