Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat.
OBJECTIVE
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) approximately equal to 1.5 m.s(-1)). Beverages (approximately 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 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.
Sims
Stacy T
ST
School of Physical Education, Department of Human Nutrition, University of Otago, Dunedin, New Zealand.
van Vliet
Linda
L
Cotter
James D
JD
Rehrer
Nancy J
NJ
D000328:Adult; D018592:Cross-Over Studies; D004311:Double-Blind Method; D015444:Exercise / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic; D009520:New Zealand; D010829:Physiological Phenomena / Q000502:physiology*; D010919:Placebos; D012964:Sodium / Q000008:administration & dosage*; D014882:Water-Electrolyte Balance / Q000502:physiology*
D010919:Placebos; D012964:Sodium