Perceived mental health literacy as a mediator between cognitive behavioural therapy (CBT) and depressive symptoms: a secondary data analysis of CoBalT trial data

Cognitive behavioural therapy (CBT) is an effective treatment for depression, but little is known about mediators of treatment response. Undertaking a secondary analysis of CoBalT trial data (CBT for treatment-resistant depression), we examined: (1) whether perceived mental health literacy (PMHL) at 6-months mediates CBT effect on depressive symptoms at 12-months; and (2) whether PMHL is an independent mediator after accounting for dysfunctional attitudes (DA) and metacognitive awareness (MA). Linear regression models were fitted between treatment allocation and outcome, analysing the impact of PMHL, DA, and MA. We examined the indirect effects of CBT on depressive symptoms via PMHL using the product of coefficients approach within a Structural Equation Modelling framework using single and multiple mediator models. Seventy-three percent of treatment effect observed on depressive symptoms was mediated through changes in PMHL, with total mediated indirect effect via PMHL, DA and MA increasing to 77%. When depressive symptoms at 3-months was included as an intermediate confounder, 53% of the effect at 12-months was mediated through PMHL at 6-months, increasing to 58% in the multiple mediator model. Mental health literacy may be key in understanding the effectiveness of CBT for depression, potentially increasing application of the understanding and skills learned, resulting in additional behaviour change.