Short-term exposure to high altitude causes decreases in total body water measured by bioelectrical impedance

A decrease in total body water (TBW) has been well documented after 24-h of high altitude (HA) exposure. The alterations in fluid balance during the initial 24-h of HA exposure have not been characterized. We hypothesized that the decrease is initiated within the first few hours of exposure. Seven male volunteers were studied at sea level (SL) and during 72-h of high altitude exposure (4,592 m) in a hypqbaric chamber on two separate occasions, with a minimum of 14 days separation between the two trials. TBW was measured by bioelectrical impedance at SL and after 4,12, 20, 36, 46, 60 and 68 hours of HA. TBW(I) was as follows (Means ± SE): SL HA4 HA12 HA20 HA36 HA46 HA60 HA68 TRIAL 1 48.5 44.8 47.2 44.8 43.3 45.1 44.3 44.6 3.0 2.3 3.3 2.4 2.4 2.8 2.8 2.5 TRIAL 2 49.9 45.9 51.1 45.0 43.2 44.8 44.4 43.4 1.2 1.4 3.1 1.8 1.7 1.9 1.6 1.9 TBW decreased within 4 hours at HA and remained decreased throughout HA exposure (p<0.05). After 20 hours at HA, there were no further reductions. No differences in TBW are evident between Trial 1 and Trial 2. We conclude that changes in TBW are detectable within 4 hours of beginning HA exposure and that no significant decreases occur after 20 hours. In addition, reexposure to HA after 14 days at SL does not alter the time course of HA TBW changes.