Sources of analytical data
Derivation of figures in tables 1 and 2
Meaning of terms
Application and limitations of tables
Extension and revision
Need for additional information
The sources consulted in preparing the tables fall into three main categories: (a) original reports concerned with research on food composition and closely related subjects; (b) summary tables and compilations dealing with food composition, mainly those intended for local or regional use in calculating the caloric and nutrient values of diets; and (c) miscellaneous sources cited for purposes other than direct food analysis, e.g., analytical methods and digestibility of foods. There are 114 of the references in category (a), 37 in (b), and 7 in (c).
The original reports (a) usually provided more explicit information on * the nature of the material analyzed, the number of specimens examined, and the analytical methods used, although this information was often incomplete. The summary tables and compilations (b) had the advantage of often giving average or "preferred" values based on the consideration of data relating to more than one sample of each food within the areas concerned. Moreover, many of these sources contain some figures which arc original, or at least not recorded in original papers available at FAO headquarters.
In arriving at the figures for foods which have been extensively studied, such as wheat, apples, citrus fruits, and eggs, main reliance was placed on summary tables. Original papers, on the other hand, provided the most important source material in formulating figures for foods such as millets, quinoa, bananas, plantains, yautias, coconuts, many of the fresh vegetables and fruits, buffalo and carabao milk, and some of the processed forms of meat and fish.
Numerous other papers, not mentioned in the bibliography, have been used indirectly. For example, a number of earlier reports from many countries .were included in the summaries from the United States of America (34, 35, 36).
Of the 151 reports used to obtain figures on the constituents in foods, 66 covered foods from' the Far East, 28 from Europe, 23 from North America, 20 from. Latin America, 12 from Africa and the Near East, and 2 from Oceania.
The values presented in the tables are generally averages based on the source material. Some are arithmetical means of numerous separate analytical determinations. More often they are weighted averages. The weights assigned depended sometimes on such factors as the relative importance in consumption of sub-groups within one class (examples of such sub-groups include varieties of groundnuts, genera of fish, group values for unspecified fresh vegetables and fruits); in many other instances they depended on the number of observations represented in the average figures given in the sources, or on a combination of these considerations.
In still other cases the values presented here are not actually averages but are "selected" or "preferred" values based on the author's judgment. Some of the summary tables contain values similarly derived (35, 58, 100, 117,130).
In deriving the figures for carcasses of certain kinds of livestock, reliance was placed mainly on data from the United States of America, the United Kingdom, and France. This course was followed because the data from these countries include detailed information on meats at the wholesale stage and show the relation between condition (fattening) and the physical and chemical composition of the carcasses. The range in condition and the corresponding nutrient composition covered by these figures appears, after a review of data from numerous other sources, to be wide enough for general application to meats in other areas.
It is common to find in the summary tables numerous values which are repetitious because several papers have taken them from the same source. For example, data from the United Kingdom and the United States of America appear in at least ten of the other compilations. Explicit reference to the presence of "borrowed" material was often lacking in these sources and frequently the identical origin was partially hidden by adaptations in the figures. In deriving values along the lines indicated above, an attempt was made to eliminate the effect of such duplication; this was done to avoid giving the weight of several values to a value which in effect is a single determination or average.
In order to make maximum use of the material contained in the original and compiled sources it was assumed that the determination of certain constituents, i.e., water, nitrogen, fat, ash, and fiber, had been made throughout by basically similar methods. In general, the methods employed seemed sufficiently consistent to warrant this assumption with regard to the major sources. Many investigators have used the general methods, if not the specific techniques, described by the Association of Official Agricultural Chemists (11) or made use of others which are closely parallel and likely to yield similar results. It is therefore sufficient to say that the terms "water," "fat," etc., are used in the tables in accordance with meanings customary in this field of research.
