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2 Introduction Since 1936, the USDA has conducted six national surveys of food consumption at roughly 10-year intervals. The early surveys measured disappearance of food from household supplies but not individual intake; however, in 1956 the protocol was changed to include individual data on recall of foods consumed over a 24-hour period. The latest Nationwide Food Consumption Survey (NFCS), conducted in 1977-1978, included both household and individual compo- nents. It included a recall of foods consumed by individ- uals at home and away from home d',ring a 3-day period. The survey did not include questions on nutrient supple- ments. The data were collected through face-to-face interviews during which individual household members were asked to report their food intake over the previous 24 hours. The respondents were then given a food diary to record their intake over the next 2 days. his individual intake component consisting of a 1-day recall and 2-day record for each individual is the basis for USDA analyses of the nutritive value of foods consumed in the United States (Peterkin _ al., 1982). These data have been reported by se.:, age, region of residence, income, race, and household characteristics (Pao et al.. 1982). For some years, the USDA used the recommended dietary allowances (RDAs) (NRC, 1980) to evaluate the adequacy of nutrient intake. However, the RDAs do not represent the true requirement of all persons. Rather, they in- clude a margin of safety to allow for variability and other factors. Therefore, the USDA staff and other food-consumption analysts have traditionally defined inadequate intake as intake below a fixed cutoff point. Some analysts use two-thirds of a specific RDA as a definition of inadequate intake; others use one-half or 7
8 three-fourths of the RDA (Peterkin et al., 1982). There have been criticisms of this approach, and no clear rationale for the selection of the particular cutoff point has emerged. Nutritional status cannot be determined from data on dietary intake alone. If appropriate criteria are used, however, these data can be used for making a preliminary evaluation of the proportion of the population that may be at risk for impaired nutritional status. The task of the Subcommittee on Dietary Intake Evaluation was to develop criteria and approaches for interpreting the nutrient intake information in the Nationwide Food Consumption Survey (NFCS). Specifically, it was asked to develop criteria for using survey data on dietary intakes within the U.S. population or subpopula- tions in order to estimate the prevalence of inadequate nutrient intake. It did not examine methods to assess individual intakes or to determine the adequacy of an individual diet. In agreement with most analysts of survey data, the subcommittee determined that to assess dietary intake at the population level, it is necessary to compare the observed dietary intakes with the require- ments for that nutrient. The subcommittee began by examining previous efforts to estimate the prevalence of inadequate nutrient intake, focusing on the scientific merits of the approaches that have been taken. Recognizing that a probability approach had been used with apparent success for similar analytic problems (e.g., analysis of data on height and weight), the subcommittee tested the feasibility of this approach for analysis of dietary adequacy. During the course of its work, it became aware of the importance of three concepts for this kind of analysis: · Because food and nutrient intakes vary from day to day, survey data on dietary intake must be adjusted to estimate statistically the distribution of usual dietary intake. Any approach to the assessment of dietary intake must take into account the mean and symmetry of the dis- tribution of nutrient requirements among persons with similar characteristics. mere is ample evidence that these nutrient requirements vary between persons in
9 similar categories of age, sex, body weight, and pregnancy and lactation status. · Because changes in physiological or functional criteria for nutrient requirements require changes in the level of dietary intake needed to meet the requirement, any approach to interpreting intake in relation to require- ment must incorporate a definition of the criterion the requirements are intended to satisfy. Tt is possible, and indeed desirable, to define multiple criteria of adequacy, multiple levels of requirement, and hence a multitiered population assessment. Chapters 3 and 4 of this report address these issues and are followed in Chapter 5 by a discussion of the proposed analytic method, including examples of appli- cations to selected nutrients. The subcommittee also recognized other important uses for data on food con~ump- tion, including the identification of food patterns asso- ciated with inadequate dietary intake and the aetermina- tion of changes in eating patterns that are likely to be acceptable, feasible, and economical for groups with poor diets. This kind of information is needed to design food assistance programs and meal patterns for these programs, to encourage improvements in nutrition education, and to design nutrition intervention programs mandated by law. Information on food consumption patterns is also essential for the development of food safety regulations, which are promulgated by the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA). These uses, although not germane to this report, are as Important as estimating the prevalence of inadequate intake and are a major function of the NFCS and other dietary intake sur- veys. The subcommittee has cautioned that the final design of future surveys must take into account all the intended uses of the data--nc~t: just assessment of the prevalence of inadequate intakes discussed in this report.
