methodology, lack of serial measurements on the same subjects, and the presence of confounding variables—like the use of exercise and the presence or absence of AMS in subjects under investigation—are some of the reasons for the dissimilarities in the findings. Moreover, there are virtually no data at extreme altitudes and after prolonged exposure to moderate altitudes. Nevertheless, it appears that acute exposure to moderate altitudes causes transient hypohydration, which is due to increased diuresis and an acute reduction in fluid intake because of decrease in thirst. The meager data available on the effects of chronic hypoxia in sheep and humans suggest that modest fluid retention may occur. Prolonged stay at extreme altitudes may cause severe salt and water retention in otherwise normal subjects. The role of hormones in normal fluid metabolism at high altitudes is unclear, but a number of hormones appear to play a role in the retention of salt and water in pathologic states like acute and subacute mountain sickness. RBF is probably reduced at high altitudes and more so at higher altitudes. Finally, exercise and increased physical activity at high altitudes favor the mechanisms that retain salt and water.
It is recommended that the acute and long-term effects of moderate and extreme altitudes on body fluid compartment and its determinants (neurohormones and renal function) need to be investigated using the same group of subjects and more sophisticated technology. The role of confounding variables like physical activity needs to be better defined. Subjects at high altitudes are almost always involved in some form of physical activity, which at times is fairly strenuous, especially for those engaged in military activity. Only by making serial measurements on the subjects would it be possible to confirm the impression that a prolonged stay may lead to some degree of fluid retention at moderate altitudes and to severe salt and water accumulation at extreme altitudes. Finally, the interaction between cold and altitude also needs to be determined. Such studies would be helpful in identifying subjects prone to high altitude-related illness and in establishing guidelines about the ''safe" length of time humans can spend at various altitudes.
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