Information required on sources of data in tables
It is important to have information on the source of the data in a food composition table to be able to check its appropriateness for the study and to confirm its authenticity. The four major categories of sources of data are:
●primary publications, e.g., peer-reviewed articles in scientific literature
●secondary publications, e.g., reviews or published compilations with compositional data
●unpublished reports ranging from analytical records to documents prepared with limited circulation, e.g., confidential reports for clients or internal use within a company
●unpublished analytical data that can be either spe-cifically commissioned analyses for the generation of nutrient data or analytical data that were not particularly generated for the purpose of generat-ing food composition data.
Data in food composition tables may be original analytical values, imputed, calculated, or borrowed. Original analytical values are those taken from pub-lished literature or unpublished laboratory reports. Unpublished reports may include original calculated values, such as protein values derived by multiplying the nitrogen content by the required factor, energy values using energy conversion factors for some con-stituents of food, and “logical” values, such as the content of cholesterol in vegetable products, which can be assumed to be zero. Imputed values are esti-mates derived from analytical values for a similar food or another form of the same food. This category includes those data derived by difference, such as moisture and, in some cases, carbohydrate and values for chloride calculated from the sodium content. Cal-culated values are those derived from recipes by cal-culation from the nutrient content of the ingredients corrected by the application of preparation factors. Such factors take into account losses or gain in weight of the food or of specific nutrients during preparation of the food. Borrowed values are those derived from other tables or databases without referring to the original source. When a value for the content of a specific nutrient in a food is not included, there is a “–” or “0” value and, when a table has no values for a particular nutrient, the value is regarded as being “not included.” In some tables, e.g., the National Nutrient Database for Standard Reference, SR, of the USDA, “0” value is a true zero, meaning the particular nutri-ent was not detected by the analytical method used; “–” indicates a missing value.
The proportion of the various types of data differs between tables and for different nutrients (Table 11.6). Details on food tables can be obtained from the Food and Agriculture Organization (http://www.faoorg/infoods). In other tables, such as those in the Netherlands, where sources of the data are given in the references, information on how the data have been obtained can also be found. However, this is not the case for all tables of food composition.