Exercise and increased physical activity soon after ascent to high altitudes can significantly alter fluid and hormonal responses (Milledge, 1992). While acute exposure to high altitudes may be associated with dehydration, increased physical activity soon after ascent to high altitudes causes significant salt and water retention (Withey et al., 1983). Therefore, studies that did not control for physical activity may have tested the effects of exercise plus hypoxia, rather than hypoxia alone. Likewise, it is unclear which studies in the literature excluded subjects who might have had early features of acute mountain sickness (AMS). Like exercise, AMS has been shown to have totally different effects on salt and water metabolism as compared to hypoxia alone (Milledge, 1992). Finally there is evidence that fluid responses at high altitudes may be species dependent (Ashack et al., 1985). Thus much of the data in the literature is confusing, controversial, and inconclusive. This chapter identifies some of the major studies on this subject and tries to present a consensus view of the effects of high altitudes on fluid metabolism.
A number of factors influence fluid metabolism at high altitudes. The most important ones are the degree of hypoxia, cold, hypobaria, and physical activity. Most workers feel that hypobaria by itself has little effect on fluid metabolism (Epstein and Saruta, 1972; Heyes et al., 1982). The effect of cold is discussed elsewhere (see Freund and Sawka, Chapter 9 in this volume) and will not be considered here. However, details of the interaction between cold and hypoxia remain to be determined.
Fluid metabolism at high altitudes has been studied in several ways. A frequent approach has been the use of simple intake-output data. Some studies have inferred changes in body fluid from the change in weight and energy balance. The best data, however, come from studies of body fluid compartments using isotope dilution techniques. Table 18-1 summarizes the findings of major published studies where body fluid compartments were measured at high altitudes. Most of these studies only report the effects of acute exposure, that is, changes occurring during the first 2 to 3 weeks at high altitudes (Frayser et al., 1975; Hannon et al., 1969; Hoyt et al., 1992; Jain et al., 1980, 1981; Krzywicki et al., 1971; Singh et al., 1990; Surks et al., 1966). Very few studies followed the effect of prolonged stay at high altitudes (Anand et al., 1993; Hoyt et al., 1991; Phillips et al., 1969; Singh et al., 1990). Human studies