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Energy expenditure

■ Intense exercise

• Discretionary

• Occupational

• Sitting, coffee, smoking Dietary-induced thermogenesis

Basal metabolic rate

70 kg Aged 25 years

70 kg Aged 70 years

Figure 1 Different components of energy expenditure in a young man and the impact of ageing. Note that the energy expenditure decreases mainly because of a decline in physical activity. There is also a decrease in the basal metabolic rate, but this predominantly reflects a reduction in the lean tissues (i.e., fat-free mass) in the elderly.

in their physical activity patterns but also in their metabolic rate at rest. PAL estimates are less useful in infants because their energy requirement includes not only their total energy expenditure but also their cost of growth; BMRs are also difficult to measure at this age, so the 2H218O method together with estimates of growth costs have been used to compile new lower estimates of energy needs than were originally estimated by the old factorial method.

Age-related changes in energy needs are important not only in children but also in adults for different reasons. Figure 1 gives an indication of the decline in energy needs during adult life. This results from the atrophy of the lean tissues, which may be related to a decline in physical activity. Lack of exercise is therefore a handicap because it directly reduces energy expenditure, and it may also lead to a slow shrinkage of tissues, such as muscle, thereby producing a long-term decline in metabolism at rest. There may also be up to an additional 5% decrease in the rate of tissue metabolism. Thus, unless people adapt their intake extraordinarily well to this progressive decline in energy output, energy storage, weight gain, and obesity are inevitable.

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