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3,900 kcal/d with an additional fat intake of 45 to 324 g (400 to 2,900 kcal/d) as butter or hydrogenated oil.

Thus, anorexia appears to be an immediate consequence of altitude exposure. The continuation of anorexia over time may be a consequence of continued negative energy balance, as is seen with starvation.

A Shift in Appetite

Anecdotal reports of climbers suggest that appetite switches from a preference for fat to carbohydrate after several days at altitude (Gill and Pugh, 1964). Carbohydrate represents the most oxygen-efficient fuel (Brooks and Fahey, 1984), and some investigations have shown that carbohydrate has a protective effect against clinical symptomology associated with hypoxia, ameliorating acute symptoms of mountain sickness (Consolazio et al., 1969), and positively affecting arterial oxygen concentration (Hansen et al., 1972) and pulmonary function (Dramise et al., 1975). In studies where composition of diet has been considered, if a voluntary switch in composition has occurred between sea level and altitude, it has most frequently been to maintain or increase absolute (Boyer and Blume, 1984; Worme et al., 1991) or relative (Guilland and Klepping, 1985) carbohydrate intake at the expense of fat and protein. A major exception to this observation are the results of the simulated ascent of Mount Everest (Operation Everest II), where subjects decreased the proportion of their diminishing food intake provided by carbohydrate (Rose et al., 1988). However, shifts in composition of diet may be dependent, at least in part, on the foods served (Hannon et al., 1976; Worme et al., 1991). In studies where food composition has been manipulated to enforce decreased carbohydrate intake at altitude, exercise performance has been adversely affected (Bigard et al., 1993).

Increased Energy Need: The Output Side of Energy Balance
Basal Energy Needs

Several reports in the literature suggest that BMR (the energy required to maintain body functions in the most minimal state) increases at altitude (Butterfield et al., 1992; Gill and Pugh, 1964; Hannon and Sudman, 1973; Stock et al., 1978), especially during the first week. The early work in this area is confounded by a lack of standardized methodology for determining BMR: some studies collected data on resting metabolic rate (measured after arising and moving around), others on true BMR (measured before arising), and still others measuring one parameter at sea level and another at altitude. In studies where valid measurements were made across several weeks, the

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