GE used for support of basal metabolic functions, voluntary activity, maintenance of body temperature, and product formation (for example, tissue growth, integument, conceptus, and milk) plus GE lost in feces, urine, and combustible gases and as waste heat.
A first measure of energy expenditure (or energy requirement) is the amount of energy required to support the basic life functions (vital cell activity, respiration, and cardiovascular distribution of blood) of an animal in repose (awake but resting and unstressed), in a postabsorptive state, and in a thermoneutral environment (no shivering or other special activity to maintain body temperature). Rubner proposed that basal energy expenditure was related to body surface area and concluded that fasting homeotherms produce 1,000 kcal of heat per square meter body surface (Kleiber, 1975). Because the surface area of a sphere is related to its volume and can be related to its weight when it has a density of 1 kg·L-1, attempts were made to relate basal energy requirements of animals to measurements of body surface area.
However, animals are not spheres and do not have a density of 1, and body-surface area measurements are difficult to reproduce consistently. Thus, a search began for a relationship between basal energy requirements and body weight. Using data published by others, Kleiber (1975) explored the concept of metabolic body size as a power function of body weight (BWn) and concluded that basal metabolic rates (BMR) of fasting adult animals varying in body weight from mice (0.021 kg) to cattle (600 kg) could be expressed in kilocalories per day as 70BWkg0.75. Nonhuman-primate data included in his calculations were derived from studies of macaques (Benedict, 1938) weighing 4.2 kg, with a BMR of 207 kcal·day-1, and chimpanzees (Bruhn and Benedict, 1936) weighing 38 kg, with a BMR of 1,090 kcal·day-1.
It should be noted that it is difficult to measure energy expenditure in the exact circumstances specified for determination of BMR. It is questionable whether ruminants reach a true postabsorptive state; Colobinae might not, and few animals appear to be stress-free during the measurement experience. Therefore, resting energy expenditure (REE) may be used instead. In studies with humans, BMR and REE differ by less than 10%, and the terms are used interchangeably (National Research Council, 1989). Prediction equations have been used for estimating BMR when analytic methods were not available (FAO/WHO/ UNU, 1985; National Research Council, 1989).
Energy expenditure (EE) and therefore energy requirement generally decreases with advancing age because of a decrease in BMR, which is characterized by loss of fat-free mass (FFM). Age-related changes probably vary in rate, timing, and extent among individuals in response to differences in physical activity, disease, and other factors. Information on rates of change in BMR and FFM is limited by study design (cross-sectional rather than longitudinal) and possibly by methodology (use of imprecise or biased methods for assessment of changes in body composition) (Murray et al., 1996). The age-related decline in BMR has been partly explained by a reduction in the quantity, as well as metabolic activity, of lean-tissue components as measured by dual-energy x-ray absorptiometry (DEXA). However, even when BMR was adjusted for differences in lean-tissue and fat components, it was significantly lower in older people (50-77 years old) by 644 kJ·day-1 (Piers et al., 1998). When the BMR of similarly aged people (average, 71 years) was measured in a respiratory chamber, BMR was significantly (P < 0.01) lower after FFM, fat mass, and sex were accounted for (Vaughn et al., 1991).
When REE of 40 healthy men and women (51-82 years old) was measured with indirect calorimetry, REE was highly correlated with FFM(r = 0.88; P < 0.001) and body weight (r = 0.85; P < 0.001); this supports the idea that active tissue mass determines daily EE (Fredrix et al., 1990). Total EE and activity level, measured by the doubly labeled water (DLW) method in combination with measurements of BMR, showed that EE was lower in elderly (68-71 years) than in younger (27-30 years old) subjects partly because of a significantly lower BMR (Pannemans and Westerterp, 1995).
When EE (adjusted for body composition and activity) was measured in two age groups (20-30 years, n = 98; 50-65 years, n = 39), older subjects had a 4.6% lower BMR than younger subjects, independently of sex, body size, body composition, and activity (Klausen et al., 1997). An effect of sex was noted among healthy men and women (over 50 years old to control for effect of menstrual status) when 24-hour EE, BMR, and sleeping metabolic rate were measured in a respiratory chamber. Men had significantly higher 24-hour EE and sleeping metabolic rates than women after adjustment for differences in fat-free mass, fat mass, and age (Ferraro et al., 1992).
Age and body composition affected energy requirements of 101 infants, 82 girls, and 27 adults when energy expenditures were scaled for differences in body size to test the effects of age and body fatness in humans (Butte et al.,