FAO Energy and Protein

FAO Energy and Protein - Energy and protein requirements

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Originated by: Agriculture and Consumer Protection Title: Energy and protein requirements. .. More details 7. FACTORS AFFECTING ENERGY AND PROTEIN REQUIREMENTS 7.1 Available energy 7.1.1 Definitions Some confusion exists over the use of certain terms connected with the energy value of foods, such as “available” and “metabolizable” energy. Excellent accounts of the historical background 1 ) and are summarized by Widdowson ( 2 ). These accounts refer to the derivation and use of the classical Atwater factors for the energy values of carbohydrate, protein, and fat. The application of these values varies slightly in tables of food composition complied in the United States of America (and other countries using a similar system) and those used in the United Kingdom. These differences will be discussed briefly below. The amount of energy contained in the energy-yielding nutrients of foods (carbohydrate, fat, and protein) and in alcohol can be expressed in two ways: ( a ) The gross energy is the energy released as heat when the particular food is completely combusted in a bomb calorimeter. This value can also be calculated if the chemical composition of the food is known, by applying the gross energy value of carbohydrate (4.1 kcal th or 17 kJ per g), fat (9.3 kcal th or 39 kJ per g), and protein (5.4 kcal th or 23 kJ per g) to the amounts of these nutrients in the food. These values are only approximations, since they will vary slightly depending on the particular carbohydrates, fats, and proteins present. The energy values given above are not those available to the body, since some of the ingested energy is lost. For protein, about 1.25 kcal th (5.2 kJ) is lost as nitrogenous substances in the urine. Therefore the “gross” energy for protein is usually taken as 4.1 kcal th (17 kJ) per g. Further losses occur, since small amounts of the nutrients are either undigested or not absorbed and are excreted in the faeces. Measurement of the gross energy content by bomb calorimetry is mainly used in balance studies, where intakes and losses in the urine and faeces are measured simultaneously. It should be also noted that when estimates of the energy cost of growth are being made, it is the values for gross energy that should be used to calculate the energy content of the tissue laid down, according to the first law of thermodynamics: F = H + W + S + L (where F = energy from food, H = heat production, W = external work, L = losses, and S = energy stored). Energy and protein requirements http://www.fao.org/DOCREP/003/AA040E/AA040E08.htm#ch7.3 1 of 16 6/23/10 9:36 AM
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The metabolic “available” energy discussed here is not to be confused with the term available energy sometimes used in the thermodynamic literature, meaning the free energy (Gibbs energy).
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This note was uploaded on 02/08/2011 for the course SMHM 1450 taught by Professor Craft during the Spring '08 term at North Texas.

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FAO Energy and Protein - Energy and protein requirements

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