| Food Composition Data: A User's Perspective (1987) |
|Managing food composition data|
|Managing food composition data at the national level|
The primary product of the NDB is the revision of Agriculture Handbook No. 8. In machinereadable form this is represented by USDA Nutrient Data Base for Standard Reference, which basically consists of the data contained in the revised sections of the handbook, supplemented by the older data for food groups that has not yet been revised. The Standard Reference tape is updated as new revisions are released and thus always represents the most up-to-date information available. Each release of the USDA Nutrient Data Base for Standard Reference is identified by a release number and the year. The most recent is Release No. 5, 1985, which covers revised sections of the handbook through no. 8-12 .
The USDA Nutrient Data Base for Standard Reference is utilized for the creation of specialized data bases, both within USDA and by users who purchase the tape for adaptation to their specific needs. A good example of a data base derived for USDA use is the Nutrient Data Base for Individual Food Intake Surveys, which is in the final stages of completion. This has been created by developing a computerized linking file that connects the survey food codes to the food codes on the USDA Nutrient Data Base for Standard Reference. For survey items that are composite foods, formulas are included by which values for the composite items can be calculated from the nutrient content of the individual components. For the forthcoming Continuing Survey of Food Intake by Individuals (CSFII), we were asked to include data for components not regularly present in the Standard Reference base. This made it necessary to expand it to include data for dietary fibre, alcohol, vitamin E, and carotenes for approximately 1,700 food items. In addition, 4,000 values for other nutrients were added to supply values not yet contained in the Standard Reference data base because the food groups have not yet been updated. We have named this expanded data base the Primary Data Set for Food Consumption Surveys.
Table 2. Proportion of nutrient values based on analytical data in Primary Data Set
|Nutrient||Percentage of values|
The linking file, the Primary Data Set, and a computerized table of retention factors are accessed by a computer program to create the Nutrient Data Base for Individual Food Intake Surveys.
In creating the Primary Data Set we were placed in the unique position of being both data users and data providers at the same time. Care has been taken to document the sources of the expanded data so that they can be further evaluated and updated as additional information becomes available. Codes have been attached to all added nutrient values to indicate whether they are from analytical data in the revised handbook, new analytical data not yet finalized for the handbook, older data from sections of the handbook not yet revised, or whether they are imputed values or assumed values of zero, such as for cholesterol in plant foods.
Besides documenting the data sources, this coding system provides a new way to measure the state of knowledge of food composition data. A quantitative measure of available analytical data for each nutrient under consideration can be calculated by determining the relative proportion of analytical to imputed values in the expanded data base. Examples of such calculations are shown in table 2. Calcium and protein are representative of nutrients that have been analysed regularly over a long period and for which analytical data are thus most available. At the other extreme is dietary fibre, for which analysed values are just beginning to be reported. Although analyses for magnesium are now commonly included in food composition studies, only limited information was available for the 1963 edition of Agriculture Handbook No. 8, and it is apparent that analytical values currently available are not as comprehensive as those for calcium. Analysed values on hand for carotene are almost entirely those determined in plant products by AOAC procedures . For the Primary Data Set the remaining values for carotene were those assumed to be present in arriving at expressions for total retinol equivalents of vitamin A.
It must be understood that the calculations shown in table 2 pertain directly only to the Primary Data Set. They indicate the basis for information on hand for calculating the composition of foods in the CSFII data base. It should be further understood that the procedure distinguishes only between analytical and imputed data, without attempting to address the reliability of the analytical data.