Objective: To systematically investigate the extent and type of theory use in physical activity and dietary interventions, as well as associations between extent and type of theory use with intervention effectiveness.
Methods: An in-depth analysis of studies included in two systematic reviews of physical activity and healthy eating interventions (k = 190). Extent and type of theory use was assessed using the Theory Coding Scheme (TCS) and intervention effectiveness was calculated using Hedges's g. Metaregressions assessed the relationships between these measures.
Results: Fifty-six percent of interventions reported a theory base. Of these, 90% did not report links between all of their behavior change techniques (BCTs) with specific theoretical constructs and 91% did not report links between all the specified constructs with BCTs. The associations between a composite score or specific items on the TCS and intervention effectiveness were inconsistent. Interventions based on Social Cognitive Theory or the Transtheoretical Model were similarly effective and no more effective than interventions not reporting a theory base.
Conclusions: The coding of theory in these studies suggested that theory was not often used extensively in the development of interventions. Moreover, the relationships between type of theory used and the extent of theory use with effectiveness were generally weak. The findings suggest that attempts to apply the two theories commonly used in this review more extensively are unlikely to increase intervention effectiveness.
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