Fluid and electrolyte homeostasis during prolonged exercise at altitude

The combined effect of exercise and altitude on fluid and electrolyte homeostasis was studied over 13 days on six male subjects eating a diet with constant sodium and potassium content. During the first 4 and last 4 days subjects were semisedentary at an altitude of 900 m. In the middle 5 days subjects exercised by hill walking for about 7 h daily at altitudes between 2,678 and 3,629 m. There was a retention of sodium (mean of 202 mM by the end of the exercise-altitude period) and a small retention of water (mean of 0.49 liters). Plasma volume increased by 0.76 liters and packed cell volume fell from a mean of 44.5 to 41.8%. There was no change in plasma sodium concentration. The retention of sodium implies an expansion in the extracellular space of 1.44 liters at the expense of the intracellular space, which decreased by a calculated 1.05 liters. These changes are similar to those resulting from comparable exercise at sea level and opposite to the effect of altitude on resting subjects.