Development and validation of a semi-quantitative food frequency questionnaire as a tool for assessing dietary vitamin D intake among Korean women

BACKGROUND/OBJECTIVES: Appropriate vitamin D status improves bone health and chronic diseases; it has shown benefits during the coronavirus disease 2019 pandemic. Therefore, assessing vitamin D status is crucial. However, limited research on vitamin D intake among Koreans complicates understanding of its consumption. This study aimed to develop and validate a semi-quantitative food frequency questionnaire (FFQ) to assess vitamin D intake among Koreans. SUBJECTS/METHODS: A vitamin D FFQ was developed to include 31 vitamin D source foods from 8 food groups frequently consumed by Korean adults. The study included 152 women residing in a major city in South Korea. From September 2020 to August 2022, intake was surveyed using a 12-day dietary record (12-day DR) across 4 seasons, and 2 vitamin D FFQs were conducted approximately 9–11 months apart (FFQ1 and FFQ2). Reproducibility (FFQ1 vs. FFQ2) and validity (FFQ2 vs. 12-day DR) were verified using Spearman’s rank correlation, weighted kappa coefficient, intraclass correlation, and Bland–Altman plots. RESULTS: The vitamin D intake of Korean women using the newly developed vitamin D FFQ was higher at FFQ1 (4.90 µg/day) and FFQ2 (4.58 µg/day) compared with the 12-day DR (4.07 µg/day). Additionally, the results for reproducibility and validity were demonstrated through the Spearman’s rank correlation coefficient (reproducibility, 0.592; validity, 0.460), weighted kappa coefficient (reproducibility, 0.379; validity, 0.284), intraclass correlation coefficient (reproducibility, 0.599; validity, 0.543), and Bland–Altman plots (reproducibility index, 3.95%; validity index, 3.95%). CONCLUSION: This study confirmed the newly developed vitamin D FFQ is reliable and valid for assessing vitamin D intake among Korean women. These results suggest the FFQ is an effective tool for dietary assessment, particularly in large-scale studies where year-round monitoring may not be feasible. Further validation in Korean men is crucial to enhance its applicability, enabling significant contributions to assessing vitamin D intake among Korean adults.