Fluid and electrolyte homeostasis during prolonged exercise at altitude

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Abstract
The combined effect of exercise and altitude on fluid and electrolyte homeostasis was studied over 13 days on 6 men eating a diet with constant Na and K 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 2678 and 3629 m. There was a retention of Na (mean of 202 mM by the end of the exercise-altitude period) and a small retention of water (mean of 0.49 l). Plasma volume increased by 0.76 l and packed cell volume fell from a mean of 44.5-41.8%. There was no change in plasma Na concentration. The retention of Na implies an expansion in the extracellular space of 1.44 l at the expense of the intracellular space, which decreased by a calculated 1.05 l. These changes are similar to those resulting from comparable exercise at sea level and opposite to the effect of altitude on resting subjects.

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