History of the Energy Value of Foods

Most of the fundamental work on the energy value of foods was carried out by the pioneer scientists Rubner (in Germany) and his pupil Atwater (in USA) at the end of the nineteenth century.

Rubner measured the heats of combustion of a number of different proteins, fats, and carbohydrates in a bomb calorimeter and also studied the heat of combustion of urine passed by a dog, a man, a boy, and a baby. He realised that the heat of combustion of protein in a bomb calorimeter was greater than its caloric value in the body because the body oxidises proteins only to urea, creatinine, uric acid, and other nitrogenous end-products, all of which can be further oxidised [5].

Atwater, in a study of 46 persons, analysed urines and measured their heat of combustion. He found that for every gram of nitrogen in the urine there was unoxidised material sufficient to yield an average of 7.9 Kcalories, equivalent to 1.25 Kcalories per gram of protein in food, if the person is in nitrogen equilibrium [6].

Atwater also extensively studied the 'availability' of nutrients and distinguished between 'available' and 'digestible' nutrients. He regarded the faeces as having two parts: undigested and therefore unabsorbed food residues, and metabolic products of digestion, consisting of desquamated cells, bacteria, and substances in the digestive juices [7]. In other experiments, Atwater analysed the foods eaten in mixed diets and the faeces of human subjects. He then compared the data with reports in the literature on the availability of protein, fat, and carbohydrate in separate classes of food. The comparison gave very good agreement between the data and indicated average factors of 4.0, 8.9 (later rounded off to 9.0), and 4.0 for protein, fat, and carbohydrate, respectively. These factors, which Atwater intended only for use in calculating calories deriving from proteins, fats, and carbohydrates in mixed diets, came to be widely used for calculating the available energy value of individual foods. These factors also form the basis for the energy value of foods reported in the Food Composition Tables.

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