Dietary supplement use can be an important contributor to total nutrient intake in addition to diet. In NHANES 2003-2004 and 2005-2006, dietary supplement use information was collected during the household interview using the Dietary Supplement Questionnaire. Information about use, frequency, type, and amount taken over the past 30 days is collected for each supplement. From this, the average daily intake of nutrients from dietary supplements may be calculated for an individual. Dietary consumption data are collected using two 24-hour recalls, one obtained in the Mobile Examination Center, and one by telephone. For further details on dietary intake and supplement variables, please see Module 6 of the Dietary Tutorial and Information about Dietary Variables in the Additional Resources Section. Because the formats of the intake measures from food and supplements are different, and due to the differences in the different types of consumption patterns observed for supplement intake compared to dietary intake, joint modeling of the total nutrient intake is challenging. In this module, we adapt the method of Carriquiry, in which the reported amounts of nutrients from dietary supplements are combined with adjusted individual mean reported amounts from (repeated) 24 hour recalls.
Two of the primary challenges of modeling nutrient intake data from 24-hour recall measures are measurement error adjustment, and positively skewed data. The method developed by researchers at the National Cancer Institute (the “NCI method”) may be used to estimate the distribution of usual intake of nutrients, meeting these challenges. (See Module 20, Task 2 for further details.) Usual intake of nutrients solely from foods and beverages may differ between supplement users and non-users. To allow the usual intake of nutrients solely from food and beverages to differ between supplement users and non-users, the reported amount of the nutrient provided from supplements may be incorporated as a covariate in the model. This allows the mean intake to differ by supplement use. Rather than using a Monte Carlo method to estimate the distribution of usual intake of the nutrient from food and beverages, a shrinkage estimator, or adjusted mean, is calculated for each person based on their 24-hour recalls. Then, the individual’s average daily nutrient intake from dietary supplements (see Module 14, Task 3 for details on how this is calculated) is added to this adjusted mean, and the empirical distribution of the sum is used to estimate the distribution of usual intake from all sources. Survey weights are incorporated in the modeling and estimated procedures. Balanced repeated replication (BRR) (Module 18, Task 4) is used to calculate standard errors.
Complete details of the NCI method and the SAS macro necessary to fit this model can be found at the NCI website.
Carriquiry AL. Estimation of usual intake distributions of nutrients and foods. Journal of Nutrition 2003;133:601-608
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