Protein. The protein values in the source material were computed from nitrogen, usually total nitrogen, by the use of various factors. The factor most commonly employed was the familiar "N x 6.25." In some of the major investigations used as source material this factor was used throughout. In others, the factor varied in accordance with available evidence regarding the nitrogen content of the proteins concerned. The latter procedure is that recommended in the Report by the FAO Committee on Calorie Conversion Factors and Food Composition Tables (56) and was followed in preparing - the present tables. For this purpose it was necessary to recalculate the figures given in. some of the sources, and to make assumptions when the factor used was not explicitly stated. Since considerable effort was made to resolve differences and to follow a consistent method of approach, the protein figures given in the tables can be regarded, for practical purposes, as having been derived by the application of the specific nitrogen factor considered appropriate to the food in question.
Specific nitrogen factors for most of the major food groups are given in the FAO Report mentioned above (56). Others are listed in Table 3.
Carbohydrates. The use of the term "carbohydrate" in the source material varied more than that of any other term. In preparing the present tables, however, the recommendations in the FAO Report (56) on the expression of carbohydrate values were followed, and discrepancies in the original material did not create any serious problems of adjustment. The value "total carbohydrate by difference" was obtained by subtracting the sum of the figures for water, protein, fat, and ash, from 100. Subtraction was done after final adjustment of the figures for the other constituents and their resolution into a single set of figures.
The carbohydrate figures for syrups are exceptions. They are based on data on sugar content, because more accurate information is available on this point than on the content of moisture and other constituents.
Calories. Calories were calculated by a procedure based on the principles enunciated in the FAO Report (56). The calorie factors employed were those given in Table 2 of the FAO Report and in other reports still unpublished which have been developed along similar lines by the Bureau of Human Nutrition and Home Economics, U. S. Department of Agriculture (21-23). This unpublished material furnished factors for a number of grains and for several groups of vegetables which are not covered in the FAO Report. For some foods - millets and sorghum, maize meal of medium-extraction, and undermilled rice, for example - it was necessary to make assumptions about digestibility in order to apply appropriate factors. The factors developed for whole wheat (56) were applied to the millets and sorghum. This seemed a reasonable procedure since these grains are often eaten as whole grains or high extraction products and contain at least as much fiber as whole wheat. The figures given for these grains are not fully applicable to the more refined forms. In the case of the intermediate forms of maize and rice, the calorie factors were obtained by interpolation between the coarser and the finer forms.
No satisfactory calorie factors are available for application to the carbohydrates in the primary products of soybeans. There is ample evidence that only a small portion of the total carbohydrate is in the form of fermentable sugars, starch, and dextrin (92, 144). Such substances as galactans, pentosans, and hemicelluloses, which are utilized poorly, if at all, by the body, represent a substantial proportion of the total. One of the few relevant biological studies (4) has indicated that the total carbohydrate in soybean flour is utilized by rats to the extent of only about 40 percent. In the absence of conclusive data on human beings the latter figure was taken to be the digestibility of the carbohydrates in the whole seeds, the flours, and several of the other major soybean products.
In the case of soybean milk, soybean curd, and soybean sauce, however, the conventional calorie factors (56) for carbohydrates based on "dry beans, peas, and nuts" (corresponding with 97 percent digestibility) were applied because when these products are prepared, most, if not all, of the insoluble carbohydrates are removed and it seems reasonable to suppose that most of the soluble forms present are well digested.
The factors used for calculating calories, apart from those given in the FAO Report (56), arc presented in Table 3. Factors used in converting nitrogen to protein and in computing calories.
Retail weight and edible portion. In Table 1 the calories per 100 grams, and the protein and fat in percentages, are given on the basis of the retail weight, after allowing for the quantities usually discarded as refuse, which is described below. This deduction has been made for convenience in application of the figures to the quantities at the retail level, i.e., "as purchased" or as brought into the kitchen. In estimating the available supplies on a per caput basis for food balance sheets, the quantities are reported in these terms, except for meats. The figures for the major meats, e.g., beef, pork, lamb and mutton, and veal are expressed here as percentages of the entire carcass, i.e., the form which is delivered by the slaughterhouse rather than as trimmed retail cuts. A special note on meats and meat fats appears on page 43.
