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•This study examined the association between vitamin D status and depressive symptoms.•Physical functioning was explored as a possible mediator in this relationship.•Two large cohorts of older persons were analyzed cross-sectionally and longitudinally.•Older women with low vitamin D status reported more depressive symptoms over time.•Decreased physical functioning may partially explain this association.

Statistical mediation analysis is an often used method in trials, to unravel the pathways underlying the effect of an intervention on a particular outcome variable. Throughout the years, several methods have been proposed, such as ordinary least square (OLS) regression, structural equation modeling (SEM), and the potential outcomes framework. Most applied researchers do not know that these methods are mathematically equivalent when applied to mediation models with a continuous mediator and outcome variable. Therefore, the aim of this paper was to demonstrate the similarities between OLS regression, SEM, and the potential outcomes framework in three mediation models: 1) a crude model, 2) a confounder-adjusted model, and 3) a model with an interaction term for exposure-mediator interaction.