This task relates to estimating total mean nutrient intakes from foods, as reflected in the 24-hour dietary recall, and dietary supplements.
In NHANES 1999-2004, the nutrient amounts in the dietary recall interview files reflect only nutrients obtained from foods and beverages, including sweetened water beverages. They DO NOT include nutrients obtained from plain drinking water. Beginning in 2005, nutrients from plain drinking water will be included in the data release.
Estimating total nutrient intake requires using data from both the 24-hour recalls and dietary supplement questionnaire. (For more information on merging supplement files, see “Module 8: Merge and Append Datasets, Task 1: Merge NHANES data.”) These two types of data have different reference periods and measurement error characteristics (see Tasks 2 and 3 in this module). Therefore, some data manipulation is required to combine and summarize the data. Also, the study sample sizes may differ because some supplement users did not complete the dietary recall interview and persons who completed the dietary recall may not be supplement users. Exploratory analyses are useful to identify the characteristics of the supplement use and dietary recall samples.
All the key concepts and caveats regarding estimating nutrient intakes from both dietary (foods and beverages) and supplement sources apply when estimating total nutrient intake. To date, no elegant procedures are available to combine these two types of data.
When estimating the mean of the population distribution of usual nutrient intakes from supplement data, no standard convention for statistical adjustment currently exists.
For this course, the sample of persons with satisfactory data for both supplements and the Day 1 recall will be selected. Then, for each person, the average daily nutrient intake from supplements will be determined and added to the nutrient intake from the 24-hour recall. Finally, a weighted mean of those values will be obtained; in this example, the Day 1 dietary recall weight will be used because it represents the variable that applies to all members of the smallest analysis subpopulation (See “Module 6: Locate Variables, Task 4: Identify Correct Sample Weights and Their File Locations” for more information). This method assumes that the sample of persons with satisfactory data on both types of data is representative of the population.
Because the units of measure are different between recalls and supplement data, ingredient units (DSDUNIT) for each nutrient of interest on the supplement files will need to be converted to units used in the dietary intake data files. For example, all calcium units should be converted to milligrams. Most of the supplement units for calcium are in milligrams, but there are some units in grams that require conversion.
Also, nutrients may be listed as compounds and need to be converted to elemental form and amounts. For example, there may be some instances of calcium carbonate, which will need to be converted to the corresponding amount of elemental calcium. This is less of a concern with the supplement data releases since 2001, but this issue occurs periodically in earlier surveys.
As in the case of estimating nutrient intakes from supplements alone, analysts must consider the possibility of missing data and whether or not to include antacids (in the case of calcium or magnesium). For further information regarding these topics, see the previous task (Task 3), “Key Concepts about Estimating Nutrient Intakes from Supplements.”
Means should be examined along with their standard errors, to get an indication of the variation about the mean. Special statistical procedures are required to get appropriate standard errors when using data from a complex sample such as the NHANES. In addition, appropriate sample weights should be applied, if the data are being used to represent the population as a whole. See “Module 13: Estimate Variance, Analyze Subgroups, and Calculate Degrees of Freedom” for further information.
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