Negative Association between Serum Vitamin D Levels and Depression in a Young Adult US Population: A Cross-Sectional Study of NHANES 2007–2018 †

Background: Vitamin D has been suggested to play a role in the development of depression, but the nature of the relationship between the two is still not fully understood. Although some studies have shown an association between vitamin D deficiency and depression, others have yielded inconsistent or inconclusive results. As a result, further research is needed to better understand the relationship between vitamin D and depression. Objectives: This study aims to assess the association between serum vitamin D and the risk of depressive symptoms in individuals aged 20 years and older in the United States. Methods: We selected 7562 participants from the 2007–2018 US National Health and Nutrition Examination Survey (NHANES). Participants’ serum vitamin D levels were determined from laboratory data, and those with a Patient Health Questionnaire-9 (PHQ-9) score of ≥ 10 were considered to have depressive symptoms. Associations between vitamin D and depressive symptoms were investigated using multiple logistic regression, subgroup analysis, and smoothed curve fitting. Results: In our study, 11.17% of the participants had depression. Multiple regression stratified analysis showed a significant inverse association between serum vitamin D and depression in the 29–39 years age group after full adjustment (OR = 0.54, 95% CI: 0.31–0.95, p = 0.0316). This result was supported by subgroup analysis and smoothed curve fitting. Conclusions: The association between serum vitamin D and depressive status in US adults varied across populations. Among those aged 29–39 years, vitamin D supplementation tended to show a lower rate of depression.