<p>Dietary supplement (DS) use is highly prevalent in the U.S. and can contribute substantial amounts of micronutrients to the one-half of U.S. adults and nearly one-third of U.S. children who use them. Despite their pervasive use, the best practices for assessing the prevalence of use of and measuring nutrient intakes from supplemental sources remain largely unknown, and currently no metric designed to assess total micronutrient exposures from foods, beverages, and DS exists. Comprehensive and robust methods for assessing the prevalence of use of and nutrient intakes from DS are critical for population-level research and surveillance. </p> <p>Therefore, the overarching aims of the research presented in this dissertation were to 1) identify the most comprehensive method for assessing the prevalence of use of and estimating nutrient intakes from DS, 2) characterize DS use and examine trends in overall and micronutrient-containing DS use among the U.S. population using multiple modes of DS assessment, 3) develop the first micronutrient-based diet quality index designed to capture usual intakes from all sources of under-consumed micronutrients among the U.S. population (i.e., the Total Nutrient Index (TNI)) and 4) to determine if the TNI is a useful measure for assessing total micronutrient exposures among the U.S. adult population.</p> <p>National Health and Nutrition Examination Survey (NHANES) 2011-2014 data among U.S. adults were employed to evaluate four combinations of methods constructed from two data collection instruments (i.e., the 24-hour dietary recall (24HR) and the NHANES in-home inventory with a frequency-based DS prescription medicine questionnaire (DSMQ)) for measuring the prevalence of use of and amounts of selected nutrients from DS. The results from the analysis indicated that a higher prevalence of DS use is reported when using frequency-based methods, like the DSMQ, but higher nutrient amounts are reported on the 24HR. Thus, combining the DSMQ with at least one 24HR (i.e., DSMQ or ≥1 24HR) provides the most comprehensive approach for measuring DS use and estimating nutrient intakes from supplemental sources among U.S. adults.</p> <p>Differences in the estimated prevalence of use of DS on the DSMQ or ≥1 24HR as compared with the DSMQ only were also noted among a nationally representative sample of the U.S. population from the 2007-2018 NHANES. When characterizing DS use and examining trends in overall and micronutrient-containing DS use among the U.S. population using multiple modes of DS assessment, increases in both overall and micronutrient-containing DS use were observed over time, but the prevalence of use was lower on the DSMQ than the DSMQ or ≥1 24HR among the total population and across all population subgroups. Therefore, the findings from this study were congruent with our previous analysis and contributed to a growing body of literature that utilizes multiple methods of DS assessment to improve the comprehensiveness of DS exposure assessment. </p> <p>The insights garnered from identifying the most comprehensive method for approximating micronutrient intakes from supplemental sources were implemented in the development and initial application of the first micronutrient-based diet quality index designed to capture usual intakes from all sources of under-consumed micronutrients among the U.S. population, named the TNI. The TNI assesses U.S. adults’ total nutrient intakes relative to the Recommended Dietary Allowance or Adequate Intake for eight under-consumed micronutrients identified by the Dietary Guidelines for Americans: calcium, magnesium, potassium, choline, and vitamins A, C, D, E. In the initial application of the TNI, based on dietary data from the 2011-2014 NHANES, the mean TNI score of U.S. adults was 75.4 out of 100, whereas the mean score when ignoring contributions from DS was only 69.0 (t-test; p<0.001). Therefore, the results from this analysis suggested that the TNI extends existing measures of diet quality by including nutrient intakes from all sources.</p> <p>However, another analysis designed to examine the construct and criterion validity of the TNI was completed using data from a nationally representative sample of U.S. adults who participated in the 2011-2014 NHANES, as well as exemplary menus reflective of high-quality diets. The results showed that the TNI yielded high scores on exemplary menus (84.8-93.3 out of 100), was moderately correlated with the Healthy Eating Index-2015 (r=0.48) and was able to differentiate between groups with known differences in nutrient intake for DS users vs. non-DS users, non-smokers vs. smokers, and those who were food-secure vs. food insecure (all <em>p</em><0.001). Moreover, correlations of TNI component scores with nutritional biomarkers for vitamins A, C, D, and E were significantly higher when compared with those obtained via the diet alone.</p> <p>Taken together, the research findings from the studies presented in this dissertation contribute evidence supporting the use of multiple modes of DS assessment to improve the comprehensiveness of DS exposure assessment, as well as the importance of capturing micronutrient exposures from all sources. These results also highlight the broad applicability of the TNI framework as a tool for comprehensively representing the totality of nutrient exposures, inclusive of DS. Our outcomes point towards a need for additional investigation into the random and systematic error that pervades DS assessment, the development of tools that can be used to describe micronutrient exposures more completely and how those relate to nutritional status, as well as further applications of the TNI. </p>