Plasma volume and the physiological response to sodium loading in men and women : a thesis submitted for the degree of Doctor of Philosophy, School of Physical Education, University of Otago, Dunedin, New Zealand
The metabolic heat generated by exercise must be dissipated to maintain body temperature within narrow physiological limits; during exercise and heat exposure, body water is lost via sweating to enable evaporative cooling of the body. When sweating takes place, total body water is reduced (without the intake of additional fluids) from each fluid compartment due to the free exchange of water between compartments with a concomitant loss of electrolytes, primarily sodium. A series of three investigations were undertaken to evaluate: 1) the efficacy of acute sodium citrate-chloride loading on endurance trained males and females as a viable means to expand extracellular fluid volume, 2) any menstrual cycle effects on renal handling of this sodium load at rest, and 3) if any subsequent hypervolaemia reduces the physiological strain of exercise in warm conditions in both genders. The first investigation examined eight endurance-trained (VO2[max]: 58 ml·kg-1·min-1 (SD 5); 36 y (SD 11)) runners in a randomized double-blind crossover study. The participants ingested a high-sodium (HighNa+: 164 mmol Na+·L-1) or low-sodium (LowNa+: 10 mmol Na+·L-1) beverage (10 ml·kg-1) before running to exhaustion at 70% VO2[max] in warm conditions (32°C, 50% RH, V[a]~1.5 m·s-1). Results indicate that HighNa+ increased PV before exercise (4.5% (SD 3.7)), calculated from Hct and [Hb]), whereas LowNa+ didn't (0.0% (SD 0.5); P = 0.04), and involved greater time to exercise termination in those who were stopped due to ethical end point of 39.5°C and volitional exhaustion (39.5°C: 57.9 min (SD 6) vs. 46.4 min (SD 4); n = 5, P = 0.04; EXH: 96.1 min (SD 22) vs. 75.3 min (SD 21); n = 3, P = 0.03; HighNa+ vs. LowNa+ respectively). At equivalent times before exercise termination, HighNa+ also involved 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)
Thesis, Dissertation, English, 2006