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

SIMS, STACY T.1,3; van VLIET, LINDA1; COTTER, JAMES D.1; REHRER, NANCY J.1,2

Medicine & Science in Sports & Exercise: January 2007 - Volume 39 - Issue 1 - pp 123-130
doi: 10.1249/01.mss.0000241639.97972.4a
BASIC SCIENCES: Original Investigations

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.

1School of Physical Education, 2Department of Human Nutrition, and 3University of Otago, Dunedin, NEW ZEALAND

Address for correspondence: Nancy J. Rehrer, Ph.D., School of Physical Education, University of Otago, PO Box 56, Dunedin, New Zealand; E-mail: nancy.rehrer@stonebow.otago.ac.nz.

Submitted for publication March 2006.

Accepted for publication July 2006.

©2007The American College of Sports Medicine