ENERGY VALUE OF FOODS . basis and derivation by Annabel L. Merrill Bernice K. Watt Human Nutrition Research Branch AGRICULTURAL RESEARCH SERVICE UNITED STATES DEPARTMENT OF AGRICULTURE Agriculture Handbook No. 74 Slishtiy revised February 1973 Far solo by 114, Sapetintandirnt af Documents. U.S. Goy•rnmenr Printing M., Washington, D.C. 20402 Pric• S1.22 cionwilic poopaid or $1.00 GPO Sao War. Stock Number 0100-02n0 - PREFACE The kilocalorie, which has been defined as the amount of heat energy needed to raise the temperature of a kilogram of water 1° C., is the unit that has been used traditionally for expressing the energy value of foods. Recently the International Bureau of Weights and Measures has recomrnended that the joule, a unit of energy applicable to electrical, work, and chemical energy, be adopted as the preferred unit for all forms of energy. The joule is derived from basic units in the International System of Units (SI) and is defined as a measure of force (newtons) times distance (metres). In the interest of uniform nomenclature, soine nutritionists have proposed that the kilojoule replace the kilocalorie. The conversion factor for expressing kcalories as kjoules, as reconunended by the Com- mittee on Nomenclature of the International Union of Nutritional Sciences, is 1 kcalorie equals 4.184 kjoules, based on the kcalorie determined at 14.6 to 15.5' C. Use of kjoules in place of kcalories as the unit of measure for energy in no way invalidates the principles underlying the Atwater system for determining energy value of foods and the energy needs and energy expenditures of man. The Atwater system is based on the demonstrated principle that the oxygen used, the carbon dioxide formed, and the energy evolved in oxidizing foods are the same whether this oxidation takes place in the body of man or in a bomb calorimeter. Furthermore, Atwater has clearly shown that by applying appro- priate factors, which allow for metabolic losses, to the contents of protein, fat, and carbohydrate in a food, the physiologically available energy value of that food can be calculated with outstanding accuracy. The results obtained by this procedure are in excellent agreement with data from mea.surements made by bomb calorimotry on food and metabolic products. Results of studies by Atwater and others could be expressed either in kcalories or in kjoules. CONTENTS Page Page Introduction 1 Part III. Derivation of current calorie factors __ _ _ 24 Part I. Sourees of food energy. 1 Physiological fuel values of foods of animal Fat 2 origin 26 Determination of fat content 2 Physiological fuel values of plant products 26 Heat of combustion 2 Products of wheat 27 Carbohydrate 2 Products of grain other than wheat. 31 Determination of carbohydrate content 2 Legumes as Heat of combustion 3 Nuts 36 Protein 4 Vegetables 37 Determination of protein content _ 1 Fruits _ 40 Heat of cornbustion 1 Miscellaneous foods_ 40 Determined versus calculated gross energy values Part IV. Application of calorie factors _ 43 of foode 5 Comparison of calculated and determined avail- Other sources of energy 6 able calories for diets 44 Organic acid 6 General factors and more specific fasitors for Alcohol 7 calculating calories in individual foods 48 Part II. Digestibility and available energy of foods— 8 Application of general factors to national food Definition of terms 8 supplies 48 Digestibility of fat, carbohydrate, and protein__ _ 8 Conclusions 51 Literature cited 51 Availability of energy from digested nutrients _ 9 Appendix. Tabular surnmary of experiments on Fat 9 digestibility of foods of plant origin by human Carbohydrate 10 subjects 58 Protein 10 Apparent digestibility and available energy 58 Alcohol_ 18 Composition and heat of combustion of foods__ - 100 Text Tables 1. Average determined boats of combustion of 12. Effects on energy metabolism of replacing fats and oils and assumed factors for fat of portions of dietary carbohydrate a»d fat by different groups of food materials 2 aloohol 22 2. Average determined heats of combustion of 13. Data used for calculating energy values of foods different carbohydrates and assumed factors or food groups by the Atwater system 25 for carbohydrates of different groups of food materials 14. Energy values of wheat flours calculated by uso 3 of specific energy factors for protein, fat, and 3. Factors for calculating protein from nitrogen 28 content of food 4 carbohydrate 4. Average determined heats of combustion of 15. Apparent digestibility and physiological fuel value of wheat flours proteids and nonproteids and calculated heat 29 of combustion of protein 5 16. Coeffloients of apparent digestibility for grain 5. Comparison of calculated heats of combustion products 32 with results of direct determinations 6 17. Comparison of determined and calculated gross 6. Fresh fruits classified Ss to organic acid content _ 7 7. Factors for heats of combustion and fuel values energy values of potatoes 38 of nutrients in different groups of food mate- 18. Summary of steps for checking available rials'and in mixed diet 10 energy values calculated by factors from 8. Summary of data showing calorie-nitrogen ratio table 13. 