Gross and Metabolizable Energy

The traditional way of measuring the energy content of foostuffs is to use a 'bomb calorimeter' in which the heat produced when a sample of food is combusted (under high pressure of oxygen) is measured. When the food is combusted, it is completely oxidized to water, carbon dioxide, and other incompletely burned elements. The total heat liberated (expressed in kilocalories or kilojoules) represents the gross energy value or heat of combustion of the food. The heat of combustion differs between carbohydrates, proteins, and fats. There are also important differences within each category of macronutrient. The gross energy yield of sucrose, for example, is 16.5 kJg-1, whereas starch yields 17.5 kJg-1. The energy yield of butterfat is 38.5 kJg-1 and of lard 39.6 kJg-1. These values have been rounded off to give 17.3 kJg-1 for carbohydrates rich in starch and poor in sugar, 39.3 kJg-1 for average fat, and 23.6 kJg-1 for mixtures of animal and vegetable proteins.

The gross energy value of foodstuffs (Table 1), however, does not represent the energy actually

Table 1

Metabolizable energy (ME) and Atwater's factors

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