The need for energy

There is an obvious need for energy to perform physical work. Work has to be done to lift a load against the force of gravity, and there must be a source of energy to perform that work. As discussed in section 5.1, the energy used in various activities can readily be measured, as can the metabolic energy yield of the foods that are the fuel for that work (see Table 1.1). This means that it is possible to calculate a balance between the intake of energy, as metabolic fuels, and the body's energy expenditure. Obviously, energy intake has to be appropriate for the level of energy expenditure; as discussed in Chapters 6, and 8 neither excess intake nor a deficiency is desirable.

Figure 1.1 shows the relationship between food intake, physical work and changes in body reserves of metabolic fuels, as shown by changes in body weight. This study was carried out in Germany at the end of the Second World War, when there was a great deal of rubble from bomb damaged buildings to be cleared, and a large number of people to be fed and found employment. Increasing food intake resulted in an increase in work output — initially with an increase in body weight, indicating that the food supply was greater than required to meet the (increased) work output. When a financial reward was offered as well, the work output increased to such an extent that people now drew on their (sparse) reserves of metabolic fuel, and there was a loss of body weight.

Quite apart from obvious work output, the body has a considerable requirement for energy, even at rest. Only about one-third of the average person's energy expenditure is for voluntary work (section 5.1.3). Two-thirds is required for maintenance of the body's functions, homeostasis of the internal environment and metabolic integrity.

Figure 1.1 The relationship between food intake, work output and body weight (Wuppertal data). From data reported by Widdowson EM, MRC Special Report series no. 275, HMSO, 1951.

Energy intake (MJ/day)

Figure 1.1 The relationship between food intake, work output and body weight (Wuppertal data). From data reported by Widdowson EM, MRC Special Report series no. 275, HMSO, 1951.

As shown in Figure 1.2, about 20% of total energy expenditure is required to maintain the electrical activity of the brain and nervous system. This energy requirement, the basal metabolic rate (BMR; section can be measured by the output of heat, or the consumption of oxygen, when the subject is completely at rest.

Part of this basal energy requirement is obvious — the heart beats to circulate the blood; respiration continues; and there is considerable electrical activity in nerves and muscles, whether they are 'working' or not. These processes require a metabolic energy source. Less obviously, there is also a requirement for energy for the wide variety of biochemical reactions occurring all the time in the body: laying down reserves of fat and carbohydrate (section 5.6); turnover of tissue proteins (section; transport of substrates into, and products out of, cells (section 3.2.2); and the production and secretion of hormones and neurotransmitters.

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