Sodium-facilitated hypervolemia, endurance performance, and thermoregulation

The purpose of this study was to investigate the effects of an immediate pre-exercise, orally ingested, sodium load (164 mEq Na+) (IPOSL), equivalent to 10 ml per kilogram of body weight, on plasma volume, endurance performance, and thermoregulation. Fourteen male participants consumed a nearly isotonic (255 mOsm . kg (-1)) IPOSL and a hypotonic (94 mOsm . kg (-1)), no-sodium, placebo beverage (Pl) equivalent to 10 ml . kg (-1) body weight in a randomized design. Subjects cycled at 70 % of maximal work rate, in a 21.0 - 23.3 degrees C lab, for 45 min while cardiovascular and thermoregulatory variables were measured. This was followed by a 15-min performance time trial. IPOSL and Pl ingestion lead to a 3.1 % expansion and a 4.7 % reduction in resting baseline plasma volume, respectively. IPOSL maintained plasma volume during exercise to a greater extent than the Pl at 15 and 30, but not 45 min. There was a significant improvement ( approximately 7.8 %; p < 0.05) in time trial performance following IPOSL. No significant differences were observed for heart rate, core temperature, rate of perceived exertion or total body sweat rate (p > 0.05). In conclusion, IPOSL ingestion increased pre-exercise plasma volumes, maintained 15- and 30-min exercise plasma volumes, and improved an endurance performance time trial better than the Pl with no apparent compromise in thermoregulation.