Energy Conversion Tables

[Cre Moonin]

Asstated in Chapter 1, the translation of human energyrequirements into recommended intakes of food and the assessment of how well theavailable food supplies or diets of populations (or even of individuals) satisfythese requirements require knowledge of the amounts of available energy inindividual foods. Determining the energy content of foods depends on thefollowing: 1) the components of food that provide energy (protein, fat,carbohydrate, alcohol, polyols, organic acids and novel compounds) should bedetermined by appropriate analytical methods; 2) the quantity of each individualcomponent must be converted to food energy using a generally accepted factorthat expresses the amount of available energy per unit of weight; and 3) thefood energies of all components must be added together to represent thenutritional energy value of the food for humans. The energy conversion factorsand the models currently used assume that each component of a food has an energyfactor that is fixed and that does not vary according to the proportions ofother components in the food or diet.

The unit of energy in the International System of Units(SI)[8] is the joule (J). A joule is the energyexpended when 1 kg is moved 1 m by a force of 1 Newton. This is the acceptedstandard unit of energy used in human energetics and it should also be used forthe expression of energy in foods. Because nutritionists and food scientists areconcerned with large amounts of energy, they generally use kiloJoules (kJ =103 J) or megaJoules (MJ = 106 J). For many decades, foodenergy has been expressed in calories, which is not a coherent unit ofthermochemical energy. Despite the recommendation of more than 30 years ago touse only joules, many scientists, non-scientists and consumers still find itdifficult to abandon the use of calories. This is evident in that both joules(kJ) and calories (kcal) are used side by side in most regulatory frameworks,e.g. Codex Alimentarius (1991). Thus, while the use of joules alone isrecommended by international convention, values for food energy in the followingsections are given in both joules and calories, with kilojoules given first andkilocalories second, within parenthesis and in a different font (Arial 9). Intables, values for kilocalories are given in italic type. The conversion factorsfor joules and calories are: 1 kJ = 0.239 kcal; and 1 kcal = 4.184 kJ.

The total combustible energy content (or theoretical maximumenergy content) of a food can be measured using bomb calorimetry. Not allcombustible energy is available to the human for maintaining energy balance(constant weight) and meeting the needs of growth, pregnancy and lactation.First, foods are not completely digested and absorbed, and consequently foodenergy is lost in the faeces. The degree of incomplete absorption is a functionof the food itself (its matrix and the amounts and types of protein, fat andcarbohydrate), how the food has been prepared, and - in some instances (e.g.infancy, illness) - the physiological state of the individual consuming thefood. Second, compounds derived from incomplete catabolism of protein are lostin the urine. Third, the capture of energy (conversion to adenosine triphosphate[ATP]) from food is less than completely efficient in intermediary metabolism(Flatt and Tremblay, 1997). Conceptually, food energy conversion factors shouldreflect the amount of energy in food components (protein, fat, carbohydrate,alcohol, novel compounds, polyols and organic acids) that can ultimately beutilized by the human organism, thereby representing the input factor in theenergy balance equation.

Food that is ingested contains energy - the maximum amountbeing reflected in the heat that is measured after complete combustion to carbondioxide (CO2) and water in a bomb calorimeter. This energy isreferred to as ingested energy (IE) or gross energy (GE). Incomplete digestionof food in the small intestine, in some cases accompanied by fermentation ofunabsorbed carbohydrate in the colon, results in losses of energy as faecalenergy (FE) and so-called gaseous energy (GaE) in the form of combustible gases(e.g. hydrogen and methane). Short-chain (volatile) fatty acids are also formedin the process, some of which are absorbed and available as energy. Most of theenergy that is absorbed is available to human metabolism, but some is lost asurinary energy (UE), mainly in the form of nitrogenous waste compounds derivedfrom incomplete catabolism of protein. A small amount of energy is also lostfrom the body surface (surface energy [SE]). The energy that remains afteraccounting for the important losses is known as “metabolizable energy”(ME) (see Figure 3.1).

Not all metabolizable energy is available for the productionof ATP. Some energy is utilized during the metabolic processes associated withdigestion, absorption and intermediary metabolism of food and can be measured asheat production; this is referred to as dietary-induced thermogenesis (DIT), orthermic effect of food, and varies with the type of food ingested. This can beconsidered an obligatory energy expenditure and, theoretically, it can berelated to the energy factors assigned to foods. When the energy lost tomicrobial fermentation and obligatory thermogenesis are subtracted from ME, theresult is an expression of the energy content of food, which is referred to asnet metabolizable energy (NME).

