Does physical activity mediate the reduction of depressive symptoms in older adults treated with behavioural activation? A secondary analysis of a cluster randomised trial

Background: Behavioural activation reduces depressive symptoms in older adults but its working mechanisms remain unknown. Physical activity may play a role by decreasing depressive symptoms in older adults. We investigated whether an increase in physical activity explains the effects of behavioural activation on depressive symptoms in older adults. Methods: Participants were 44 adults aged 65–88 years, who participated in a cluster randomised trial evaluating the effectiveness of an eight-week behavioural activation treatment (n = 25) compared with eight weeks of treatment as usual (TAU; n = 19) in primary care. Mediation analysis was used to evaluate whether the effects of behavioural activation on depression were mediated by physical activity as assessed by trunk accelerometer. Results: Physical activity did not mediate the effect of behavioural activation on depression because physical activity was not related to type of treatment (p = 0.336). An increase in physical activity was associated (R²_adj = 0.075) with a decrease in depressive symptoms, regardless of type of treatment (p < 0.05). Conclusion: In this sample, physical activity did not explain the difference in effectiveness between behavioural activation and treatment as usual. In line with the literature, our results showed an association of a small increase in physical activity with a decrease in depressive symptoms, regardless of treatment modality. Larger studies are needed to replicate these findings. Trial registration: Netherlands Trial Register, NL5436.