Regulation of Food Intake and Energy Storage
Stability of the body’s total mass and composition over long periods requires that energy intake match energy expenditure. Only about 27 per cent of the energy ingested normally reaches the functional systems of the cells, and much of this is eventually converted to heat, which is generated as a result of protein metabolism, muscle activity, and activities of the various organs and tissues of the body. Excess energy intake is stored mainly as fat, whereas a deficit of energy intake causes loss of total body mass until energy expenditure eventually equals energy intake or death occurs.
Although there is considerable variability in the amount of energy storage (i.e., fat mass) in different individuals, maintenance of an adequate energy supply is necessary for survival. Therefore, the body is endowed with powerful physiologic control systems that help maintain adequate energy intake. Deficits of energy stores, for example, rapidly activate multiple mechanisms that cause hunger and drive a person to seek food. In athletes and laborers, energy expenditure for the high level of muscle activity may be as high as 6000 to 7000 Calories per day, compared with only about 2000 Calories per day for sedentary individuals. Thus, a large energy expenditure associated with phys-ical work usually stimulates equally large increases in caloric intake.
What are the physiologic mechanisms that sense changes in energy balance and influence the quest for food? Maintenance of adequate energy supply in the body is so critical that there are multiple short-term and long-term control systems that regulate not only food intake but also energy expenditure and energy stores. In the next few sections we describe some of these control systems and their operation in physiologic conditions, as well as in obesity and starvation.
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