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˙O2max: 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% V˙O2max in warm conditions (32°C, 50% RH, Va ≍ 1.5 m·s−1). 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 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.