Evaluating an intervention’s theoretical basis can inform design modifications to produce more effective interventions. Hence the present study’s purpose was to determine if effects from a multicomponent lifestyle intervention were mediated by changes in the psychosocial constructs decisional balance, self-efficacy and social support.

Delta Body and Soul III, conducted from August 2011 to May 2012, was a 6-month, church-based, lifestyle intervention designed to improve diet quality and increase physical activity. Primary outcomes, diet quality and aerobic and strength/flexibility physical activity, as well as psychosocial constructs, were assessed via self-report, interviewer-administered surveys at baseline and post intervention. Mediation analyses were conducted using ordinary least squares (continuous outcomes) and maximum likelihood logistic (dichotomous outcomes) regression path analysis.

Churches (five intervention and three control) were recruited from four counties in the Lower Mississippi Delta region of the USA.

Rural, Southern, primarily African-American adults (n 321).

Based upon results from the multiple mediation models, there was no evidence that treatment (intervention v. control) indirectly influenced changes in diet quality or physical activity through its effects on decisional balance, self-efficacy and social support. However, there was evidence for direct effects of social support for exercise on physical activity and of self-efficacy for sugar-sweetened beverages on diet quality.

Results do not support the hypothesis that the psychosocial constructs decisional balance, self-efficacy and social support were the theoretical mechanisms by which the Delta Body and Soul III intervention influenced changes in diet quality and physical activity.