boredom in menus, reluctance to work on a full stomach, lack of fluid availability, odd meal times, anxiety, or self-imposed dieting. In Chapter 20 of this volume, Hoyt and Honig present a review of 11 recent field studies. In these studies the reported energy intakes using dietary records were about 2,400 kcal/d, although approximately 3,600 kcal/d were provided. This intake represented a consumption of only about two-thirds of the rations provided, which created a theoretical energy deficit of almost 1,000 kcal/d.
The anorexia commonly experienced at high altitudes may contribute to this difficult situation. There is some speculation that the anorexia of altitude may serve to limit sodium and water intake with the teleologic consequence of adjusting body stores for better acclimatization (Hoyt and Honig, 1996). However, no research in human subjects has been able to attribute the anorexia of acute mountain sickness to a possible physiological mechanism.
The metabolic fuels used to supply energy requirements during military operations at high altitudes are derived from both dietary sources and preexisting body stores. Most troops have fat stores sufficient to cover an energy deficit of 1,000 kcal/d for 1 to 2 months (fat stores equivalent to about 30,000 to 60,000 kcal, or about 9 kg of fat mass) (Sahlin, 1986). However, glycogen stores, which determine to some extent the endurance capacity of the individual, are more limited. Carbohydrate needs, based on a mean Respiratory Exchange Ratio (R) of 0.85, which represents a carbohydrate contribution to energy supply of about 70 percent (Kleiber, 1961), would need to be at least 400 g/d. Normal rations available to troops supply 580 to 680 g carbohydrate per day (AR 40-25, 1985; see Table 1-3); however, data presented by Hoyt and Honig (see Chapter 20 in this volume) suggest actual carbohydrate intakes of less than 300 g/d. Thus, inadequate carbohydrate intake may exist in troops working at high altitudes, a state which might contribute to diminished performance.
The failure to match dietary fuel supply with physiological need results in a shift in metabolic fuel use toward fat. Increased energy needs at high altitudes may increase the use of body fat under these circumstances of energy deficit. Acclimatization results in a decrease in muscle glycogen utilization (Young et al., 1982) and a decrease in muscle lactate (Green et al., 1989) in individuals functioning under an energy deficit, suggesting to Hoyt and Honig that acclimatization leads to an increase in fat utilization. However, in individuals maintaining adequate energy intake, the situation may be reversed (Roberts et al., in press a).
Estimation of energy requirement in the field is difficult. Hoyt and Honig (see Chapter 20 in this volume) describe a new ambulatory monitor and method that involves estimation of energy expenditure based on body weight, the time during each stride that a single foot contacts the ground (foot contact time), and the nature of the terrain being covered. The resulting estimate is based on the energy cost of supporting body weight and the rate at which this force is generated. The rate of force generation can be estimated as total body