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ENERGY BALANCE AT ALTITUDE

The Scenario of Weight Loss at Altitude

Weight loss at altitude is accompanied by anorexia (Boyer and Blume, 1984; Consolazio et al., 1968, 1972; Guilland and Klepping, 1985; Hannon et al., 1976; Kryzywicki et al., 1971; Pugh, 1962; Rose et al., 1988; Westerterp et al., 1992; Whitten and Janoski, 1969) and often diuresis (Boyer and Blume, 1984; Guilland and Klepping, 1985; Koller et al., 1991; Krzywicki et al., 1971), both thought to begin with acute exposure. The weight loss occurs continuously throughout exposure in most studies (Rose et al., 1988); the diuresis may be a transient response (Boyer and Blume, 1984). Where monitored, nitrogen balance (a measure of the maintenance of lean body mass) has usually been negative (Guilland and Klepping, 1985; Hannon et al., 1976), which suggests a breakdown of lean tissue. Basal metabolic rate (BMR) is elevated over sea-level values initially but falls toward sea level over time (Hannon and Sudman, 1973; Stock et al., 1978). Studies on fuel utilization at altitude show glycogen sparing (preferential use of a fuel other than glycogen) and increased circulating glycerol and triglycerides with exercise after acclimatization (Young et al., 1982). These data have been interpreted to mean that the metabolic fuel chosen for maintenance of energy needs at rest and during exercise changes from carbohydrate to fat with acclimatization (Young et al., 1982).

The Scenario with Starvation: Consequences of Negative Energy Balance

This picture of weight loss during acute exposure to altitude sounds suspiciously similar to the series of events accompanying weight loss due to starvation. Starvation may be defined as a state where energy intake does not match energy need or as a state of negative energy balance. Ancel Keys in his seminal work on starvation in male conscientious objectors (Keys et al., 1950) described the sequelae of events in response to diminished energy intake, which included negative nitrogen balance accompanied by a decline in lean body and fat mass and by a concurrent decrease in BMR. Keys and coworkers (1950) also noted a decrease in overall activity level, which was thought to represent a mechanism to conserve existing energy stores. Further work on starvation by other investigators has shown a shift in energy metabolism toward mobilization and utilization of fat and ketone bodies under circumstances of negative energy balance (Saudek and Felig, 1976). The ultimate adaptation to inadequate energy intake was the failure of appetite (Keys et al., 1950).



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