Vrijens, D. M., & Rehrer, N. J. (1999). Sodium free ingestion decreases plasma sodium during exercise in the heat. Medicine and Science in Sports and Exercise, 31(5), Supplement abstract 1613.

This study assessed whether replacing sweat losses with sodium-free fluid lowered plasma sodium concentration and precipitated the development of hyponatraemia. Male endurance athletes (N = 10) cycled for one hour to estimate fluid needs. Test trials consisted of three hours at 55% VO2max, at 34 degrees Celsius and 65% relative humidity. Fluid replacement was with distilled water or a sodium (18 mM/l) sports drink (Gatorade).

Six Ss did not complete the water trial, and four did not complete the sodium trial. One S developed hyponatraemia during the water trial. Plasma sodium decreased significantly more with water than with Gatorade.

This investigation demonstrated that when sweat losses are large, decreased plasma sodium concentration results from replacement of sweat losses with water. The development of hyponatraemia can be precipitated, particularly in individuals who have decreased urine production during exercise.

Implication. The consumption of a sodium-containing beverage is recommended during extended bouts of exercise when sweat loss is likely to be great.

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