The major determinants of fluid metabolism at high altitudes include fluid intake and urine output, alterations in neurohormones, and renal function.
Intake-output data at high altitudes are often too crude and unreliable to assess fluid balance accurately. Nevertheless, such data do provide useful information that helps in the understanding of fluid metabolism at high altitudes. A number of studies have shown that intake is reduced at high altitudes whenever fluids are allowed ad libitum but is normal when intake is strictly enforced (Claybaugh and Wade, 1987). Decrease in appetite and thirst, lack of available water, and physical exertion probably all contribute to reduced intake. However, the chief mechanism appears to be a decrease in the sensation of thirst (Jones et al., 1981a, b). Reduced fluid intake can account for approximately 2 to 3 liters of negative fluid balance during the initial few days at high altitudes. The exact mechanism of reduced thirst is also unclear. Changes in antidiuretic hormone (ADH) cannot fully explain it, since thirst is reduced even in patients with diabetes insipidus (a condition characterized by the inability to produce or respond to ADH) (Jones et al., 1981a, b). Similarly, changes in circulating atrial natriuretic peptide (ANP) and angiotensin II do not seem to play a major role in the reduced sensation of thirst (Jones et al., 1981b). Changes in the central release of these hormones may be important, but this has never been proven. The sensation of thirst returns to normal over a period of a few days despite continued stay at high altitudes.
Changes in urine output are also common at high altitudes, but the data are controversial. This laboratory reviewed 57 studies where urine output was measured at high altitudes. The data suggest that there is usually a transient diuresis at high altitudes lasting, on an average, 3 to 4 days. Diuresis is common at moderate altitudes, in subjects who do not exercise soon after transfer to high altitudes, in subjects who do not develop AMS, and in subjects for whom fluid intake is strictly enforced. Diuresis is also seen in acute experiments at sea level when subjects breathe hypoxic gas mixtures (Ashack et al., 1985). In contrast, anti-diuresis commonly accompanies severe hypoxia, at extreme altitudes, in subjects who exercise soon after arriving at high altitudes, and in those who are prone to and later develop AMS.
Other factors that might lead to dehydration at high altitudes include an increase in insensible loss because of low humidity and hyperventilation. The magnitude of this effect is not clear. Malabsorption of various nutrients has also been described at high altitudes (Chinn and Hannon, 1969), but the effect of this phenomenon on fluid metabolism has never been evaluated.