Kavouras, S. A., Arnaoutis, G., Gioxari, A., Kollia, M., Anastasiou, C. A., & Sidossis, L. S. (2006). Sodium intake during prolonged exercise in the heat may prevent hyponatremia. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1506.

This study investigated the effects of sodium in fluids ingested during prolonged exercise in the heat on fluid and electrolyte balance. Physically active but untrained males (N = 7) completed a 3-hour exercise protocol (30 minutes of cycling, 30 minutes of walking) in the heat (30C), a set of calf raises exercise protocol (8 x 30 repetitions) and a 45 minutes of steep, brisk walking (5.5 km/h, 12% grade), while consuming one of the experimental drinks: HNa (carbohydrate-electrolyte drink containing 37 mmol/l Na); LNa (carbohydrate-electrolyte drink containing 23 mmol/l Na); MW (mineral water containing <0.3 mmol/l), and PL (placebo of distilled water with a non caloric and Na containing color and flavor). The experimental drinks were administered to balance fluid losses due to sweating.

HNa and LNa prevented mild hyponatremia (<135 mEq/l) that was observed during MW and PL. While no differences were found between HNa and LNa in plasma sodium, plasma osmolality was greater during the HNa than the rest of the drinks. Heart rate and lactate levels were similar among the trials. None of the volunteers experienced any clinical symptoms, such as dizziness or muscle cramping.

Implication. Sodium intake during prolonged exercise in the heat plays a significant role in preventing hyponatremia when fluid intake matches sweat loss.

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