1 Additional energy is needed for gainsof body tissue, any increase in energy stores, growth of the foetus duringpregnancy, production of milk during lactation, and energy losses associatedwith synthesis/deposition of new tissue or milk.

Some energy is also lost as the heat produced by metabolicprocesses associated with other forms of thermogenesis, such as the effects ofcold, hormones, certain drugs, bioactive compounds and stimulants. In none ofthese cases is the amount of heat produced dependent on the type of foodingested alone; consequently, these energy losses have generally not been takeninto consideration when assigning energy factors to foods. The energy thatremains after subtracting these heat losses from NME is referred to as netenergy for maintenance (NE), which is the energy that can be used by the humanto support basal metabolism, physical activity and the energy needed for growth,pregnancy and lactation.

ME has traditionally been defined as “food energyavailable for heat production (= energy expenditure) and body gains”(Atwater and Bryant, 1900), and more recently as “the amount ofenergy available for total (whole body) heat production at nitrogen and energybalance” (Livesey, 2001). By contrast, net metabolizable energy(NME) is based on the ATP-producing capacity of foods and their components,rather than on the total heat-producing capacity of foods. It can be thought ofas the “food energy available for body functions that require ATP”.The theoretical appeal of NME for the derivation of energy conversion factorsrests on the following: substrates are known to differ in the efficiency withwhich they are converted to ATP, and hence in their ability to fuel energy needsof the body. These differences in efficiency are reflected in the differencesbetween heat production from each substrate and that from glucose; they can bedetermined stoichiometrically and can be measured. Furthermore, foods replaceeach other as energy sources in the diet and in intermediary metabolism on thebasis of their ATP equivalence (which is reflected in NME), rather than on theirability to produce equal amounts of heat (which is reflected in ME). For more ofthe derivations of and differences between ME and NME see the detaileddiscussions of Warwick and Baines (2000) and Livesey (2001).

Just as a large number of analytical methods for food analysishave been developed since the late nineteenth century, so have a variety ofdifferent energy conversion factors for foods. In general, three systems are inuse: the Atwater general factor system; a more extensive general factor system;and an Atwater specific factor system. It is important to note that all of thesesystems relate conceptually to (ME) as defined in the previous section. Ageneral factor system based on NME has been proposed by Livesey (2001) as analternative to these systems.

The Atwater general factor system was developed by W.O.Atwater and his colleagues at the United States Department of Agriculture (USDA)Agricultural Experiment Station in Storrs, Connecticut at the end of thenineteenth century (Atwater and Woods, 1896). The system is based on the heatsof combustion of protein, fat and carbohydrate, which are corrected for lossesin digestion, absorption and urinary excretion of urea. It uses a single factorfor each of the energy-yielding substrates (protein, fat, carbohydrate),regardless of the food in which it is found. The energy values are 17 kJ/g (4.0kcal/g) for protein, 37 kJ/g (9.0 kcal/g) for fat and 17 kJ/g (4.0 kcal/g) forcarbohydrates.[9] The Atwater general system alsoincludes alcohol with a rounded value of 29 kJ/g (7.0 kcal/g or an unroundedvalue of 6.9 kcal/g) (Atwater and Benedict, 1902). As originally described byAtwater, carbohydrate is determined by difference, and thus includes fibre. TheAtwater system has been widely used, in part because of its obvioussimplicity.

A more extensive general factor system has been derived bymodifying, refining and making additions to the Atwater general factor system.For example, separate factors were needed so that the division of totalcarbohydrate into available carbohydrate and fibre could be taken into account.In 1970, Southgate and Durnin (1970) added a factor for available carbohydrateexpressed as monosaccharide (16 kJ/g [3.75 kcal/g]). This change recognized thefact that different weights for available carbohydrate are obtained depending onwhether the carbohydrate is measured by difference or directly. In recent years,an energy factor for dietary fibre of 8.0 kJ/g (2.0 kcal/g) (FAO, 1998) has beenrecommended, but has not yet been implemented.

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