Sodium Supplementation Maintains Serum Sodium Concentration and Improves Cognitive Function in Endurance Athletes during Exercise

Abstract

Some athletes lose large amounts of fluid and sodium via heavy sweating and may be at risk of decreasing serum sodium (serumNa) concentration when hydration status is maintained and sodium is not ingested. PURPOSE: Determine the effect of sodium supplementation on serumNa and cognitive function in endurance athletes with exceptionally high sweat sodium losses. METHODS: Heat acclimated endurance-trained males (n=13, 33.1+5.9y, Mean+SD) with high sweat sodium (sweatNa) losses (sweat rate x sweat sodium concentration, 106+33mEq/h) cycled a stationary ergometer for 3h at 60% of VO2max in a warm environment (33.0+0.8°C DB, 27.5+1.5°C WB, 53.6+3.2% RH, 4.1+0.7m/s wind). Subjects completed two trials, separated by 5-10 d, in which placebo (PL) or sodium chloride (NA) (18.3+4.7g) was ingested via capsule to match individual whole body sweat sodium losses, which were determined in a preliminary trial (2-7d prior). Body mass was maintained during the trials by drinking water with carbohydrate (6.9+0.7L) to match fluid losses (e.g. sweat and urine) during exercise. An absorbent patch (10x12cm) was placed on the right forearm, scapula, thigh, and calf for 30 min at 30 and 120 min of the trial to determine sweatNa concentration. A subset of subjects (n=8) completed the Stroop Color-Word Test before and after each trial. RESULTS: Pre-trial serumNa was 143.5+1.3 and 143.9+1.9 mEq/L for NA and PL. SerumNa declined significantly during PL (-6.5+1.7 mEq/L; p=0.001) but not during NA (-1.1+2.2, p>0.05). Body mass did not change in either trial (0.0+0.5 vs. 0.3+0.5 kg, NA vs. PL, p>0.05). The rate of sweatNa losses was similar in both trials (113.2+31.8 vs. 112.3+35.9 mEq/h, NA vs. PL, p>0.05). Response time for correct responses to the Stroop Color-Word Test improved from baseline during NA (1.21+0.17 vs. 1.07+0.17s, pre- vs. post-test, p=0.016), but did not change during PL (1.10+0.13 vs. 1.12+0.15s, p>0.05). CONCLUSION: These results demonstrate that serumNa decreases significantly during 3h of moderate intensity exercise in a warm environment in athletes with high sweat sodium losses when body mass is maintained. SerumNa is maintained and postexercise response time to Stroop Color-Word Test is improved from baseline when sodium supplementation is provided to match sweatNa losses. This study was funded by the GSSI, Barrington, IL.