There are two different ways to consider food sources of nutrients—as “important” vs. “rich” sources. Rich sources are those foods with the greatest concentration of a nutrient; important sources are those that contribute the most to a population’s dietary intake. For example, sardines are a rich source of calcium, but they are not a very important source in the US diet, because they are consumed relatively infrequently. Fluid milk, on the other hand, is both a rich and important source of calcium because it contains high levels of calcium (rich) and it is consumed frequently in the population (important). A food composition table or database can provide information about rich sources of nutrients, whereas population intake as well as food composition data are needed to identify important sources. This task demonstrates the latter.
Food sources of nutrients are identified using the 24-hour recall data, because these data provide the necessary detail regarding all foods consumed and contain nutrient values associated with each food.
Identifying food sources of nutrients involves ratios, because it deals with the proportion of a given nutrient that is supplied by a given food. More specifically, it involves a ratio of means, because the percentage contribution of each food to the population’s total consumption is of most interest.
A ratio of means provides a single aggregate value for the whole population, so individual values are not available for each person. However, if subgroup differences are of interest then separate ratios of means can be estimated for each group.
When data from one or two 24-hour recalls are used to estimate a ratio of means, the mean in the numerator and the mean in the denominator can each be considered an estimate of usual intake (for reasons stated in Task 1 of “Module 14: Estimate Population Mean Intakes”). Therefore, no specific statistical adjustments are necessary.
Identifying important food sources of nutrients requires grouping the foods reported in the survey using either pre-existing schemes or one developed for the purpose. It is important to list all the food groups used in the analysis when reporting results, because decisions regarding how the foods are grouped can have a major influence on the relative contributions of one food group vs. another. For information about some pre-defined food grouping schemes, see “Module 4: Resources for Dietary Data Analysis” of the Survey Orientation Course.
In identifying important food sources of nutrients, use the ratio of means and be clear about how foods are grouped.
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