## 4 Discussion
Therefore, our data support the current findings that increased dietary vitamin D intake and serum 25(OH)D levels in women may be beneficial for cardiovascular diseases. The US Endocrine Society defines vitamin D deficiency as circulating 25(OH)D levels less than 20 ng/mL, and it recommends a target 25(OH)D concentration of 30 ng/mL to ensure vitamin D pleiotropic effects [7; 56]. According to this definition, only 3.9% of the participants in this study were vitamin D sufficient. As a result, the majority of Korean adults have inadequate serum 25(OH)D levels. Between 2008 and 2014, the serum 25(OH)D levels decreased and vitamin D deficiency increased significantly in both males and females, according to the KNHANES [32; 36; 38]. However, the vitamin D intake was not adequately increased [36; 37]. A variety of measures, including vitamin D consumption education and the development of vitamin D-fortified foods, must be implemented in Korea to improve the vitamin D nutritional status. A continuous follow-up study is being conducted to determine the effect of vitamin D intake and the resulting benefit on serum vitamin D levels and related health factors such as MetS. The 2013\(-\)2014 KNHANES has a large sample size, but as a cross-sectional study, it cannot reveal the precise causal relationship between vitamin D status (intake and serum level) and MetS components. The absence of supplement vitamin D intake data is a weakness of both this study and the KNHANES for vitamin D research, as many adults currently take vitamin D supplements for bone health. In addition, a one-day, 24-h recall method could not calculate the average daily intake of the participants. Although the season for drawing blood is evenly distributed, repeated measurements of 25(OH)D for individuals may provide a more accurate picture of the long-term exposure. Furthermore, no research has been conducted on the effect of exposure time on the serum 25(OH)D concentration. Interestingly, vitamin D intake did not differ according to the serum 25(OH)D categories, suggesting that vitamin D intake does not precisely reflect the serum 25(OH)D levels, at least in the present study. Despite the fact that there is a discrepancy between serum 25(OH)D and dietary vitamin D intake in terms of serum biomarkers and statistical analysis of MetS, this study is worthwhile to investigate the assessment of dietary vitamin D intake for health caution or a healthier lifestyle. Finally, this study used a newly supplemented and established vitamin D content DB with 2013\(-\)2014 KNHANES data to identify factors related to the serum 25(OH)D level, vitamin D intake, and MetS. Thesefindings demonstrated that maintaining a healthy serum 25(OH)D level can improve MetS, particularly abdominal obesity, TG levels in men, and LDL cholesterol levels in women. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13063748/s1](https://www.mdpi.com/article/10.3390/app13063748/s1)]([https://www.mdpi.com/article/10.3390/app13063748/s1](https://www.mdpi.com/article/10.3390/app13063748/s1)), Table S1: Contribution of vitamin D-rich food groups towards the daily mean intake of vitamin D by categories of the serum 25(OH)D concentration. Conceptualization, J.A.C., S.Y.L. and S.S.; methodology, S.-I.Y., S.S. and J.-Y.M. ; software, S.-I.Y., S.Y.L., S.S. and J.-Y.M. ; validation, J.A.C., S.-I.Y. and J.-Y.M. ; formal analysis, S.-I.Y. and J.-Y.M. ; investigation, J.A.C. and S.-I.Y. ; resources, J.A.C. ; data curation, S.-I.Y. and J.M; writing--original draft preparation, S.-I.Y. and J.-A.C.; writing--review and editing, J.A.C. and S.-I.Y. ; visualization, S.-I.Y. and J.-Y.M. ; supervision, J.A.C. ; project administration, J.-Y.M. ; funding acquisition, J.A.C. All authors have read and agreed to the published version of the manuscript. This research was supported by the Ministry of Education of the Republic of Korea and the National Research Foundation of Korea (NRF-2022R1A2C1091570 and NRF-2020R1A5A8017671) and Chungnam National University Research grant.