In Table 2 the more detailed composition of the edible portion of each food is given, together with the quantity of refuse; the latter is in terms of the retail weight.
The values in Table 1 are all based on values in Table 2.
The term "refuse" in this report refers to such inedible parts as bones of meat, egg shells, hard fruit pits and tough rinds, and also the thin skins of fruits and roots and the outer leaves of some vegetables, which arc commonly removed in preparation.
The composition of any food, as produced or consumed, may vary to a considerable extent. It is influenced by botanical variety, cultural conditions, climate, processing, and a number of other factors. It therefore follows that the values contained in any working table of food composition can be only approximations to the true values of the foods to which they are applied, in different circumstances, for estimating calorie and nutrient content. This is true even of tables designed for use in a limited area and based on the direct analysis of samples of foods consumed within that area.
In tables designed for use on a wide geographical scale, such as the present tables, little account can be taken of local variations. It is, however, to be observed that the range of variation in the composition of natural foods is roughly similar in different areas. Moreover, average values, based on the analysis of a large number of samples of a similar type, are usually approximately alike in different countries and regions. In general, figures reached by the methods described in this report, and derived from an extensive series of observations and determinations, are likely to be closer approximations, in a wide range of circumstances, than local figures, if the latter are based on the examination of a meager number of samples.
Among the most striking local differences in nutrient content are the "man-made" differences, those due to processing being of particular importance. In cereals such as rice and maize the effect of processing on nutrient content is often greater than differences in composition of the foods as grown. It is true, for example, that certain types of maize or wheat contain more or less protein or fat than the average for the species as a whole; maize hybrids now being produced in the United States contain less protein than the varieties they are replacing. Differences in composition resulting from milling, however, may be of greater significance. Differences in the quantities of fat removed from meats, as well as differences in the fattening of domestic animals, arc also of major importance.
For reasons already stated, the retail values (Table 1) are less accurate than those relating to the edible portion (Table 2). Much less attention has been given to estimates of "refuse," or waste in preparation, than to the determination of constituents in the edible portion. More than half the primary sources of analytical data used in the preparation of the tables contained no information on this important subject. The' omission would not be serious if estimates of the weight of the edible material were always available in calculating the composition of diets. Unfortunately, this information is often lacking, even in dietary surveys (except the most detailed ones).
It is usually necessary to apply estimates of losses in the form of "refuse" in family consumption studies as well as in investigations of consumption of larger population groups. The importance of the factor "refuse" in dietary calculations may be illustrated by the fact that the errors of estimation in the percentages of refuse (peeling) in such foods as potatoes, sweet potatoes, and cassava are relatively greater than the errors of estimation in the figures for the constituents in the edible portion. Available data on the nutrients in the edible portion are usually derived from many more samples and the figures from various sources show better agreement with respect to these constituents. Whenever good, i.e., truly representative, local data on the percentage of refuse are available, more accurate results will be obtained by calculating the quantities of edible material and applying the figures in Table 2 instead of those in Table 1.
The most obvious gap in the present tables is the lack of any information on minerals and vitamins. Little work on these nutrients has been undertaken so far by the Nutrition Division, but it is planned to start the collection of information on these nutrients for summarizing as soon as possible.
Revision of the figures in the present tables is also planned to include better values for certain foods on which present information is inadequate and to add figures for a number of important foods not covered in this report. Notes starred in Table 2 indicate a few of the foods for which values were based on especially meager evidence. For a few refuse estimates on which any direct observations whatsoever were lacking, values (shown in parentheses) were imputed from analogy with similar foods.
Further improvements will be facilitated by any developments which lead to greater uniformity in analytical techniques and additional knowledge about the digestibility of foods.
FAO's Nutrition Division would appreciate receiving additional information, old or new, on refuse and on the nutrient content of the edible portion. Data on minerals and vitamins, especially those accompanied by detailed information on the character of the analytical samples, are greatly needed. The Division would also welcome suggestions concerning any foods which are omitted in the current tables but which are considered important enough to warrant inclusion in a later version.