Sodium Loading Aids Fluid Balance and Reduces Physiological Strain of Trained Men Exercising in the Heat

SIMS, S. T., L. van VLIET, J. D. COTTER, and N. J. REHRER. Sodium Loading Aids Fluid Balance and Reduces Physiological Strain of Trained Men Exercising in the Heat. Med. Sci. Sports Exerc., Vol. 39, No. 1, pp. 123–130, 2007. Purpose: This study was conducted to determine whether preexercise ingestion of a highly concentrated sodium beverage would increase plasma volume (PV) and reduce the physiological strain of moderately trained males running in the heat. Methods: Eight endurance-trained (V u O2max :5 8 mLIkg j1 Imin j1 (SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment. Runners ingested a high-sodium (High Na + : 164 mmol Na + IL j1 ) or low-sodium (Low Na + : 10 mmol Na + IL j1 ) beverage (10 mLIkg j1 ) before running to exhaustion at 70% V u O2max in warm conditions (32-C, 50% RH, Va $ 1.5 mIs j1 ). Beverages (~757 mL) were ingested in seven portions across 60 min beginning 105 min before exercise. Trials were separated by 1–3 wk. Heart rate and core and skin temperatures were measured throughout exercise. Urine and venous blood were sampled before and after drinking and exercise. Results: High Na + increased PV before exercise (4.5% (SD 3.7)), calculated from Hct and [Hb]), whereas Low Na + did not (0.0% (SD 0.5); P = 0.04), and involved greater time to exercise termination in the six who stopped because of an ethical end point (core temperature 39.5-C: 57.9 min (SD 6) vs 46.4 min (SD 4); P = 0.04) and those who were exhausted (96.1 min (SD 22) vs 75.3 min (SD 21); P = 0.03; High Na + vs Low Na + , respectively). At equivalent times before exercise termination, High Na + also resulted in lower core temperature (38.9 vs 39.3-C; P = 0.00) and perceived exertion (P = 0.01) and a tendency for lower heart rate (164 vs 174 bpm; P = 0.08). Conclusions: Preexercise