Energy Balance Definition

Overall energy balance is given by the following equation:

energy intake = energy expenditure + A energy stores

Thus, if the total energy contained in the body (as fat, protein, and glycogen) is not altered (i.e., A energy stores = 0), then energy expenditure must be equal to energy intake. In this case, the individual is said to be in a state of energy balance.

If the intake and expenditure of energy are not equal, then a change in body energy content will occur, with negative energy balance resulting in the utilization of the body's energy stores (glycogen, fat, and protein) or positive energy balance resulting in an increase in body energy stores, primarily as fat.

The difference between the concepts of energy balance and heat balance is presented in Figure 1.

Model of Energy Balance: A Dynamic State

There are multiple reciprocal direct and indirect influences of energy intake on energy expenditure and vice versa: for example, energy intake influences

Figure 2 Simple model of energy (E) balance. Long-term constancy of body weight through the regulation of energy balance is achieved through a highly complex network of regulatory systems operating through changes in food intake, in energy expenditure, and body energy content (i.e., change in body composition).

Figure 2 Simple model of energy (E) balance. Long-term constancy of body weight through the regulation of energy balance is achieved through a highly complex network of regulatory systems operating through changes in food intake, in energy expenditure, and body energy content (i.e., change in body composition).

resting energy expenditure by increasing postprandial and dietary-induced thermogenesis, whereas changes in energy expenditure via a modification of physical activity is susceptible to influence energy intake to maintain energy balance. In order to assure an accurate regulation of body stores, a double control is essential (see below). Body weight and body composition are not invariant with time but small corrections of both input and output from day-to-day or week-to-week, assure energy homeostasis (Figure 2). When attempting to explain the actual responses in energy balance and weight regulation in real life, we need to recognize that several factors may be operating at once on both sides of the energy balance equation. Compensatory adjustments occur in both intake and output, so unravelling the importance of one or other adjustment is not easy in man.

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