euhydration levels, but not at rest when core temperature was not elevated. Likewise, Ryan and colleagues (1998) found that dehydration (approximately 3 percent of body weight) did not influence gastric emptying or intestinal absorption during exercise without marked heat strain. Their subjects had final exercise core temperatures of 38.5°C (101.3°F), which was approximately the core temperature where the subjects of Neufer and colleagues (1989a) began to clearly demonstrate reduced gastric emptying.
Altitude exposure will result in dehydration because of elevated respiratory water losses (approximately 200 mL/day above the usual baseline of 250 mL/day), hypoxia-induced diuresis, reduced fluid consumption (approximately 2 to 3 L over several days), and possibly elevated sweating from the high metabolic rates needed to traverse rugged mountain terrains (Anand and Chandrashekhar, 1996; Hoyt and Honig, 1996). The net effect is a total body water deficit reduction during altitude exposure (Anand and Chandrashekhar, 1996; Hoyt and Honig, 1996). In lowlanders exposed to moderate altitude (> 2,500 m), hypoxia will rapidly initiate diuresis that continues for several days (Anand and Chandrashekhar, 1996; Hoyt and Honig, 1996). This diuresis and the factors discussed above decrease total body water and plasma volume in proportion to the elevation of ascent (Sawka et al., 2000). Mechanisms responsible for the resultant hemoconcentration include diuresis, natriuresis, and dehydration, as well as loss of circulating plasma protein (Anand and Chandrashekhar, 1996; Hoyt and Honig, 1996; Sawka et al., 2000). This hemoconcentration is isoosmotic (unless sweat-induced dehydration contributes) and exceeds the reduction in total body water because it is largely oncotically mediated (Sawka et al., 1996b).
Body water reduction and hemoconcentration are believed to provide several physiological benefits by contributing to the increased oxygen content (Sawka et al., 2000) and perhaps reduced risk of mountain sickness (e.g., Acute Mountain Sickness, pulmonary edema, cerebral edema) (Anand and Chandrashekhar, 1996). The effects of dehydration on mountain sickness and performance decrements at altitude have not been studied.
Body fluid losses in cold climates can be as high as losses in hot climates due to high rates of energy expenditure and use of heavy clothing (Freund and Young, 1996). Fluid losses during cold exposure are commonly thought to result from cold-induced diuresis