Association between vitamin D and depressive symptoms varies by season: Results from the German Health Interview and Examination Survey for Adults (DEGS1).

BACKGROUND

Findings from epidemiological studies regarding seasonal variations in the association between vitamin D status and depression are inconsistent.

METHODS

Cross-sectional analysis of data from 6331 participants aged 18-79 years in the nationwide representative German Health Interview and Examination Survey for Adults 2008-2011 (DEGS1). Associations between 25-hydroxy-vitamin-D (25(OH)D) serum levels in quartiles and current depressive symptoms as measured by the Patient Health Questionnaire depression module (PHQ-9) and defined as severity of depressive symptoms (PHQ-9 score, range 0-27 points) and elevated depressive symptoms (PHQ-9 score ≥10 points) were analysed using multivariable linear and logistic regression adjusted for sociodemographic, biological and lifestyle factors. Analyses were stratified by summertime (May to October) and wintertime (November to April) because of evidence for interaction with season (p≤0.01).

RESULTS

In crude analyses, vitamin D status was inversely associated with both depression outcomes in summertime but not in wintertime. After adjustment for potential confounders, a significant association with severity of depressive symptoms remained in summer, with 0.73 point lower PHQ-9 scores in the highest versus lowest quartile. The association between 25(OH)D quartiles and elevated depressive symptoms in summertime was less strong and no longer significant in fully adjusted models.

LIMITATIONS: Participants with severe depression may be underrepresented in DEGS1. Residual confounding cannot be excluded.

CONCLUSION

25(OH)D serum levels were inversely associated with current depressive symptoms in summer but not in wintertime. The fact that the association is stronger in summertime suggests that vitamin D deficiency may be a consequence rather than a cause of depression.