| Total daily energy expenditure is a sum of basal metabolic rate, the thermic effect of food and the energy used up in physical activity.
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| Energy expenditure can be measured by direct calorimetry which relies on the measurement of heat production. Indirect calorimetry is based on the measurement of the rate of oxygen consumption (V02). The ratio of VCO2 to VO2 is known as the Respiratory Exchange Rate (RER) or Respiratory Quotient. For carbohydrates RER = 1 and for fat RER = 0.7.
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| Energy expenditure required to maintain body function at a complete rest is called the basal metabolic rate (BMR). BMR is sex-, age-, and body weight-dependent. At rest, energy is required for membrane transport (30% of the total), protein synthesis and degradation (30%), and for maintaining temperature, physical activity and growth. Certain organs use particularly high amounts of energy: in a 70-kg person brain metabolism constitutes approximately 20% of basal metabolic demand, liver 25% and muscle 25%. On the other hand, in very-low-birth babies brain is responsible for as much as 60% of the BMR, liver for 20% and muscle for only 5%.
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| In health, physical activity is the most important changeable component of energy expenditure. It is normally expressed as multiplies of the BMR. Examples of energy expenditure associated with different activities are given in Table 21.2. Energy requirement also depends on sex and age (Table 21.3).
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