44 of urine bašed on early studies of energy 19. Comparison of determined and calculated metabolism and digestibility 12 available energy values of various types of 9. Daily food intake in the experiments from which diets 46 Atwater originally obtained the calorie- nitrogen ratio of 7.9 for urine 16 20. Factors for digestibility, heats of combustion, 10. Daily nutrient intake in the experirnenta from and physiological fuel values of nutrients in which Atwater originally obtained the cakorie- food groups as used in present-day mixed nitrogen ratio of 7.9 for urine-.. 17 diets 49 11. Comparison of data for available energy 21. Comparison of energy values for different obtained by direct determination only and dietary patterns calculated with spocific and in part by calculation 19 with general calorie factors 50 Appendix Tabks 22. Use of digestibility data to determine coeffi- 23. Apparent digestibility, otc.—Continued cients of apparent, digestibility and available Vegetables, vegetable products 94 energy 59 Fruits 98 23. Apparent digestibility and available energy of Miscellaneous 98 foods of plant origin for human eubjects-- 60 24. Composition and heat of combustion of food Grainvgrain products 60 items used in experiments on human digest- Legumes and nuts _ 87 ibility (table 23) 100 iv Energy value oF Foods . basis and derivation INTRODUCTION Accurate evaluation of the energy value of foods tions and misuse when applied to individual foods is essential for dealing with problems of normal and different types of diets (114, 115).2 nutrition, undernutrition, or obesity. The classic The Food and Apiculture Organization, faced investigations of Professor W. O. Atwater and his with the urgency of assessing energy values of food associates at the Storrs (Conn.) Agricultural Ex- supplies in various countries and population groups, periment Station some 50 years ago provided the convened an ad hoc cornmittee of experts in 1947 ba.sis used in this country for measuring the energy to study the problems involved and to make values of food. The general calorie factors 4, 9, 4 recommendations. While endorsing the Atwater developed from that work gained widespread method as one that in the light of present knowl- acceptance, and until recently they were used for edge is suitable if properly used, the committee calculating the calories shown in official food com- pointed out the limitations of the use of general position tables. Properly applied, these general. factors and the need for more specific calorie factors provide a satisfactory measure of available factors (65) when dealing with individual foods. energy in average diets and food supplies in this These developments have pointed to the need country. Following Atwater's period little eaten- for summarizing the kinds of information Atwater tion was given to methods of calculating food used, the steps followed in his procedure for deter- energy and to the details of Atwater's procedure. mining fuel values of food, and the need for revising However, in recent years attention has again calorie data for foods to take aceount of additional turned to the important problems of determining research aecuinulating since his time. This pub- and meeting man's energy needs. In attempts to lication has been prepared to provide more back- alleviate food shortages experienced during and gronncl information on food energy data than that following World War II consideration was given given in current textbooks and food tables and to first to meeting energy needs in stricken areas. show the basic data drawn upon in deriving the Maynard, who represented this government in revised calorie factors now used in tables of food various interallied food-planning groups, pointed composition in this country. Except for a few out the necessity of nriderstanding the bases of recent revisions, factors derived as shown in this the different methods for estimating energy values publication have been used in U. S. Department of in use in Canada, the United Kingdom, and in Agriculture Handbooks No. 8 (185) a,nd No. 34 this country. On several occasions he called at- (100) and in various other sources, ineluding food tention to the correct application of the general tables published by the k ood and Agriculture calorie factors 4, 9, 4 and pointed Oti t, their lirnita- Organization (36). PART I. SOURCES OF FOOD ENERGY The chief food sources of energy to the human urea. Thus protein is incompletely oxidized in body are fat, carbohydrate, and protein. Fats the body, whereas it can be completely oxidized/in and carbohydrates contain carbon and hydrogen the calorimeter. The heat released by oxidation which can be oxidized to their end products, CO2 of food in the bomb calorimeter is its heat of and H20, both in the bomb calorimeter and in the combustion and is a measure of its gross energy body.