Changes in body energy content occur through changes in the balance between daily intake and energy expenditure. Energy intake is episodic, derived primarily from the carbohydrates, proteins, and fats in foods consumed. Total daily energy expenditure for theoretical and analytic purposes can be divided into several components
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The resting metabolic rate (RMR) represents the largest portion of daily energy expenditure (60 to 75%) and is a measurement of the energy expended for maintenance of normal body functions and homeostasis. These processes include resting cardiovascular and pulmonary functions, the energy consumed by the central nervous system, cellular homeostasis, and other biochemical reactions involved in the maintenance of resting metabolism.
Another term to describe basal levels of energy expenditure is basal metabolic rate (BMR). While at the Mayo Clinic, Dr Boothby defined and popularized use of the BMR for the diagnosis of thyroid disorders. He defined this function as the energy expended by an individual bodily and mentally at rest in a thermoneutral environment 12 to 18 hours after a meal. Much to the inconvenience of the patient, measurements were done during the early morning hours when, in addition, the circadian rhythm of oxygen consumption was known to be low. Because of the increase in metabolism caused by the muscular and mental unrest introduced by this procedure, it is unlikely that the true basal metabolism was often measured. Therefore, for practical and conceptual reasons, the BMR is now rarely measured. In its place, we now measure what is referred to as the resting metabolic rate (or resting energy expenditure), which may be (but is not always) higher than the BMR.
The RMR is primarily related to the fat-free mass of the body and is also influenced by age, gender, body composition, and genetic factors. For example, the RMR decreases with advancing age (2 to 3%/decade), which is primarily attributed to the loss of fat-free mass. Males tend to have a higher RMR than females because of their greater body size. The dependency of the RMR on body composition must be considered when individuals of different age, sex, and physical activity status are compared. Other processes, such as sympathetic nervous system activity, thyroid hormone activity, and sodium-potassium pump activity, contribute to the variation in the RMR among individuals. (See WHO equations for predicting basal metabolic rates from body weights and heights for different age groups and both sexes and their derived data in Appendix TabJe.III_-_10:b, Table IJJ:10.-.c, Xable...I.I.I.-10-d and Tabje. . . !.II.-10-e).
The thermic effect of feeding (TEF) is the increase in energy expenditure associated with food ingestion. The TEF represents approximately 10% of the daily energy expenditure and includes the energy costs of food absorption, metabolism, and storage. The magnitude of the TEF depends on several factors, including the caloric content and composition of the meal as well as the antecedent diet of the individual. Following meal ingestion, energy expenditure increases for 4 to 8 hours, its magnitude and duration depending on the quantity and type of macronutrient (i.e., protein, fat, or carbohydrate).
The TEF has been divided into subcomponents: obligatory and facultative thermogenesis. The obligatory component of the TEF is the energy cost associated with absorption and transport of nutrients and the synthesis and storage of protein, fat, and carbohydrate. The "excess" energy expended above the obligatory thermogenesis is the facultative thermogenesis and is thought to be partially mediated by sympathetic nervous system activity.
The TEF also decreases with advancing age and may be associated with development of insulin resistance ( 10). It is presently unclear how exercise training influences the TEF, although there is clearly some interaction between physical exercise and TEF. There is presently no evidence that gender influences postprandial thermogenesis.
The most variable component of the daily energy expenditure is the thermic effect of physical activity ( 11). The component includes the energy expended above the RMR and the TEF and includes the energy expended through voluntary exercise and the energy devoted to involuntary activity such as shivering, fidgeting, and postural control. In sedentary individuals, the thermic effect of activity may be as low as 100 kcal7sol;day; in highly active individuals it may approach 3000 kcal/day (see Chapter..47). Thus, physical activity represents a significant factor in the daily energy expenditure in humans because it is extremely variable and subject to voluntary control. Physical activity tends to decrease with advancing age; this decrease in physical activity may be associated with a loss of fat-free mass and an increase in adiposity. Males in general tend to have a greater caloric expenditure associated with physical activity than females, partially because of the greater energy cost of moving a larger body mass. Average values of the energy cost of different grades of physical activity for men and women are given in Appendix Table iy..-A.-11-.D. and Table UL-A.-JJ-.-.E..
The RMR, TEF, and physical activity often overlap during the course of a normal day. Although daily variations in energy balance put individuals in a slight energy deficit or surplus, maintenance of a stable body weight depends on tight coupling of energy intake and energy expenditure over long periods of time. It is presently unclear which psychologic and/or physiologic factors influence the coupling of energy intake with energy expenditure to maintain energy balance.
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