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national surveys of food intake by households or individuals. Food supply is estimated by adding the quantity of food imported to the quantity produced within a country and then subtracting the sum of food exported, destroyed by pests during storage, and put to nonfood use (e.g., in the production of industrial alcohol). The final figure is divided by the total population to obtain the average per-capita food availability. The results are estimates of foods that disappear into wholesale and retail markets; they fail to account for food wasted before consumption, food fed to pets, and home-grown foods (when the latter is not included in production data). Nutrients available in the food supply are usually estimated from standard food composition tables.

Data on per-capita food availability provide useful leads for further research on the relationship of diet to disease, because they enable investigators to compare rates of chronic diseases among countries with marked differences in mortality rates from chronic diseases and in the availability of specific nutrients in their food supply. These cross-sectional comparisons do not control for confounding factors, nor can they be used to show associations between diet and disease in individuals. The food supply in the United States has been monitored by the U.S. Department of Agriculture (USDA) since 1909, and the information gathered has been used to estimate trends in food use (see Chapter 3).

In household food inventories, food consumed is estimated by recording the difference between inventories of foods on hand at the beginning and end of the study period—usually 1 week—and accounting for food purchased or otherwise brought into the house. Average per-capita intake is estimated by dividing total household food intake by the number of people in that home.

Per-capita intakes by different age-sex groups in U.S. households are provided by national surveys conducted by USDA (USDA, 1984, 1987) and the U.S. Department of Health and Human Services (Carroll et al., 1983). These are also discussed in Chapter 3.

Individual Dietary Data

Food supply data and household food inventories supply only rough estimates of foods available and cannot be used to determine intakes of individuals. Methods most often used to assess individual intakes are food records and dietary recalls, both of which include diet histories and food frequency questionnaires.

The food record method requires participants to measure and record types and amounts of all foods and drinks consumed over a specified time. In some studies, all foods are weighed. In others, measuring cups and spoons and a ruler are used to assess dimensions. Food models, volume models, and photographs have also been used.

The 24-hour recall method requires that respondents report the types and amounts of foods they consumed over the previous 24-hour period. Information is obtained by face-to-face (in-person) interview or by telephone.

Diet history methods rely on interviewers or questionnaires to estimate the usual diet (or certain aspects of the diet) of subjects over a long period. The objective is to obtain a picture of habitual intake, which is more likely to be related to slowly developing diseases than is the intake over a time as short as 24 hours, which cannot represent the customary or usual intake. The classic diet history method used by Bertha Burke (1947) included a 3-day food intake record, a 24-hour recall, and an accounting of the frequency of food intakes over a period of 1 to 3 months. This method is rarely used today in its entirety. A less intensive version consists of two steps. First, an interviewer obtains detailed information about usual diet and portion sizes, e.g., what is usually consumed for each meal and for snacks. Then, to improve recall and obtain a more complete picture of habitual food practices, the interviewer helps the respondent review a detailed list of foods and adds anything omitted (Fehily, 1984).

Some diet histories are obtained through questionnaires, administered by an interviewer or completed independently by the respondent, that ask about the number of times each listed food is consumed (and sometimes the amounts) over a specified period, such as a few weeks or a year. This is often called the food frequency method. Few or many food items may be listed on the questionnaire. For example, in studies of the association between diet and cancer, the questionnaire may focus only on foods that provide a nutrient of particular interest.

In most case-control studies to determine the etiology of chronic diseases, investigators have recognized the difficulty of determining past dietary intake and have assumed that the current diet (or the diet prior to the onset of symptoms of the disease) reflects past intake sufficiently well to identify associations of dietary factors with disease (Morgan et al., 1978). However, recall of a diet from the distant past (17 to 25 years ago) or the

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