1995). As humans increase in weight and fatness from infancy to adulthood, energy requirements increase as a power function (BW0.63) of body weight.

The capabilities of aging people need not diminish if they maintain a healthy, active lifestyle. Energy requirements and EE of healthy, active older people (63-77 years) and younger people (average, 28 years) were reported in a study of men receiving a diet with a defined formula for 47 days under controlled conditions. Energy expenditure while they were at rest: 1.22 × BMR, and while sitting quietly: 1.30 × BMR, were the same for older and younger men (Calloway and Zanni, 1980). Moderate activity, such as walking on the level at about 2.5 mph, cost 4.51 ± 0.34 (mean ± SD) kcal·min-1 (about 1.4 × BMR). Cycling at a comfortable load (300-400 kpm) cost only slightly more energy than did walking for both age groups. Metabolizable energy intake required to maintain a constant BW for these men, who were sedentary except for 30 min of cycling per day, was 2,554 ± 222 kcal·day-1, or about 1.6 × BMR. The minimal maintenance ME requirement (ambulatory but inactive) of healthy older men was 1.5 × BMR, the same as for younger men, and similar to the averages of 1.55 × BMR for adult macaques and 1.56 × BMR for adult baboons of various ages (Table 2-1). The estimate of total daily EE, determined by multiplying energy costs of a given level of activity by the individual estimate of BMR, was 1.55 × BMR for sedentary men, as reported by Almendingen et al. (1998) in describing methods for predicting individual energy intakes.

The “factorial approach” to estimating ME requirements as multiples of BMR was based on the factorial method used to determine protein requirements (Payne and Waterlow, 1971). It provides a way of partitioning the ME required for maintenance into BMR, activity, and heat increment (Lloyd et al., 1978; National Research Council, 1981b; FAO/WHO/UNU, 1985). The idea has been expanded to encompass estimates of total EE whether determined according to dietary intake, DLW, or other indirect measures of energy requirements or expenditures (Roberts, 1996; Shetty et al., 1996; Scholler, 1998; DeLany, 1998). A measure of error can be introduced into the estimate in that activity (work) is determined by mass and distance traveled in the horizontal or vertical planes and is not a function of age or gender (Mathers, 1997).

When cross-sectional energy-balance measurements were made on groups of rhesus monkeys (Macaca mulatta) 6.5-7.0 years old, 8.5-10 years old, and over 24 years old, the 24-hour EE tended to decrease with age when it was expressed in absolute or BWkg 0.75 terms (Lane et al., 1995). Absolute EE (mean ± SD) declined for the juvenile, adult, and aged ad libitum-fed control groups to 1,008 ± 326, 853 ± 188, and 603 ± 148 kcal·d-1, respectively. Energy expenditures expressed in relation to BWkg 0.75 for the groups declined in a similar manner 194 ± 64, 167 ± 32, and 122 ± 46 kcal·d-1, respectively. There was no significant effect of age on either measurement. In another study of young (7-9 years), middle- (13-17 years) and older-aged (> 23 years) rhesus monkeys, energy expenditure (kJ/min) tended to decrease with age but the decrease was not significant. In this study, older animals spent less time in vertical movement and thus had the lowest energy expenditure (Ramsey et al., 2000).

The ME intake required for maintenance must provide the chemical energy to meet basal metabolism, thermoregulation, and activity energy costs (Lloyd et al., 1978; McNab, 1986; Scott, 1986; Robbins, 1993a; Torun et al., 1996). In other words, ME intake must equal heat production. Such biologic factors as sex, growth, age, health, and reproductive status affect energy requirements of nonhuman primates. Evidence suggests that some nocturnal primates have lower relative basal requirements than diurnal primates (Ross, 1992). Although the maintenance energy requirement is often defined as the energy intake that sustains a constant BW, care must be taken in using weight as the sole criterion of energy balance because body composition may change, particularly with age (Robbins, 1993a). Similarly, the expression of food intake data per kilogram BW rather than per unit of metabolic body size can lead to variable conclusions, especially when small and large animals or those with different body compositions are compared (Brody, 1945; Ausman et al., 1985).

In humans and rats, about 60-75% of the ME supplied by the diet is used to meet BMR requirements (Lloyd et al., 1978; Rothwell and Stock, 1981; Curtis, 1983; FAO/ WHO/UNU, 1985). About 5-10% is used to support the thermogenic effect (heat of digestion) of food (Mayes, 1996; Forsum et al., 1981). The heat increment (HI) associated with digestive and metabolic processes is energy that cannot be used for productive purposes but can be used to help to maintain body temperature in cold environments. Except for temperature extremes, the influence of environmental temperature on apparent digestibility of the energy in food is relatively small compared to that of differences in food composition (Curtis, 1983). It also is difficult to measure because climatic effects are often confounded with amounts of food consumed and the foods selected, choices that may vary seasonally among free-living mammals (National Research Council, 1981a; McNab, 1986).

The ambient temperature range in which thermoregulation occurs without increasing metabolic heat production is termed the thermoneutral zone and is bounded by the upper and lower critical temperatures (Curtis, 1983; Robbins, 1993a). As ambient temperature rises above the upper critical temperature, metabolic heat production increases because of the energy-demanding processes, such as panting and sweating, required for heat dissipation. Declines in ambient temperature below the lower critical temperature require increased metabolic heat production by such activi-

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