Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat

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 (VO2max: 58 mL(.)kg(-1.)min(-1) (SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment. Runners ingested a high-sodium (High Na+: 164 mmol Na+.L-1) or low-sodium (Low Na+:10 mmol Na+.L-1) beverage (10 mL(.)kg(-1)) before running to exhaustion at 70% VO2max in warm conditions (32 degrees C, 50% RH, V-a approximate to 1.5 m(.)s(-1)). Beverages (similar to 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 Het 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 degrees 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 degrees 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 ingestion of a high-sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceived strain during exercise and increased exercise capacity in warm conditions.